Dictionary of Terms¶
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z
A¶
Abbe's number¶
There are at least three versions of the Abbe number, which is an important parameter of optical glasses:
where indexes at \(n\) describe corresponding standard wavelengths for optical purposes (cf. nu-value).
Absorbance¶
The characteristic of the absorption of light by a semi-transparent medium. It is calculated by the following equation:
where \(I\) is the intensity of light transmitted through the medium, \(I_{0}\) is the intensity of incident light, \(A\) is the optical density. \(A = k/d = \alpha/0.4343d\) where \(k\) is the absorption coefficient, \(\alpha\) is the decimal absorption coefficient and \(d\) is the distance of transmission of light through the medium.
Absorption coefficient¶
The characteristic of a decrease in intensity of light going through the layer of a semi-transparent medium 1 cm thick. It is defined by the Beer-Lambert law:
where \(I\) is the intensity of light transmitted through the medium, \(I_{0}\) is the intensity of incident light, \(d\) is the thickness of the layer, \(k\) is the absorption coefficient.
Sometimes the Beer-Lambert law is written in the form:
In such cases, a decimal absorption coefficient \(\alpha\) is used for characterization of medium absorption.
As follows from the above equations, \(\alpha = 0.4343k\). Both \(k\) and \(\alpha\) are expressed in \(cm^{-1}\).
Acoustical properties¶
A group of properties in SciGlass which includes values of sound velocity and sound absorption.
Activation energy¶
This term is used to describe the amount of energy necessary for an atom, vibrating in the vicinity of a certain position in a glass-forming network, to acquire to surpass a potential barrier surrounding it and move to another position. When a group of atoms can form two or more stable configurations, the energy of vibrations needed for this group of atoms to change from a certain configuration to another is also called the activation energy. Activation energy can be determined by the temperature dependence of a property whose value is directly connected with surpassing potential barriers by atoms or ions of glass. These are the so-called transport properties. When the structure of the substance does not depend on temperature (meaning the height of potential barriers does not depend on temperature), the temperature dependence of any transport property is described by the following equations:
or
where \(\rho\) is a property, \(T\) is the temperature in K, \(\rho_{0}\) is a constant (extrapolation of value \(\rho\) to infinite temperature), \(R\) is the gas constant, \(k\) is the Boltzmann constant, and \(E_{\rho}\) is the activation energy. It is calculated either for one mole of the substance (Eq.(1)) or for one atom or atomic group (Eq.(2)). If the investigated property is proportional to the mobility of ions (atoms) of the substance (coefficients of diffusion of ions, atoms, or gaseous molecules; electric conductivity), then one should use a minus sign on the right side of the two equations; if the property is inversely proportional to mobility (viscosity, electrical resistivity) then one should use a plus sign.
It should be noted that because both equations are valid only when potential barriers do not change with temperature, the use of the term activation energy to describe temperature dependencies of transport properties of glass-forming liquids in temperature ranges below 1000-1200\(^{\circ}\)C has no physical meaning. In these ranges values of \(E_{\rho}\) could be considered as purely empirical constants and their values have no bearing on energy characteristics of transport processes.
Annealing point¶
The temperature at which the internal stress is substantially relieved in 15 min. It corresponds to the equilibrium temperature at which the glass has a viscosity of 1013.4 poises, as measured by the loaded-fiber method, using the following equation:
where \(\eta\) = viscosity in P, \(m\) = load in gram, \(g\) = acceleration due to gravity = 980 \(cm/s^{2}\), \(l\) = heated or effective length of fiber in centimeters, \(r\) = fiber radius in centimeters, and \(dl/dt\) = elongation rate in \(cm/s\).
The annealing point generally corresponds to the upper end of the annealing range. [Standard definition (ASTM; C162-52)]
Note
According to (Volf, 1961) [see References] the annealing point corresponds to viscosity 1013 P.
Annealing range¶
The temperature interval between the annealing point and the strain point.
B¶
Bulk modulus¶
By analysis¶
Composition of glass presented by analysis. Usually the difference between values of the concentration of components given by analysis and the real concentration of the components does not surpass \(\pm\)0.3-0.5 wt.% for each component (for components with low concentrations, the error of analysis can be much lower).
By batch¶
Composition of glass presented by calculating compositions using the amount of raw materials used for preparation of the glass batch. The difference between the calculated content of various components in a glass and the real content of these components could depend on many factors and vary among different studies over very broad ranges - from several tenths of a percent to several percent. The main factors are the volatility of batch materials and the glass itself in the course of its melting as well as the dissolution of crucible material in the glass. The last factor can be excluded by using crucibles made from precious metals (Platinum, Platinum-Rhodium etc.). The first factor is sometimes accounted for by researchers who add especially volatile components in amounts large enough to compensate for volatility losses. Such compensation is not made often and the authors usually do not mention such procedures.
C¶
Chemical durability¶
The lasting quality (both physical and chemical) of a glass surface. It is frequently evaluated, after prolonged weathering or storing, in terms of chemical and physical changes in the glass surface, or in terms of changes in the contents of a vessel. [Standard definition (ASTM; C162-52)]
Classes of chemical durability¶
In many standards of chemical durability tests (see acid resistance tests, alkali resistance tests, dimming resistance tests, staining resistance tests, water resistance tests, weathering resistance tests) the division of all studied glasses into several classes (sometimes a term group is also used) of chemical durability is proposed. Sometimes the use of a class of chemical durability (acid class, alkali class, hydrolytic class etc.) can be considered as a convenient way to form an idea on the level of chemical stability of a glass without detailed inspection of a particular method of measurements and obtained results. At the same time one should have in mind that non-critical use of such simplified characteristics of chemical durability can lead to various misunderstandings. It can be shown by the following example.
Hydrolytic classes (see water resistance tests) can be determined by the DGG method using the amount of residue substance in the solution being in the contact with the studied glass and by DIN 12111 basing on the determination of the concentration of alkali ions by titration of the solution. Alkali borosilicate glasses with low concentration of alkali oxides and high concentration of boron oxide have hydrolytic class I according to DIN 12111 and hydrolytic class III or even IV according to DGG method (Volf, 1961: see References).
Note also that even if the method of measurements in two different standards is the same, the proposed classification of glasses in these standards could be different. For example (see Acid resistance tests), glasses which according to Soviet standard GOST 21400 (this standard was specially developed for testing chemical glass ware) belong to III acid class, according to classification adopted in DIN 12116 belong to I acid class.
Compressibility¶
The inverse of bulk modulus (see elastic properties).
Critical cooling rate¶
The minimal cooling rate of a substance initially existing in a liquid state which ensures obtaining this substance in a glassy state without visible signs of crystallization. The higher the tendency of the substance to crystallize, the higher its critical cooling rate.
Critical temperature¶
The maximum temperature of a phase-separation cupola.
