Assignment
Engineering ceramics & glasses
Properties of GLASS
Introduction
A glass is an
inorganic non metallic material that does not have a crystalline structure.
Such materials are said to be amorphous and are virtually solid liquids
cooled at such a rate that crystals have not been able to form. Typical glasses
range from the soda-lime silicate glass for soda bottles to the extremely high
purity silica glass for optical fibers. Glass is widely used for
windows, bottles, glasses for drinking, transfer piping and recepticles for
highly corrosive liquids, optical glasses, windows for nuclear applications
etc.
The main constituent
of glass is silicon dioxide (SiO2). The most common form of
silica used in glassmaking has always been sand. Sand by itself can be fused to
produce glass but the temperature at which this can be achieved is about 1700o
C. Adding other chemicals to sand can considerably reduce the temperature
of the fusion. The addition of sodium carbonate ( Na2CO3
) known as soda ash, in a quantity to produce a fused mixture of 75%
Silica (SiO2) and 25% of sodium oxide (Na 2O), will
reduce the temperature of fusion to about 800o C.
However, a glass of this composition is water soluble and is known
as water glass. In order to give the glass stability, other
chemicals like Calcium Oxide (CaO) and magnesium oxide (MgO) are needed.
The raw materials used for introducing CaO and MgO are their carbonates,
limestone (CaCO3) and dolomite (MgCO3), which when
subjected to high temperatures give off carbon dioxide leaving the oxides in
the glass.
OPTICAL PROPERTIES OF GLASS
Glasses are among the few solids that transmit visible light
• Thin film oxides might, but scattering from grains limit their
thickness
• Glasses form the basic elements of virtually all optical systems
• World-wide telecommunications by optical fibers
• Aesthetic appeal of fine glassware- 'crystal' chandeliers
• High refractive index/birefringent PbO-based glasses
• Color in cathedral windows, art glass, etc.
1.
Refractive Index~(velocity of light in vacuo, or air)/(velocity of
light in medium)
Snell's
Law:
Note: unit less quantity
·
n (air) = 1.0003
·
water = 1.33
·
sapphire = 1.77
·
diamond = 2.42
·
f-SiO2 = 1.458
·
heavy flint = 1.89
Internal Reflection:
Critical angle (Brewster's angle) θc below which light is totally
reflected:
Note: larger n means greater θc, and so more light (from a broader
distribution of incident angles) will be internally reflected. High index
materials (diamonds, PbO glasses) look 'brilliant' when facets are cut so that
internal reflection returns light from large faces that originally collected
the light.
Note too: internal reflection is important for transmission of light down
an optical fiber.
The main dispersion is expressed by (nF-nc) and (nF'-nc’). The
Abbe-number is defined:
The refractive index of optical glass changes with the temperature. The
tem-perature coefficient of the refractive index, (Δn/ ΔT) abs., is measured at
20°C intervals between –40~80°C in a vacuum, using an interference-dilatometer
to detect changes in both optical path length and dilation of the specimen. The
light source used is a He-Ne gas laser (632.8nm).
For calculation of the temperature coefficient of the relative refractive
index (Δn / ΔT) rel. in air at 101.325 kPa, the following equation is given:
Ideally, the optical properties of glass are isotropic through fine
annealing. Birefringence may be observed, however, when external forces are
applied or when residual stresses are present (commonly the result of rapid
cooling).
The optical path difference δ (nm) associated with birefringence is
linearly proportional to both the applied tensile or compressive stress, σ (105
Pa) and the thickness d (cm) of the specimen and is given by the
following equation:
The
proportionality constant, B (10-12 / Pa), in this equation is proper constant
of each glass type and referred to as the stress-optical coefficient.
Stress-optical
coefficients are obtained by measuring the optical path difference caused at
the center of a glass disk with He-Ne laser light, when the disk is subject to
a compressive load in a diametral direction.
The
transmittance characteristics of optical glasses in this catalog are expressed
by two terms. One is "Internal Transmittance" and the other is
"Coloration Code".
Internal
transmittance (τ) refers to transmittance obtained by excluding reflection
losses at the entrance and exit surfaces of the glass. Internal transmittance
values over the wavelength range from 280 to 1,550nm are calculated from
transmittance measurements on a pair of specimens with different thicknesses.
Internal
transmittance values obtained for 5mm and 10mm thick glasses are given as τ5mm
and τ10mm.
The
internal transmittance τ for glass with arbitrary thickness d can be obtained
from these values by using:
where ô0
refers, to the internal transmittance given in the tables for glass with
thickness d0 equal to either 5 mm or 10 mm.
Optical
glasses exhibit almost no light absorption over a wavelength range ex-tending
through the visible to the near infra-red. The spectral transmittance
characteristics of optical glasses can be simply summarized with the coloration
code λ80/λ5.
The
coloration code is determined in the following way. The internal transmittance
of a specimen with thickness 10 ± 0.1mm is measured from 280nm to 700nm.
Wavelengths are rounded off to the nearest 10nm and expressed in units of 10nm.
λ80 is the wavelength for which the glass exhibits 80% transmittance while λ5
is the wavelength at which the glass exhibits 5% transmittance. For example, a
glass with 80% transmittance at 398 nm and 5% transmittance at 362nm has a
coloration code 40 / 36
The coloration code is generally applied for transmittance control of
optical glasses.
Fig. Designation of
the Coloration Code in Spectral Transmittance curve.
Thermal Properties
The glass
transformation temperature 'T'g refers to the temperature at which the glass
transforms from a lower temperature glassy state to a higher temperature
super-cooled liquid state.
This behavior
is illustrated in Fig. below which shows
thermal expansion measured as a function of temperature. A differential thermal
dilatometer is used for the measurement as it maintains a uniform temperature
distribution within the furnace to ±1°C. As illustrated in the figure, the
transformation temperature is determined by the intersection point of the two
tangents of the high and low temperature ranges of the thermal expansion curve.
In the
thermal expansion curve shown in Fig. above, the Sag Temperature (Ts) is
defined as temperature at which thermal expansion stops increasing and actually
begins to decrease with increasing temperature. This behavior is not due to an
intrinsic property of the glass but is rather due to deformation of the glass
under the load applied in these measurements. The viscosity of the glass at Ts
corresponds to about 1010 to 1011 dPa•s.
The strain
point, T1014.5, represents a temperature at which internal stresses in a glass
are relieved after a few hours. The viscosity of the glass at that temperature
corresponds to about 1014.5 dPa•s.
The
annealing point, T1013, represents a temperature at which internal stresses in
a glass are relieved after a few minutes. The viscosity of the glass at that
temperature corresponds to about 1013 dPa•s.
The
softening point, T107.6, represents a temperature at which a glass
begins to remarkably soften and deform under its own weight. The viscosity of
the glass at that temperature corresponds about 107.6 dPa•s.
The thermal
conductivity ë is the quotient obtained by dividing the density of heat flow
rate by the temperature gradient, that is, the quotient obtained by dividing
the heat quantity transferring through a unit area in a unit time, by the
temperature difference per unit distance, and expressed in W / (m•K).
Note. 1 W /
(m•K) = 8.600 0 x 10-1 kcal / (h•m•°C) = 2.388 89x 10-3
cal / (s•cm•°C)
The
specific heat, cp, is the quotient obtained by dividing the heat capacity of a
substance by the mass, that is, the heat quantity required for increasing the
temperature of a substance of unit mass by one unit (1K or 1°C) and expressed
in kJ / (kg • K).
References
2.
Shelby Chapter 10, Optical Properties, Cer103
Notes, R.K. Brow
