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The technology of glass and ceramics - Hlavac J.

Hlavac J. The technology of glass and ceramics - Oxford, 1983. - 429 p.
Download (direct link): tehnologyofglass1983.djvu
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surface more frequently occur on cooling down, while those due to heating
up are oriented at an angle of about 45 degrees to the surface, so that
corners are spalled off and the block becomes "rounded".
There are two possible approaches to the selection of materials from
the standpoint of thermal shock resistance. The first is suitable for
glass and fine dense ceramics and was discussed in Section II. 5. 2. With
these materials, it is necessary to avoid formation of primary cracks
which originate at the surface and propagate rapidly into the interior
where they are the causc of extensive fracture. In this case, the
favourable properties include high strength and high thermal
conductivity, and low elasticity modulus and expansion coefficient
values.
In the case of porous and distinctly heterogeneous materials such as
refractories and composites, a crack may be formed by thermal shock on
the surface, but further propagation is stopped at the pore or at the
grain boundary so that total fracture does not occur although the
strength of the object is reduced. The problems of crack stability and of
the conditions of propagation were dealt with by Hasselmann (1969), who
laid the foundations of new concepts for evaluating the resistance of
materials to abrupt changcs in temperature. According to these concepts,
the thermal shock resistance of refractories is characterized by the
parameter
R = Ey'V-
(110)
where E is the elasticity modulus, yeff is the fracture surface work, oy
is fracture stress and ju is Poisson's ratio. The requirements with
respect to elasticity modulus and strength here contrast with the
previous case of glass or porcelain; high values of E and low values of
strength are favourable.
The selection of refractories resisting total break up by thermal
shock is nowadays usually based on assessment of the relevant properties.
The practical tests consisting of alternate heating and cooling of
specimens and of determining losses of materials due to spalling show
unsatisfactory accuracy. Strength decrease determination is more
suitable, but even its results show considerable scatter.
Experience indicates that there exist substantial differences in
thermal shock resistance of one type of refractory due to particle size
of grog, its amount and to firing temperature. In agreement with the
above concepts, the shock resistance is usually higher when the grog is
coarse, the ware porous and the glass content low.
366
For instance, in the case of fireclay, the shock resistance decreases
with increasing firing temperature and decreasing grog particle size.
Silica refractories are sensitive to changes in temperature at lower
temperatures and very resistant to thermal shocks at high temperatures
(cf. thermal expansion). As a result of high expansion, magnesite has a
poor thermal shock resistance, so that its quality in this respect
depends much on the correctly chosen grain size composition of the raw
material mix. A roJe is similarly played by the foreign substances
present: A1203 acts favourably by its reaction producing spinel. One
should aJso take into account that the resistance of alJ refractories may
be strongly affected by infiltration of melt or molten carry-over into
surface layers. The resistance is usually impaired in this way. A high
thermal shock resistance is exhibited by siJicium carbide and graphite
refractories (high thermal conductivity) and by fused silica (very low
thermal expansion).
Porosity and permeability. Refractories always contain pores, the
practical effects of which depend on whether they are closed (isolated)
or open. In the latter case, capillary phenomena occur when the pores are
less than 1 mm in diameter.
Apparent porosity* is very important in connection with flux corrosion
and with thermal insulation which may be unfavourably affected by
permeation of hot gases. The total volume of open pores can be
determined, for example, by measuring the amount of absorbed water, but
this value provides no information on pore size which is important with
respect to these properties. The permeability for gases is therefore
measured. The test specimen is fixed in a special vessel so that it
allows a gas or air to be driven through. The gas permeation rate and the
pressure difference are measured. The permeability coefficient is then
given by the equation
K=^~
(111)
qApt
where Vis the volume of gas passed through during time t, tj is the gas
viscosity, I is the specimen thickness and q is its section area, Ap is
the pressure difference.
When the actual porous substance is replaced with a model containing
only cylindrical pores of radius r arranged in the direction of gas How,
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