Crown optical glass¶
An optical glass with low dispersion and low index of refraction, usually forming the converging element of an optical system. Any optical glass possessing a Nu-value of at least 55.0; or any optical glass with a Nu-value between 50.0 and 55.0 having a refractive index greater than 1.60. [Standard definition (ASTM; C162-52)]
Crystal glass¶
A colorless glass, highly transparent, frequently used for art or tableware. [Standard definition (ASTM; C162-52)]
Crystallization¶
Crystallization of glass-forming melts takes place at temperatures below their liquidus temperatures and is accompanied by the evolution of an amount of heat (heat of crystallization) which depends on the melt composition and the nature of the crystal phase. The difference between the liquidus temperature and the temperature at which crystallization takes place is called the level of supercooling of the melt.
The process of crystallization can be divided into two main stages: nucleation (formation of crystals with critical radii) and crystal growth. The nucleation rate is determined by the number of crystals forming per unit of time in a unit of melt volume (in the case of bulk nucleation) or on a unit of melt surface (in the case of surface nucleation). Two general kinds of nucleation are recognized: homogeneous nucleation, where the nucleation takes place in a homogeneous melt, and heterogeneous (catalyzed) nucleation, where nuclei are formed on the surface of foreign particles existing in the melt.
The rate of crystal growth is determined as the rate of movement of the interface between the crystal and the melt. Again, two kinds of crystal growth rates are recognized. For surface crystal growth, the growing crystals cover the whole surface after a period of time which results in the formation of a continuous surface layer. After that the crystal growth rate is determined as a rate of the growth of the crystal layer. The other rate is bulk crystal growth.
The dependences on supercooling of both crystal growth rate and nucleation rate have maxima at certain values of supercooling; the maximum nucleation rate corresponds to greater supercooling (to a lower temperature) than that of crystal growth rate. The greater the difference between the temperatures corresponding to the two maxima, the more stable the crystallization of the melt appears.
In the case of isothermal hold of a melt, the nucleation rate reaches its stationary value after a certain time after the beginning of the hold. During this time a stationary distribution of the dimensions of heterogeneous fluctuations corresponding to the temperature of the hold takes place. This time is usually described by the so called induction period which is determined by the intersection of the extrapolated linear part of the dependence of the formed nuclei on time and the X-axis.
The term induction period of crystallization is used also for characterization of the minimal time of isothermal hold after which the amount of the crystal phase becomes large enough for experimental detection. Sometimes the value of the induction period is also determined by the extrapolation of the time dependence of certain characteristics of crystallization (dimensions of a crystal, thickness of a crystallized layer etc.) to the X-axis. However, in this case, the physical meaning of this value becomes more complicated and less clear.
See also temperature dependence of crystallization parameters.
D¶
Deformation point¶
The temperature observed during the measurement of expansivity by the interferometer method, at which viscous flow exactly counteracts thermal expansion. The deformation point generally corresponds to a viscosity in the range from 1011 to 1012 poises. [Standard definition (ASTM; C162-52)]
See also dilatometric softening temperature.
Density¶
Mass of a unit volume of a substance. In the Database, they are given for values at 20, 600, 800, 1000, 1200, and 1400\(^{\circ}\)C.
Dielectric loss angle (loss angle)¶
The angle \(\delta\) between the excitations and response vectors of a circle diagram describing the response of a body in alternating electrical or mechanical fields. \(tan\space\delta\) is the ratio of imaginary to real parts of complex dielectric constant or complex elastic modulus. Sometimes \(tan\space\delta\) is also called the loss angle: see (Varshneya, 1994) in References.
Dielectric properties¶
A group of properties in SciGlass which includes values of dielectric permittivity and dielectric losses (see loss angle).
Diffusion¶
The process of moving ionic or molecular species in a substance from an area with a higher concentration to an area with a lower concentration of that species. See Diffusion coefficient.
Diffusion coefficient¶
The coefficient describing intensity of diffusion. According to Fick's first law of diffusion (in one dimension):
where \(J\) is the net number of atoms moving in the direction of the gradient of concentration of diffusing particles and is expressed in moles per unit area per unit time, \(dc/dx\) is the gradient of concentration (concentration \(c\) expressed in moles per volume), \(D\) is the diffusion coefficient.
Note
The above equation is correct in cases when only one kind of mobile atoms is present in the medium.
Diffusion of gases¶
Data on diffusion of gases are contained in the Database in the group Diffusivity, permeation & solubility of gases. Tables containing diffusion data for any individual gas of interest can be found with the help of the Subject Index.
See also Gas permeation and Gas solubility.
Dilatometric softening temperature, Mg¶
The temperature at which the length of a sample in a dilatometer reaches a maximum value when considerable external force is applied to the sample at constant rate during heating and begins to decrease as the temperature continues to rise. The appearance of the maximum on the dilatometric curve is connected with the parallel influence of sample dilatation with increasing temperature and sample deformation due to the viscous flow. The position of the maximum can correspond approximately to a certain viscosity of the studied glass. Usually this viscosity is assumed to be equal to 1011 P. However this value depends a great deal on the external load applied to the sample (the greater the load, the higher the corresponding viscosity), on the area of sample section (the greater the area the lower viscosity), on the value of TEC for the studied substance in its liquid state (the higher TEC the lower viscosity), on the rate of heating and on the previous thermal history of the sample (the higher the heating rate the lower viscosity; the lower the fictive temperature of the substance the lower viscosity). Thus, the actual viscosity values at dilatometric softening temperatures could differ from the value stated above as greatly as one order of magnitude.
See also deformation point.
Dispersion¶
Variation of refractive index with wavelength of light. See also R-value and Nu-value. [Standard definition (ASTM; C162-52)]
E¶
Elastic properties¶
In an isotropic body the following correlations between elastic properties are valid:
where \(E\) is the Young's modulus (the ratio of the linear stress to the linear strain in cases where a uniaxial stress is applies to a body), \(G\) is the shear modulus (the ratio of the shear stress to the shear strain), \(K\) is the bulk modulus (the ratio of hydrostatic stress to the volumetric strain), and \(\mu\) the Poisson's ratio (the ratio of the transverse strain to the longitudinal strain). Thus, it is enough to know any two of the four main elastic constants to describe the elastic behavior of glass.
Recently in some papers dealing with glass elasticity, the fifth elastic constant, \(L\), is used:
Electrical conductivity¶
The characteristics of the ability of a substance to conduct electrical current. It is the conductance of the cube when two opposite surfaces are connected to electrodes (cf. electrical resistivity). Units of electrical conductivity are reciprocal to those of electrical resistivity: Ohm-1·cm-1 in CGS and S/m in SI.
Electrical resistivity¶
The specific value characterizing the electrical resistance of a substance. It is the resistance of the cube when two opposite surfaces are connected to electrodes:
where \(I\) is the current, \(U\) is the applied potential, \(S\) is the cross-sectional area, \(d\) is the distance between electrodes, and \(\rho\) is the resistivity. Units of electrical resistivity: Ohm·cm in CGS and Ohm·m in SI.
