Effect of Crystallization

Since the thermal expansion coefficient is a volume averaged function of the contributions of each of the phases present in a sample, formation of crystals with thermal expansion coefficients which are very different from the initial glass can radically alter the thermal expansion coefficient of the composite. Formation of crystals can also change the values of Tg and T by changing the composition of the residual glass, and by preventing deformation of the sample under the push-rod load. 

Commercial lithium aluminosilicate glass-ceramics provide excellent examples of such behavior. The initial glass used for production of transparent cookware, for example, has a thermal expansion coefficient of 4 ppm K , Tg 730 °C, and T = 760 °C. After processing, the thermal expansion coefficient is == 0.5 ppm K and Tg and T can no longer be detected on an expansion curve below 1000 °C. Heat treatment results in the formation of a lithium aluminosilicate crystal which has a very low thermal expansion coefficient. Removal of lithium from the residual glassy phase also decreases the thermal expansion coefficient of that phase, while simultaneously increasing the transformation and softening temperatures. 

It is also possible for a sample to begin to crystallize during a thermal expansion measurement as the temperature approaches or just passes Td. If this occurs, deformation of the sample may cease and the sample will begin to expand again. In other cases, a crystalline phase may melt, allowing a rapid decrease in viscosity and an abrupt softening of the sample. 

Sketch the thermal expansion curves for (a) a well-annealed homogeneous glass and (b) a quenched glass of the same composition. 

Use the additivity factors below to calculate the thermal expansion coefficient for the indicated glass compositions, (all coefficients are x 10 K wt% ) 

---

In the discussion of Table 4.1, we acknowledged that there might be some uncertainty in the values of the quantities tabulated, but we sidestepped the origin of the uncertainty. In the next section we shall consider one of these areas the effect of crystal dimensions of the value of T j,. 

Size-dependent Crystal Growth. A number of empirical expressions correlate the apparent effect of crystal size on growth rate (30). The most commonly used correlation uses three empirical parameters to correlate growth rate with crystal size ... 

Gupta [28] presents results on the effect of crystal orientation on shock propagation in LiF crystals. This work supports earlier studies and shows... 

The effect of crystal agglomeration is generally to cause concave curvature on the log population density versus size plot (see Figure 6.11b). 

Chianese, A., Di Berardino, F. and Jones, A.G., 1993. On the effect of crystal breakage on the fine crystal distribution from a seeded batch crystallizer. Chemical Engineering Science, 48, 551-560. 

Randolph, A.D., 1969. Effect of crystal breakage on crystal size distribution from a mixed-suspension crystallizer. Industrial and Engineering Chemistry Fundamentals, 8, 58. 

Effect of Crystal Defects on Corrosion—General Considerations... 

Firstly, they might be expected to have an effect when corrosion occurs under conditions of active (film-free) anodic dissolution and is not limited by the diffusion of oxygen or some other species in the environment. However, if the rate of active dissolution is controlled by the rate of oxygen diffusion, or if, in general terms, the rate-controlling process does not take place at the metal surface, the effect of crystal defects might be expected to be minimal. 

Finally, it should be noted that in both cases the effect of crystal defects and microstructural features must, in general, be to tend to make the corrosion less uniform and more localised. 

Since corrosion is essentially a reaction between a metal and its environment, the very significant effect of crystal defects and metallurgical structure on certain corrosion phenomena is to be expected. It is no more possible to... 

Geometry optimization was performed on the discrete cluster unit to eliminate the effects of crystal packing and interactions with titanium ions. The calculations were carried out in local spin density approximation using a SPARTAN 5.0.3 package (Wavefanction, Inc., Irvine, CA 92612 USA). 

This relation is only valid for a crystal with isotropic /-factor. The effect of crystal anisotropy will be treated in Sect. 4.6.2. The function h(6) describes the probability of finding an angle 6 between the direction of the z-axis and the y-ray propagation. In a powder sample, there is a random distribution of the principal axes system of the EFG, and with h 6) = 1, we expect the intensity ratio to be I2J li = I, that is, an asymmetric Mossbauer spectrum. In this case, it is not possible to determine the sign of the quadmpole coupling constant eQV. For a single crystal, where h ) = — 6o) 5 delta-function), the intensity ratio takes the form... 

Effect of Crystal Anisotropy on the Relative Intensities of Hyperfine Splitting Components... 

Au microcrystals Effect of crystal size on recoil-free fraction... 

After each peak has been described by the parameters of a model function, the convolution in Eq. (8.13) can be carried out analytically. As a result, equations are obtained that describe the effects of crystal size, lattice distortion, and instrumental broadening38 on the breadth of the observed peak. Impossible is in this case the separation of different kinds of lattice distortions. 

The effect of crystal size of these zeolites on the resulted toluene conversion can be ruled out as the crystal sizes are rather comparable, which is particularly valid for ZSM-5 vs. SSZ-35 and Beta vs. SSZ-33. The concentrations of aluminum in the framework of ZSM-5 and SSZ-35 are comparable, Si/Al = 37.5 and 39, respectively. However, the differences in toluene conversion after 15 min of time-on-stream (T-O-S) are considerable being 25 and 48.5 %, respectively. On the other hand, SSZ-35 exhibits a substantially higher concentration of strong Lewis acid sites, which can promote a higher rate of the disproportionation reaction. Two mechanisms of xylene isomerization were proposed on the literature [8] and especially the bimolecular one involving the formation of biphenyl methane intermediate was considered to operate in ZSM-5 zeolites. Molecular modeling provided the evidence that the bimolecular transition state of toluene disproportionation reaction fits in the channel intersections of ZSM-5. With respect to that formation of this transition state should be severely limited in one-dimensional (1-D) channel system of medium pore zeolites. This is in contrast to the results obtained as SSZ-35 with 1-D channels system exhibits a substantially higher... 

The effect of crystal field splitting is easily seen by studying the absorption spectrum of [Ti(H20)6]3+ because the Ti3+ ion has a single electron in the 3d orbitals. In the octahedral field produced by the six water molecules, the 3d orbitals are split in energy as shown in Figure 17.3. The only transition possible is promotion of the electron from an orbital in the t2g set to one in the eg set. This transition... 

The effect of crystal size, 2r, in STOP is demonstrated in Figure 10. These data for three zeolites having similar activity, but with crystal sizes differing by nearly two orders of magnitude, show a significant increase in para-xylene selectivity with increasing crystal size. The primary product selectivity is enhanced and secondary isomerization is retarded. 

Figure 10. Effect of crystal size, 2r, on p-xylene selectivity in toluene disproportionation by HZSM-5.

An overview of the effect of crystal structure on color application properties of organic pigments was published [7]. 

---