Microfracture from thermal expansion anisotropy—I. Single phase systems

Grain size-microcracking trends are examined in the three pseudobrookite oxides MgTi2O5, Fe2TiO5, and Al2TiO5. These trends are toward smaller critical grain sizes, lower elastic moduli, and lower strengths with increased thermal expansion anisotropy. Extension of an energy criterion relates the micro-cracking transition to the inverse square of the maximum difference in single-crystal thermal expansion coefficients, Δαmax-2.

Grain Size/Microcracking Relations for Pseudobrookite Oxides

The theory of heat conduction in ceramics by phonons, and at high temperatures also by infrared radiation, is reviewed. The phonon mean free path is limited by three-phonon interactions and by scattering of various imperfections. Point defects scatter high-frequency phonons; extended imperfections, such as inclusions, pores, and grain boundaries, affect mainly low-frequency phonons. Thermal radiation is also scattered by imperfections, but of a larger size, such as splat boundaries and large pores. Porosity also reduces the effective index of refraction. For films there are also external boundaries, cracks, and splat boundaries, depending on the method of deposition. Examples discussed are cubic zirconia, titanium oxide, and uranium oxide. Graphite and graphene sheets, with two-dimensional phonon gas, are discussed briefly.

Theory of thermal conduction in thin ceramic films

Expressions are derived for the effect of penny-shaped cracks of various orientations on the thermal conductivity of solid materials.

Effect of Cracks on Thermal Conductivity

The relation between grain size and grain-boundary microcracking during cooling in aluminum titanate ceramics was studied. Microcracking temperature was determined by the measurement of thermal contraction and expansion, which was accompanied by acoustic emission. When the ceramics were cooled at a rate of 6°C/min, stress relaxation did not occur below the sintering temperature of 1500°C. The relation between the temperature difference from the sintering temperature to the microcracking temperature and the grain size showed good agreement with the prediction based on the energy criterion of grain-boundary microcracking.

Grain‐Boundary Microcracking Due to Thermal Expansion Anisotropy in Aluminum Titanate Ceramics

The effect of microcracking on the thermal diffusivity of polycrystalline Fe2TiO5 subjected to a range of annealing treatments was investigated. At fine grain size (∼1 μm), the thermal diffusivity exhibited the decrease with increasing temperature common for dielectrics. Extensive microcracking in the larger-grain-sized materials significantly decreased their thermal diffusivity. On heating, the microcracked materials exhibited increased thermal diffusivity at elevated temperatures which can be attributed primarily to microcrack closure and healing; on cooling, they exhibited a pronounced hysteresis, attributable to irreversible crack opening and closing. Thermal cycling closed the hysteresis curves, which suggests permanent changes in microcrack morphology. It appears that microcracking is a promising technique for tailoring ceramic materials to a combination of high thermal shock resistance and good insulating capability.

Effect of Microcracking on the Thermal Diffusivity of Fe2TiO5

The laser flash technique was used to study the effect of grain size on the thermal diffusivity of polycrystalline MgTi2O5 from 25° to 800°C. Microcracking decreased the thermal diffusivity by as much as a factor of two with the decrease increasing with increasing grain size. When specimens were heated then cooled, the thermal diffusivity exhibited a horizontal flat figure-eight hysteresis. This hysteresis, which appeared to be a function of the thermal history, was attributed to a balance between crack healing, or closure, at high temperatures and the growth of existing cracks or the formation of new cracks during cooling.

H. J. Siebeneck

Papers

Effect of Microcracking on the Thermal Diffusivity of Fe2TiO5

Effects of Grain Size and Microcracking on the Thermal Diffusivity of MgTi2O5

Thermal Diffusivity of Nonstoichiometric Titanium Dioxide

Thermal diffusivity gradients in thick glassy carbons

Effects of grain size and microcracking on the thermal diffusivity of magnesium dititanate (mgti2o5)