Extinction coefficient¶
A characteristics of the influence of a certain coloring component on the optical absorption of a glass or melt at the selected wavelength. There are two kinds of extinction coefficients: one is used for determination of optical density \(D\), the other to determine the absorbance \(A\):
where \(\varepsilon_{d}\) and \(\varepsilon_{n}\) are the extinction coefficients (in \(l \cdot mole^{-1} \cdot cm^{-1}\)), \(C\) is the concentration of the coloring component (in \(mole \cdot l^{-1}\)), \(d\) is the thickness of the sample (in \(cm\)).
F¶
Fictive temperature¶
The temperature which is used for characterization of the structure of a glass-forming melt. The notion of fictive temperature \(T_{f}\) was formulated by Tool (1946) and, since then, has been used broadly to describe properties of glass-forming substances inside and below the glass transition region. Tool defined the fictive temperature of a substance in a non-equilibrium state as the actual temperature of the same substance in the equilibrium (liquid) state whose structure is similar to that of the non-equilibrium substance. This definition can be illustrated by the following figure which shows a method to determine \(T_{f}\) for any substance using the value of a property of this substance, when temperature dependencies of this property for glassy (\(\alpha_{g}\)) and liquid (\(\alpha_{l}\)) states are known.
It follows from the definition above that, for the glassy state, \(T_{f}\) does not depend on temperature, and, for the liquid state, \(T_{f}\) is always equal to the actual temperature of a substance. For rate cooling \(T_{f}\) below the glass transition region is equal to \(T_{g}\), obtained at the same cooling rate.
It should be noted that in most cases, the structure of a glass is not similar to the structure of the same substance in a liquid state at any temperature. Thus, for any \(T_{f}\) of glass, there could be an infinite number of different structures of glasses which would have the same property value but behave differently during subsequent heating or long isothermal holds (for more detailed description of the phenomenon, see Chapter 2 in Volume 2 of the User Guide). Quite often \(T_{f}\) determined for different properties of the same glass are not equal to each other. Nevertheless, the fictive temperature is a very convenient characteristic of glass and is widely used in literature on glass properties.
Flint optical glass¶
An optical glass with a high dispersion and high index of refraction, usually forming the diverging elements of an optical system. Any optical glass possessing a Nu-value less than 50.0 or any optical glass with a Nu-value between 50.0 and 55.0 having a refractive index less than 1.60. [Standard definition (ASTM; C162-52)]
Fracture toughness¶
The term "fracture toughness" is conventionally used for description of the critical stress intensity factor \(K_{1c}\) which is equal to (\(EG_{1c}\)) where \(E\) is the Young's modulus and \(G_{1c}\) is the energy required to extend the flaw from some initial size to the critical size.
G¶
Gas permeation¶
The passage of gas atoms through a glass-forming substance. The corresponding characteristic of the substance is called its permeability \(K\). It is connected to other characteristics of the interaction of the substance with gases by the following equation:
where \(D\) is the diffusion coefficient of gas atoms and \(S\) is the gas solubility.
Gas solubility (Solubility of gases)¶
The ratio of the saturation concentrations of species dissolved in a glass to that in the gas phase.
Glass¶
Glass is an inorganic product of fusion which has cooled to a rigid condition without crystallizing. [Standard definition (ASTM; C162-52)]
Note
In recent years there has been a definite tendency to broaden the meaning of the term glass and have it cover all kinds of non-crystalline solids (Varshneya, 1994: References). In SciGlass, however, the ASTM definition is strictly followed. All information presented inside SciGlass concerns the properties of non-crystalline solids cooled to a rigid condition from temperatures higher than the glass transition region, or the properties of glass-forming melt.
Glass formation¶
Selecting this item in the Property Group of Search tables feature, the user can obtain tables with ranges of glass formation in binary and ternary systems (1% of the tables contain System Type: >= 4 components) and with data on critical cooling rates. See also regions of glass formation.
Glass-forming melt¶
A melt which can be cooled without crystallization at a rate which is enough for producing a glass in sufficient quantities for its intended use. See regions of glass formation and critical cooling rate.
Glass transition region¶
The temperature range where the temperature dependencies of properties of a glass-forming substance, which are specific for a liquid state, transform gradually into temperature dependencies of properties specific for a glassy state. As a convenient and theoretically reasonable approximation it is possible to determine the width of the glass transition region by the width of a hysteresis loop obtained for the same cooling and heating rates and for the same property. A deficiency of this approach is the fact that the borderlines of hysteresis loop depend on the precision of measurements of the corresponding property. However, in most cases, it is not essential to know the exact positions of the boundaries of the glass transition regions and the method mentioned above is quite acceptable. The position of the glass transition region for any selected glass-forming substance depends on the rates of temperature change and, to a certain extent, on the selected property.
Glass transition temperature, Tg¶
One of the temperatures belonging to the glass transition region. Strictly speaking, \(T_{g}\) should be determined by using the temperature dependence of a property obtained at the cooling of a glass-forming substance. In this case, it is equal to the temperature corresponding to the point of intersection of temperature dependence for a liquid (metastable) state and the glassy state. However, in most cases it appears experimentally more simple to determine a certain value similar to \(T_{g}\) in the course of heating a glass sample. In the literature, this value is also called Tg. To distinguish these two values the notations \(T_{g}^{-}\) (\(T_{g}\) obtained under cooling) and \(T_{g}^{+}\) (\(T_{g}\) obtained under heating) will be used below. \(T_{g}^{-}\) is a function of the cooling rate: the lower the rate, the lower is the glass transition temperature. The value of \(T_{g}^{-}\) coincides with the value of fictive temperature \(T_{f}\) for glass obtained by cooling at the same rate as that used for determining \(T_{g}^{-}\).
The value of \(T_{g}^{+}\) is determined as the temperature of the point of intersection of temperature dependence of the property for the glassy state and the extrapolation of the straight line found by experimenter on the property dependence at high-temperature part of the glass transition region. According to the theory of glass transition, any linear temperature dependence of property within any portion of a glass transition region could not be found. Thus, unlike the \(T_{g}^{-}\) values, the values of \(T_{g}^{+}\) have no clear physical meaning and depend on many experimental facts and subjective approaches of experimenters to evaluation of experimental results. Nevertheless, these values give an idea about the influence of composition on the position of the central part of the glass transition region and therefore are quite useful for many applications. The value of \(T_{g}^{+}\) depends not only on the heating schedule of the measured sample but on the schedule of preliminary cooling the sample as well. The standard conditions of measurements of \(T_{g}^{+}\) are as follows: cooling and heating should be performed at the same rate; usually these rates are equal to 3 K/min.
For most glasses \(T_{g}^{-}\) values are reasonably close to temperatures at which the glass viscosity is equal to 1013.7 -1014 P. For \(T_{g}^{+}\) it is usually assumed that it corresponds to 1013.3 or just to 1013 P. One should have in mind that these estimations are quite approximate and could easily lead to an error equal to half an order of magnitude. However, due to the fact that for most glasses the temperature dependence of viscosity in the corresponding viscosity range is very steep (usually the change in temperature by 20-25 K leads to the change in viscosity by one order of magnitude) even such rough estimation of viscosity values at \(T_{g}\) could be considered as an acceptable one.
