IV. ConclusionsI t is instructive to summarize briefly the results of the studies on the transport mechanisms observed during the welding together of monocrystalline spheres of zinc oxide. A t low temperatures a solid-state diffusional transport predominates.' The transport is the result of the nonstoichiometric nature 01" zinc oxide and involves a relatively low activation energy (-35 kcal per mole). There is evidence that the carriers are excess zinc ions. Oxygen partial pressure and impurity content of zinc oxide aflect the rate of welding. Differences in chemical potential of the zinc ions mav arise either as a result of surface oxidation (chemical sintering) or as a consequence of the presence of the small convcx neck are& (physical sintering). I n both cases the bulk of the spheres is the source of the carriers, but their sink is reprcsented by the surface of the sphere for the former, and by the neck area for the latter case. Between '700" and !)OO°C chemical sintering is predominant, whereas physical sintcring through bulk diffusion is appreciable a t about 1 0 O O O to 1050°C. At higher temperatures ( 1 l O O O to 1?50°C) the diffusional transport is greatly overshadowed by the transfer of niatter through the surrounding gas phase. Siritering then becomes dependent on the rate of evaporation of zinc oxide. In principle, three cases are possible. The transport through the gas phase may not be slowed down by convective phenomena. The rate of transfer is then controlled by the rate of evaporation and is independent of total pressure and gaseous flow rate, and it involves a rather high activation energy (I09 kcal per mole). This mechanism tends to occur a t total pressures of less than I atm. A t higher prcssurcs, diffusive control of the rate of evaporation may set in. Sintering becomes dependent on the total pressure (natural convection) and, a t high flow velocities, should be proportional to a fractional power of the flow velocity of I / T x 104 (OK-!)Fig. 4. Variation of the time to reach x/a = 0.3 vs. reciprocal of the temperature at 1 atm pressure. the carrier gas (forced convection). The energy values involved in solid diffusion and evaporation are the factors which determine the type of sintering to be followed. The three elastic compliance coefficients of synthetic periclase were determined in the kilocycles per second frequency range by a resonance method. Polycrystalline elastic moduli on dense-formed MgO were measured. The calculated isotropic elastic moduli for polycrystalline MgO obtained from the single-crystal constants are in good agreement with experimental values measured on the dense MgO at the theoretical density point. The measured Young's modulus and the shear modulus of polycrystalline MgO are found to be 30.50 and 12.90 X loLL dynes per cm2 respectively. The results of the present investigation are compared with the earlier work in the megacycles per second frequency range. A theoretical analysis is made to establish the validity of the present values. Schemes of averaging the sing...

has not been reported in the literature. Figure 4 shows the spectrum of monoclinic zirconia with two absorption bands, one at 270 cm-' and another at 230 cm-l. The absorption band occurring at 270 c m -I is fairly asymmetric indicating the possibility of another absorption band lying close to it.The infrared transmission spectrum of the cubic zirconia is quite transparent in this region and gave no indications of any absorption when recorded as a capillary film in Nujol 0 c IV. DiscussionThe infrared spectrum of ZrOz is quite characteristic and from this study would seem to be useful for identification and in solid state research. Infrared spectra are quite dependent Vol. 47, No. 12 A method was developed for hot-pressing 80 to 95% dense, polycrystalline specimens of commercially available, unstabilized, monoclinic zirconia. T h e elastic moduli of these specimens were measured by a sonic method in t h e range 25' to 100O'C. Application of mathematical expressions developed for the porosity dependence of elastic moduli of refractory oxides led to the calculation of Young's and shear moduli of this material at zero porosity in this temperature range. . 1 R. M. Spriggs, "Expression for Effect of Porosity on Elastic Modulus of Polycrystalline Refractory Materials, Particularly Aluminum Oxide," J . Am. Ceram. Soc.,44 I121 628-29 (1961). R. M. Spriggs and L. A. Brissette, "Expressions for Shear Modulus and Poisson's Ratio of Porous Refractory Oxides," J . Am. Ceram. Soc., 45 [4] 198-99 (1962). a R. M. Spriggs, L. A. Brissette, and T. Vasilos, "Effect of Porosity on Elastic and Shear Moduli of Polycrystalline Magnesium Oxide," J . Am. Ceram. SOL, 45 [8] 400 (1962).

A method is described for studying the thermal shock characteristics of a brittle material. An analysis of the thermal stresses developed in a homogeneous isotropic solid sphere has led to the formulation of an equation relating the physical properties of the body to the temperature difference causing failure and time to maximum stress in a single‐cycle unsteady‐state test. The thermal shock test consisted of plunging a sphere at one uniform temperature into a medium at another temperature. If fracture occurred, the time to fracture was recorded. A large number of tests were run to determine the temperature difference which caused 50% of the spheres to fracture. The thermal shock relationships were tested using a high‐alumina body. The physical properties relating to the thermal shock equations were measured, and calculated temperature differences causing failure and times to maximum stress were compared with measured values. Sufficient agreement was found to lend support to the theory. Summary A method has been developed for studying the thermal shock characteristics of a brittle substance. The method consists of a single‐cycle test of unsteady‐state nature. Two testing conditions have been selected, one having a rather high surface heat transfer coefficient in a liquid bath and the other having a small finite surface heat transfer coefficient in an air bath. These two conditions are at either extremes regarding the thermal shock usually given a substance in practice. The only fair agreement found in the calculated and experimental data indicates that further investigation is necessary. The importance of certain factors, such as time to maximum stress, which was found to be in rather good agreement for the salt bath heat transfer, cannot be overlooked. There are many applications in the high temperature‐high stress field in which the concept of time to maximum stress might well be examined. For example, when repeated high‐temperature heatings are made on a refractory piece, the cycling might be arranged so that the time to maximum stress was never reached for the particular heating cycle, although the (ΔT) was higher than that necessary to cause failure. Much is to be learned from this type of study which may be applied to actual situations. It must be emphasized, however, that this work has been conducted on one single set of conditions and the factors found here do not necessarily apply to other situations.