The most common exceptions are the values of \(T_{g}\) for phase-separated glasses (see primary glass transition temperature in phase separated melts). They can be much lower than the temperatures corresponding to the above-stated viscosity values.
Note
\(T_{g}^{+}\) is always higher than \(T_{g}^{-}\). Values of both \(T_{g}^{+}\) and \(T_{g}^{-}\) determined for the same glass and at the same cooling rate but by using temperature dependencies of different properties can be different.
Glassy state¶
Thermodynamically unstable but kinetically stable state of a non-crystalline substance obtained by cooling a liquid to a rigid condition without crystallization (cf. glass).
H¶
Heat capacity¶
Heat capacity is the amount of heat (joules or calories) required to increase the temperature of a body by 1 K. Heat capacity can be either specific (related to a unit mass) or molar (related to a unit mole). In most cases, it is measured at constant pressure and in this case is denoted by \(C_{p}\).
The group of tables in the Database can be selected by checking the Heat capacity checkbox in the Property Group list. It includes not only tables containing heat capacity data, but also tables with data on temperature dependencies of enthalpy \(H\). Virtually, these tables also contain data on heat capacity (\(C_{p}=dHdT\) where \(T\) is temperature in C or K).
I-K¶
Induction period¶
see Crystallization.
Internal friction¶
Characteristic of the energy loss of sinusoidal vibrations in a substance due to its inelastic deformations. It is equal to \(2\pi \cdot tan\delta\), where \(tan\delta\) is the loss tangent (see Quality factor).
Ion diffusion¶
Diffusion of ions inside the glass or into the glass bulk from the surface.
Note
Most data on the diffusion of oxygen in any form are included in the group of tables called Diffusion, permeation & solubility of gases. To select tables containing data on the diffusion of a certain ion, one can use the Subject Index.
L¶
Linear thermal expansion coefficient, TEC¶
The relative change in length of a sample per 1 K of change in temperature. Very often abbreviated LTEC or simply TEC. It is usually presented as a mean value within a certain temperature interval:
where \(\alpha\) is the mean TEC, \(l\) is the initial length of the sample, \(\Delta l\) and \(\Delta T\) are the changes in length and temperature of the sample. In principle, when one obtains a curve describing the temperature dependence of the sample length one can determine the true TEC at a certain particular temperature: \(\alpha=(dl/dt)/l\).
Liquid state¶
Thermodynamically stable (above liquidus temperature) or metastable (below liquidus temperature) state of a non-crystalline substance. It should be noted that this thermodynamic definition of liquid state does not agree with the most common definitions of liquids as substances, the main characteristic of which is the fluidity. According to the thermodynamic definition a liquid could be thoroughly rigid (for example, after a very long isothermal hold, a glass-forming substance can reach a metastable state at viscosity of 1015 P or higher).
In this Database, the thermodynamic definition of the term liquid state is used.
Liquid-liquid phase separation¶
Separation of a liquid into two liquid phases at composition and temperature ranges where such separation leads to a decrease in the free energy of the liquid. The rate of phase separation depends on the composition and temperature. The rate of phase separation is approximately proportional to the viscosity of the initial phase (at the first stages of the process), or the more viscous phase (if the less viscous phase is in droplets), or the less viscous phase (in all other cases). The process of liquid-liquid phase separation tends to produce two separate layers, a layer with lower density being on the top of the other one.
However, if the temperature of phase separation is low enough (i.e. the viscosity of the corresponding phase is high), the phase separated liquid or glass could only reach (even in cases of the longest possible periods of heat treatment) micro-heterogeneous structure (with dimensions of phase formations from hundreds of angstroms to hundreds of microns) remaining macro-homogeneous. High viscosity of liquids is, in most cases, characteristic to liquids in a metastable state (below liquidus temperatures) i.e., as a rule to glass-forming liquids. In such cases, the term metastable phase separation is often used.
Liquidus temperature, Tliq¶
The maximum temperature at which equilibrium exists between the molten glass and its primary crystalline phase. [Standard definition (ASTM; C162-52)]
Littleton temperature (softening temperature)¶
The temperature at which a uniform fiber, 0.5 to 1.0 mm in diameter and 22.9 cm in length, elongates under its own weight at a rate of 1 mm per min. when the upper 10 cm of its length is heated in a prescribed furnace at the rate of approximately 5\(^{\circ}\)C per min. For a glass of density near 2.5 \(g/cm^{3}\), this temperature corresponds to a viscosity of 107.6 poises. [Standard definition (ASTM; C162-52)]
Softening temperature is often called the Littleton softening temperature or the Littleton temperature or Littleton Point (L.P.) (the term used in SciGlass).
Sometimes the term Softening temperature is used for description of the temperature corresponding to viscosities near 1011 poises obtained by analyses of a dilatometric curve. This can lead to some confusion. It is more correct to use the term Dilatometric softening temperature.
Long glass¶
A comparative term signifying a slow-setting glass. [Standard definition (ASTM; C162-52)]
Loss coefficient (loss factor)¶
The imaginary part (\(\varepsilon ''\)) of the complex dielectric constant of a body in an alternating electrical field (cf. loss angle). According to the definition \(\varepsilon ''=\varepsilon ' \cdot tan\delta\) where \(\varepsilon '\) is the real part of the complex dielectric constant and \(tan\delta\) is the tangent of loss angle.
Low-temperature border of annealing range¶
This term is equivalent to the term Strain point.
M¶
Mean dispersion¶
Difference of refractive indices nF – nC for F and C wavelengths (see Standard wavelengths for optical properties). The quantity is used to determine the Nu-value.
Melting temperature¶
The range of furnace temperatures at which melting takes place at a commercially desirable rate, and at which the resulting glass generally has a viscosity between 101.5 to 102.5 poises. For purposes of comparing glasses, it is assumed that a glass at melting temperature has a viscosity of 102 poises. [Standard definition (ASTM; C162-52)]
Metastable phase separation (liquid-liquid phase separation)¶
see Liquid-liquid phase separation
Methods of chemical durability measurements¶
No other glass property is measured by such an abundance of methods as chemical durability. The same is true for the ways of expressing the characteristics of chemical durability of glasses.
Some methods have been developed for studying the mechanisms of processes which take place on a glass surface under the influence of various reagents; other methods - for comparison of various experimental or commercial glasses with each other (for example when new compositions are developed or the best existing compositions are selected); special methods are used to determine the ability of glass articles to function in the given conditions.
It is necessary to differentiate the chemical durability of the glass as a material (of the bulk of a glass) from that of the natural surface of glass articles. In quite a few cases these characteristics can be substantially different. Chemical durability of the bulk of glass is determined solely by glass composition and structure. At the same time, chemical durability of the surface of a glass article depends on many other factors as well, namely on the method of forming, annealing conditions (including conditions of the surrounding atmosphere), treatment by various reagents, storage conditions etc.
The most relevant information about the ability of glass articles to work under the conditions they were intended for can obviously be determined by testing them in those particular conditions. However, due to the comparatively high chemical stability of most commercial glasses, it is necessary to use accelerated tests which increase the temperature of the tests, increase the tested surface, or both.
See also surface area to solution volume effect and classes of chemical durability.
For details on the main measurement methods, see (Adachi, 1980; Bacon, 1968; Bezborodov, 1977; Hench, 1977; Holland, 1966; Jones, 1941; Newton, 1985; Scholze, 1991; Volf, 1961; Volf, 1969; Walters and Adams, 1968: References)
Microhardness¶
The characteristic of the deformation of the glass surface under the influence of a pyramidal diamond indenter. The size of indentation is usually of the order of several micrometers. In most cases, two kinds of pyramids are used, namely Vickers and Knoop pyramids. Their forms are presented in the figure:
The Vickers hardness number (VHN) = 1.8544F/D2 kgf/mm2
where F is the force in kgf and D is the average diagonal of impression in mm.
The Knoop hardness number (KHN) = 14.23F/L2 kgf/mm2
where L is the length of the long diagonal (see figure).
The difference between values obtained by these two kinds of measurements is not great and usually is within the limits of the scatter of data reported by various authors for the same glass composition.
Thus, in the Database all microhardness data are presented together independently on the kind of indenter used for the corresponding measurements.
Miscibility gap¶
The range of composition of a melt where the melt tends to form two coexisting phases (see Liquid-liquid phase separation). For a given system, the position of the miscibility gap depends on temperature and pressure. As a rule, the length of the miscibility gap increases with decreasing temperature. Dependences of properties on composition within miscibility gaps are usually considerably more pronounced and complicated than the same kind of dependencies outside the miscibility gap.
Molar extinction coefficient¶
The value describing the influence of certain chemical species on the optical absorption of a medium. It is defined by the equation:
where \(k\) is the absorption coefficient, \(c\) is the concentration in mol/liter, and \(\varepsilon\) is molar extinction coefficient which is expressed in liter/(mol \(\cdot\) cm).
Molar volume¶
The volume of one mole of glass. Molar volume is calculated by the following equation:
where \(M\) is the molar weight of glass and \(d\) is density.
Molar weight of glass¶
The averaged value of molar weights of glass components (oxides, halides etc.):
where \(m_{i}\) is the mole percent of the \(i\)-th component, \(M_{i}\) is the molar weight of the \(i\)-th component, \(M\) is the molar weight of glass.
Morphology of phase separated glasses and melts¶
In the case of Liquid-liquid phase separation, two basic types of structure of glasses can be enumerated: the droplet structure (one phase is continuous, the other one is discontinuous) and the two-matrix structure (both phases are continuous). If one moves from one end of a tie-line (100% of phase I) to the other one (100% of phase II) he encounters subsequently three structural regions:
(1) droplet structure - phase I forms matrix, phase II forms droplets
(2) two-matrix structure
(3) droplet structure - phase I forms droplets, phase II forms matrix.
Accordingly, if one studies the dependence of the so-called structure sensitive properties (viscosity, specific resistivity, diffusion, chemical durability) on the composition along a tie line he usually finds a sharp change in property values within a narrow range of composition. It is connected with the fact that the viscosity of a phase separated glass-forming melt is determined by the viscosity of the high-viscosity phase if it forms a matrix and by the viscosity of the low-viscosity phase if the high-viscosity phase forms droplets. The same is true for electrical conductivity and diffusion. Chemical durability is determined by a low-durable phase, if this phase forms matrix and by a high-durable phase, if the low-durable phase forms droplets. Changes in properties due to changes in the structure of a phase separated glass are considerable and can amount to several orders of magnitude. Near the composition where changes in phase structure take place, the structure could depend on the previous thermal history of the sample.
It follows that, in the course of the initial stages of phase separation, properties of a studied glass could change considerably during its heat treatment inside the immiscibility cupola. The direction of such changes depends on the structure of the glass after the first stages of the phase separation are completed.
N¶
N-values¶
\(n\), \(n_{D}\), etc. are abbreviations for the refractive index (see Refractive index), generally used with a subscript indicating the spectral line, for example, \(n_{D}\) = index of refraction for \(D\) or sodium line.
[Standard definition (ASTM; C162-52)]
Note
For characteristics of the spectral lines, see Standard wavelengths for optical properties.
Nu-value¶
Expressed by the Greek letter \(\nu\) or by the English letter \(V\). Refers to the reciprocal dispersion power as follows:
[Standard definition (ASTM; C162-52)]. See also Abbe's number.
Nucleation¶
see Crystallization
Nucleation rate¶
see Crystallization
O¶
Optical density¶
The characteristic of the absorption of light by a semi-transparent medium. It is calculated by the following equation:
where \(I\) is the intensity of light transmitted through the medium, \(I_{0}\) is the intensity of incident light, \(D\) is the optical density. \(D = 0.4343k/d = \alpha/d\) where \(k\) is the absorption coefficient, \(\alpha\) is the decimal absorption coefficient, and \(d\) is the distance of transmission of light through the medium.
Optical properties¶
A group of properties in the Database of SciGlass which includes Refractive index, dispersion, and absorption and transmission of optical waves.
P¶
Phase-separation cupola¶
An area in composition-temperature space inside which a glass-forming system separates into two phases. Usually, the range of compositions having a tendency toward phase separation decreases with increasing temperature.
Pochettino viscometer¶
The device is based on the method of measuring high viscosity values at conditions of pure shear. The investigated glass is introduced into a ring-shaped space between two concentric cylinders; the shearing force is applied axially, and the axial displacement of the inner cylinder towards the outer cylinder is measured by interferometry.
Poisson's ratio¶
When a uniaxial stress is applied to an element of an elastic substance, it produces a strain in the direction x of the applied stress and a strain of the opposite sign in two other directions, y and z. The ratio of the strain in directions y or z to the strain in direction x is called the Poisson's ratio.
Primary glass transition temperature in phase separated melts¶
Generally two coexisting phases in phase separated melts (see Liquid-liquid phase separation) have different values of viscosity and, hence, different glass transition temperatures. In the course of enthalpy measurements by DTA or differential scanning calorimetry methods, one can always observe thermal effects corresponding to two \(T_{g}\). In the course of dilatometric measurements, it is only possible when the more viscous phase forms a matrix. In all these cases, the lower \(T_{g}\) is called the primary glass transition temperature and denoted as \(T_{g}^{-}\) while the higher \(T_{g}\) is called the secondary glass transition temperature and denoted as \(T_{g}^{+}\).
Q¶
Quality factor¶
The characteristic of energy losses in a substance to which sinusoidal vibrations are applied:
where \(G''\) and \(G'\) are the components of complex shear modulus \(G^{*}\) (\(G^{*}= G' + j G''\)),
R¶
R-value¶
The partial dispersion ratio given by the following equation:
[Standard definition (ASTM; C162-52)]
Rate cooling¶
Cooling at a constant rate.
Rate heating¶
Heating at a constant rate.
Refraction¶
Specific refraction \(R\) is calculated by the following equation:
where \(n\) is refractive index and \(d\) is density.
Molecular refraction is calculated by the following equation:
where \(M\) is the molar weight of glass.
Refractive index¶
The ratio of the velocities of light in a vacuum to that in the substance. It is dimensionless.
Regions of glass formation¶
The areas of composition in chemical systems which can be cooled from stable liquid state to solid state without crystallization, i.e. could form glass. Corresponding chemical systems are called glass-forming systems. In any glass-forming system the exact dimensions of the regions of glass formation depend on cooling rates. The higher the rate, the larger the region. If a melt is cooled in air the cooling rate depends on the dimensions of a sample. The smaller the sample, the higher the rate and the broader the region of glass formation. See also critical cooling rate.
Relaxation time¶
Relaxation is the process of moving to equilibrium for a system driven out of equilibrium. Relaxation time \(\tau\) is a constant characterizing the speed of this moving. In the simplest case when the relaxation of the system can be described by only one relaxation time and this value does not change with time during the whole relaxation process, this process is described by the following equation:
where \(y(0)\), \(y(t)\), and \(y(\infty)\) are the values of a certain parameter of the system changing in the course of relaxation process at the beginning of the process, at a certain time t in the course of this process, and at infinite time (equilibrium), respectively.
For liquids and glasses, a broad distribution of relaxation times is specific for any composition and relaxation process. In this case the equation used for calculation of a relaxation process depends on the particular kind of distribution. During last twenty years, however, it was found that all kinds of relaxation (electrical, mechanical, etc.) in any amorphous substance lead to the possibility of description of the relaxation process by the so-called stretched exponent:
where \(b\) and \(\tau_{K}\) are constants. Value of \(b\) characterizes the distribution of relaxation times: at \(b=1\), there is no distribution (eq.(2) in this case is equivalent to eq.(1)), the smaller is \(b\) the broader is the distribution. Value of \(\tau_{K}\) is a mean value of relaxation times (index \(K\) is the first letter of the name of Kohlrausch, who was the first who proposed to use a stretched exponent for the description of a relaxation process in glass).
S¶
Sag point¶
One of the temperatures proposed for simple and quick determination of temperature dependence of glass viscosity. There are two methods of the measurements of the sag point.
One method was proposed by Spinner et al., 1957 (see References). According to this method, a long fiber is placed lengthwise on a platinum holder with the row of vertically positioned supports. The holder is inserted into the core of the furnace where a certain gradient of temperature is kept permanently. The sag point is defined as the temperature at which a fiber, 0.5 to 0.8 mm in diameter, horizontally supported at ½-inch intervals will sag under its own weight in 25\(\pm\)5 min. As it was shown by the authors of the method for all of 9 studied glasses Sag Point belonged to the range between the Deformation Point and Softening Point, in most cases nearer to the Deformation Point.
The other method of measurements of the Sag Point was proposed by Hirayama, 1962 (see References). According to this method, a horizontally positioned glass fiber 3 mm in diameter is fixed in the holder by one end and can sag under the weight of the free end which length is 2.5 cm. The Sag Point is defined as the temperature at which the glass fiber starts to deflect at heating at the rate 4 K/min. Approximately the Sag Point is equal to the temperature corresponding to 1010 P.
SA/V effect (surface area to solution volume effect)¶
The rate of chemical attack on a glass surface by various reagents depends not only on the characteristics of the reagent (including its pH), temperature, pressure, time, and the characteristics of glass surface but also on the ratio of glass surface area (SA) to the volume of the reagent (V). The ratio SA/V can influence the results of the attack quite strongly. At the same time, many authors of publications on chemical durability of glasses fail to report the value of this ratio (cf. descriptions of methods for measuring chemical durability).
There are two factors connected with the influence of SA/V ratio on chemical durability of glass which should be taken into account.
The first factor plays an important role in cases of water-resistant tests of alkali-containing glasses. The greater the SA/V ratio, the more intensive is an increase in pH resulting from leaching alkali oxides from a glass surface. An increase in pH usually leads to a significant increase in glass dissolution rate. The best way to eliminate the influence of this factor is to keep pH value constant, i.e. to perform the measurements in pH-static conditions (Bunker et al., 1984; Roshchina et al., 1995: see References).
The second factor is important for any kind of measurements. The greater the SA/V ratio, the higher is the rate of increase of the concentration of glass components dissolving in a solution which hinders the rate of dissolution. The dissolution stops altogether when the concentration of a corresponding component in a solution becomes quite near to the saturation concentration.
Shear modulus¶
The ratio of shear stress to shear strain. The term rigidity modulus is also used.
Short glass¶
A comparative term signifying a fast-setting glass. [Standard definition (ASTM; C162-152)].
Silica glasses¶
Both in the silica glass industry and science it is accepted to divide the whole spectrum of commercial transparent silica glasses into four types depending on the method of their production and on the content of impurities.
1st type - hydroxyl-free glasses obtained by melting from natural or synthetic quartz or cristobalite in electrical furnaces. Such glasses contain impurities inherited from the initial raw material (Li, Na, K, Al, Fe, Ti, Ca etc.) and virtually no water.
2nd type - glasses obtained by melting of grains from natural or synthetic quartz in hydrogen-oxygen flames or flames of natural gas. Glasses of this type contain impurities inherited from the quartz and a certain amount of impurities of structural water (several hundred ppm).
3rd type - glasses obtained by high-temperature hydrolysis of the volatile compounds of silicon. Glasses of this type are characterized by very low content of metal impurities but contain a considerable concentration of structural water and chlorine. Also included are glasses obtained by the two-stage method and containing an appreciable amount of OH groups.
4th type - glasses obtained by the method of high-temperature oxidation of SiCl4. Glasses of this type contain a very small amount of metal impurities and virtually no structural water. However, they contain high concentrations (several hundred ppm) of chlorine impurities. Also included are water-free glasses obtained by the method of high-temperature hydrolysis of volatile compounds of silicon using the two-stage technique.
Special distilled water¶
Special distilled water shall be freshly prepared before use, shall be free of dissolved gases (CO2, etc.), free of heavy metals and have a specific conductivity not exceeding 2·106 Ohm-1cm-1 at 20\(^{\circ}\)C, when tested immediately prior to use.
In German standards: 1·106 Ohm-1cm-1
Standard wavelengths for optical properties¶
(Pye at al., 1977 ): see References
| \(\lambda\),nm | Fraunhofer's symbol | Light source | Color |
|---|---|---|---|
| 365.01 | i | He | UV |
| 404.66 | h | Hg | violet |
| 435.84 | g | Hg | blue |
| 479.19 | F' | Cd | blue |
| 486.13 | F | H | blue |
| 546.07 | e | Hg | green |
| 587.56 | d | He | yellow |
| 589.3 | D | Na | yellow |
| 643.85 | C' | Cd | red |
| 656.27 | C | H | red |
| 706.52 | r | He | red |
| 768.2 | A' | K | red |
| 852.11 | s | Cs | IR |
| 1013.98 | t | Hg | IR |
Strain point¶
Or lower annealing temperature point, is the temperature corresponding to the viscosity of 1014.5 P. It is the temperature at which the stress in a glass is substantially removed in about 4h. (Volf, 1961): see References.
Strength of glasses¶
Strength is the ability of solids to withstand fracture under the influence of internal forces. Strength measured under the influence of compressive forces greatly surpasses the strength under tension. Theoretical (maximal) strength of glass is determined by the nature of interaction of its atoms. However, the real strength of glass samples or articles is usually 2 to 3 orders of magnitude lower than the theoretical one. The main reason for this is the existence of microcracks on the glass surface.
Application of tensile stresses to a sample leads to a concentration of stresses on the tips of the cracks. The intensity of this concentration depends on the depth of a crack \(\delta\) and its shape. Under the influence of a tensile stress the depth of the crack is continuously increased. Simultaneously the stresses at the crack tip increase also. Increase in humidity of the surrounding atmosphere leads to an increase in the rate of crack growth. When stresses at the tip of the crack surpasses the limiting strength of the glass, the fracture of the glass takes place. The value of tensile stress (\(\alpha_{t}\)) leading to the fracture of a sample is determined by the following equation:
where Y is a coefficient dependent on the geometry of the crack, \(K_{1c}\) is the critical stress intensity factor (it is often called the fracture toughness) which is constant for the material. The fracture of a sample does not take place immediately after the application of the stress, but only after a certain time \(t\), during which the crack reaches a critical length. If the load applied to a sample is a constant one, the above-mentioned time is denoted by \(t_{s}\) and is called the static fatigue. If the applied load increases with time, the corresponding value is denoted by \(t_{d}\) and is called the dynamic fatigue. The Dimensions and shapes of microcracks on a glass surface are statistically distributed and experimental characteristics of strength have a clear statistical character. An increase in a sample length increases the probability of appearance of more dangerous cracks on its surface. Accordingly, an increase of the sample length leads to a decrease of the sample strength. This dependence can be described by the Weibull distribution function.
It follows from the above said that the strength of glass depends not only on the composition of glass but also on quite a few other factors: condition of glass surface, humidity of atmosphere, rate of sample loading, time of application of the load etc. Depending on these factors the measured values of strength of the glass of the same composition could vary within a very broad range.
Stress-optic coefficient (Brewster's constant)¶
Ideally, homogeneous glass is isotropic. However, in the presence of non-hydrostatic stresses, it becomes non-isotropic. It means that perpendicular vibrations of a non-polarized light are going through the glass sample with different velocities. Accordingly, the sample is characterized by two different refractive indexes (develops a double refraction). A constant which shows correlation between the optical path difference between vibrations along two perpendicular axes (with minimal and maximal velocities) and stresses which gave rise to this difference is called the stress-optic coefficient, or Brewster's constant:
where \(B\) is the stress-optic coefficient, \(\sigma\) is the stress, \(d\) is the thickness of the sample, and \(\Delta\) is the optical path difference which is calculated by the following equation:
where \(\delta n\) is the difference in velocities of two above-mentioned vibrations and \(K\) is a coefficient depending on the selected units.
Learn more about the Brewster (B) unit in Wikipedia
Surface electrical conductivity¶
The conductivity of a square on the glass surface when a voltage is applied to two opposite sides. It is expressed in \(S\) (Ohm-1).
Surface tension¶
Amount of work (or energy) needed to form a unit surface.
T¶
Temperature dependence of crystallization parameters¶
In cases of high supercooling (at temperatures well below the maxima of nucleation and crystallization rates) temperature dependencies of both induction period and crystal growth rate can be approximated with a reasonable precision by the following equations:
where \(\tau\) is the induction period, \(V\) is the crystal growth rate, \(R\) is the gas constant, \(T\) is temperature in K, \(\tau_{0}\), \(V_{0}\), \(E_{t}\), and \(E_{v}\) are constants. \(E_{t}\), and \(E_{v}\) are often called activation energies.
Thermal conductivity¶
Thermal conductivity is the amount of heat transmitted per unit cross-sectional area of a body per unit time under the influence of a unit temperature gradient. In an opaque body, the process of heat transmittance is obeyed to Fourier's first law, and the corresponding equation can be written as follows:
where \(\lambda\) is the thermal conductivity (phonon thermal conductivity), \(S\) is the area of a cross-section, \(dT/dx\) is the temperature gradient. For a semi-transparent body (to such kind of bodies, most of the glasses belong) at high enough temperatures (for example, higher than 400\(^{\circ}\)C), the process of heat transfer connected with radiative heat transfer takes place. This process depends on a number of factors in a rather complicated way. In some cases, it is possible to use Fourier's first law also for description of this process with a reasonable approximation. In this case the corresponding coefficient in the above presented equation is called the photon thermal conductivity. In most cases, photon thermal conductivity is calculated by using IR-absorption spectra.
In the Database, only tables with data on phonon thermal conductivity are compiled.
Thermal diffusivity¶
For solution of problems of non-stationary heat transfer, it is more convenient to use instead of thermal conductivity values the values of thermal diffusivity \(a\):
where \(\lambda\) is the thermal conductivity, \(\rho\) is the density, and \(C_{p}\) is the heat capacity at constant pressure.
Note
Thermal diffusivity is usually denoted by lowercase \(\alpha\) (alpha), but a, h, \(\kappa\) (kappa), K, and D are also used (see: https://en.wikipedia.org/wiki/Thermal_diffusivity).
Thermal endurance (Thermal shock resistance)¶
The relative ability of glassware to withstand thermal shock. [Standard definition (ASTM; C162-52)]. Thermal endurance (the other term - thermal shock resistance) is expressed in K which describes the maximum difference between temperature of heated glassware or a sample of certain standard dimensions and the temperature of cool water into which the studied object could be quickly inserted without any visible indication of disintegration.
Thermal expansion coefficient¶
see linear thermal expansion coefficient and volume thermal expansion coefficient.
Tie-line¶
A straight line inside the immiscibility gap connecting compositions of two equilibrium phases coexisting with each other. All compositions of melts inside the immiscibility gap which are described by points positioned on a tie-line tend to separate into two coexisting phases with compositions corresponding to the ends of the tie-line after the completion of the phase separation process. All structure sensitive properties (viscosity, electrical conductivity, diffusion, chemical durability) demonstrate S-shaped dependence on composition along the tie-line (see morphology of phase separated glasses and melts). These dependencies are qualitatively the same for all isothermal sections of immiscibility cupola and all tie-lines inside them.
It is to be noted that \(T_{g}\) values for each of the coexisting phases does not depend on the amount of the phase in the glass and usually does not depend appreciably on the morphology of the phase (rare exceptions involve cases where the pressure inside the droplets formed by one of the phases differs considerably from the atmospheric ones). Thus, in most cases, the lines of constant \(T_{g}\) for phase separated glasses in ternary systems coincide with tie-lines. This feature of \(T_{g}\) in phase separated glasses and melts is a basis for the simplest method of determinations of the direction of tie-lines.
Tk-100¶
The temperature corresponding to the specific electrical resistivity of a substance equal to 108 Ohm·cm or 106 Ohm·m.
Transport properties¶
The properties connected with activated changes in positions of atoms or ions, i.e., viscosity and other rheological properties, electrical properties, and diffusion (see activation energy). Structural relaxation, which is connected with activated rearrangements in mutual positions of atoms and crystallization, can be also considered as a transport property. A number of features of temperature and time dependencies are the same for all transport properties.
U-V¶
Viscosity¶
Viscosity \(\eta\) is a measure of the resistance of the melt to shear deformation with time. It is a coefficient of proportionality in the following equation:
where \(f\) is the shear force, \(dν/dx\) is a gradient of the velocity of shear deformation, and \(S\) is an area of the melt perpendicular to this gradient. In the CGS system, the unit is written as dyn·cm-2 and is called the poise (commonly abbreviated as P). It is still the most commonly used viscosity unit and is used as a main unit in SciGlass. In the SI system, the viscosity unit is written as Pa·s.
1 Pa·s=10 P. In a considerable number of recent publications on viscosity of glass-forming melts the unit dPa·s is used, because values expressed in this unit are equal to those expressed in P.
The viscosity data in the Database are given in P and temperature values in \(^{\circ}\)C.
Note
We are planning to implement a unit converter in the web version. At the moment, the original unit: P is still used in the Database.
Viscosity standard points¶
In addition to results of direct measurements of viscosity one can find in the literature a lot of data which give certain information about viscosity values obtained by some specific ways of measurements of the rate of melt deformation or indirect ways of estimation of viscosity values. Results of such measurements are usually expressed in values of temperatures corresponding to a certain viscosity values. These temperatures are often called viscosity standard points.
The most broadly used viscosity standard points are softening temperature (Littleton Point) corresponding to viscosity 107.6 P and glass transition temperature, corresponding in most cases to viscosity 1013 -1013.5 P.
Other viscosity standard points for which a considerable amount of data could be found in SciGlass are as follows: Annealing Point, Dilatometric Softening Temperature, Sag Point, and Strain Point.
Vogel-Fulcher-Tammann equation¶
The abbreviation: VFT equation is often used. This equation is most widely used to describe the temperature dependence of viscosity:
Where \(\eta\) is viscosity, \(T\) is temperature in K, \(A\), \(B\), and \(T_{0}\) are constants for each particular melt.
The specific feature of this equation is the possibility to describe with a reasonable precision the temperature dependencies of viscosity in the range 102 -1013 P for many types of glasses. Especially well, this equation could be used for the description of the corresponding dependencies of multi-component silicate melts. For some binary silicate melts and for a considerable number of non-silicate melts, the use of this equation can lead to greater errors.
Info
In the Visualization feature, you can plot the viscosity table data and use the VFT function to perform a curve fitting.
We are also seeking for other good equations (models) like the Adam Gibbs equation to make better prediction (fitting). Please contact us if you know how or would like to help.
Volume thermal expansion coefficient¶
The relative change in volume of a sample per 1 K of change in temperature. Sometimes an abbreviation VTEC is used. Usually it is presented as a mean value within a certain temperature interval:
where \(\beta\) is mean VTEC, \(V\) is the initial volume of the sample, \(\Delta V\) and \(\Delta T\) are the changes in volume and temperature of the sample.
W-X¶
Weathering resistance¶
The term weathering is used to denote the interaction of glass with water, CO2, SO2, or any other components of the atmosphere. Two main types of weathering are recognized: dimming and staining.
The term dimming refers to the formation of hazy film of soluble salts on a polished surface of a sample which is in contact with the humid atmosphere of water vapor or mist but not in contact with water. All glasses are subject to such dimming if they are kept in a moist atmosphere for a long period of time, but the rate and degree of dimming vary with different glasses.
The mechanism of dimming is as follows. The glass surface absorbs water from the surrounding atmosphere resulting in the formation of a very thin layer of water. Alkali ions from the glass are dissolved in this layer forming a solution with high pH. Alkali attack of the layer leads to etching of the glass surface. The products of the dissolution of the glass, namely SiO2, Na2CO3, NaOH and some others could precipitate on this surface. The main methods of measurements: determination of the hygroscopicity of the glass by an increase in the sample weight, measurements of the level of dimness by a haze meter, measurements of a decrease in the coefficient of reflection, etc.
The term staining refers to the formation of a silica-rich surface layer due to the removal of modifier oxides (alkali oxides, PbO, BaO, etc.) in the course of the influence of water or acid solution. This silica-rich layer has a smaller refractive index than the bulk glass does. As a result, the layer shows an interference color ranging (depending mainly on the thickness of the layer) from brown to blue. One of the main methods of evaluating the staining ability of a glass is the time needed to reach an optical thickness equal to ¼ of the wavelength corresponding to the center of the spectra of visible light. It corresponds to the blue color of the layer in a reflected white light.
Weibull distribution function¶
Describes, mathematically, the dependence of strength of glass on the dimensions of the sample (usually it is applied to the description of the dependence of glass fiber strength on the fiber length). This function connects the probability of sample fracture (\(F\)) with its length (\(L\)) and the value of tensile stress (\(\alpha_{t}\)):
where \(m\) is a constant.
If one knows the value of \(sigma_{t1}\) for a given probability of the fracture of a fiber with length \(L_{1}\), she/he can easily calculate the value \(\sigma_{t2}\) corresponding to the same probability of the fracture of a fiber with length \(L_{2}\):
Thus, by using results of measurements of the strength of comparatively short fibers (for example, with length 20-100 m) one can judge the strength of fibers with length of several kilometers.
Working point¶
The temperature corresponding to viscosity of 104 P. At this temperature the glass is sufficiently soft for the shaping (blowing, pressing) in a glass-forming process. (Volf, 1961): see References.
Working range¶
The range of surface temperature in which glass is formed into ware in a specific process. The "upper end" refers to the temperature at which the glass is ready for working (generally corresponding to a viscosity of 103 to 104 poises), while the "lower end" refers to the temperature at which it is sufficiently viscous to hold its formed shape (generally corresponding to a viscosity greater than 106 poises). For comparative purposes, when no specific process is considered, the working range of glass is assumed to correspond to a viscosity range from 104 to 107.6 poises.
[Standard definition (ASTM; C162-52)]
Y-Z¶
Young's modulus¶
The ratio of the linear stress to the linear strain in cases when a uniaxial stress is applied to a body.