Thermal shock resistance of ultra-high temperature ceramics including the effects of thermal environment and external constraints

The thermal shock resistance of ceramics depends on the materials mechanical and thermal properties, also is affected by component geometry and external factors and so on. Therefore, the thermal shock resistance of ceramic materials is the comprehensive performance of their mechanical and thermal properties corresponding to the various heat conditions and external constraints. In the present work, a thermal shock resistance model of the ultra-high temperature ceramics which considered the effects of thermal environment and constraints was established. With this model, the influence of constraints on the thermal shock resistance and critical fracture temperature difference had been studied and an effective idea to improve thermal shock resistance for ceramic material and structure was found. Furthermore, the model was validated by finite element method.     

Phonon scattering and thermal conduction mechanisms of sintered aluminium nitride ceramics

From thermal diffusivity measurements of sintered AIN at temperatures ranging from 100 to 1000 K, the phonon mean free path of AIN was calculated in order to investigate phonon scattering mechanisms. The calculated mean phonon scattering distance was increased with decreasing temperature. The mean phonon-defect scattering distances were respectively limited to about 50 nm at temperatures ranging from 100 to 270 K and about 30 nm at temperatures ranging from 100 to 700 K, for AIN specimens with a room-temperature thermal conductivity of 220 and 121 Wm^–1 K^–1 containing 0.1 and 1.4 wt % oxygen, respectively. These short phonon-defect scattering distances were considered to correspond to the separation of oxygen-related internal defects in AIN grains. Calculation of the mean phonon scattering frequencies indicated that the phonon scattering is dominated by phonon-defect scattering at temperatures below 270 K for an AIN specimen with an oxygen content of 0.1 wt %, and at temperatures below 350 K for an AIN specimen with an oxygen content of 1.4 wt %.     

Effect of the temperature dependence of thermal properties on the thermal shock tests of ceramics

Thermal stress generated during a thermal shock is closely related to the fracture of ceramics. An attempt has been made to obtain thermal stress in a specimen by numerical calculation. The temperature dependence of thermal conductivity and diffusivity were introduced to realize the practical thermal conditions. The maximum thermal stress, _max ^* , was recognized at the Fourier number, but differed from the temperature dependence. Correlative equations of _max ^* and _max ^* with the Biot number, _i, under cooling or heating tests, have been proposed. These equations resulted in the exact _max ^* and _max ^* compared with the previous equations, in which temperature dependence was ignored. The thermal shock resistance parameter was expressed by the correlative equations of _max ^* in order to suggest adequate experimental conditions and specimen size. A comparison of the measured and calculated time to failure of the specimen led to confirmation of the fracture criterion. The measured time disagreed with the calculated one, if the fracture by thermal shocking was not predominant. The correlative equations were also useful to select the kind of ceramics subjected to thermal shocking.     

Test methodology for the thermal shock characterization of ceramics

An experimental methodology is proposed to evaluate the thermal shock resistance of ceramics. A technique based on infrared heating has been developed to perform systematic and well controlled thermal shock experiments. This novel technique was used to evaluate the resistance of yttria-stabilized zirconia–alumina foams to thermal loads. Foams of varying cell sizes were subjected to thermal shock and the damage was evaluated using retained strength and non-destructive elastic modulus measurements. The transient thermal gradients and the resulting thermoelastic stresses in the foams were predicted using finite element analysis and the extent of damage was correlated to the maximum thermal strains generated in foams.     

Effect of thermal cycling on the crack-growth resistance of fiber glasses

We study the influence of thermal cycling on the crack-growth resistance and fracture of fiber glasses based on epoxy binders unidirectionally reinforced by solid or hollow fibers. Specimens were subjected to 15 thermal cycles according to the following procedure: First, they were held for 5 min in liquid nitrogen at 77°K and then, for 60 min, in air at 293°K. Prior to failure, the structure of the specimens in the initial state and after thermal cycling was studied with an optic microscope. The fracture surfaces of destroyed specimens were analyzed with an REM-200 electron microscope. It was discovered that, after thermal cycling, the crack-growth resistance of fiber glasses with solid fibers is lower than the crack-growth resistance of fiber glasses with hollow fibers. In addition, for the same reinforcement, the type of the applied lubricant practically does not affect the degree of decrease in crack-growth resistance. We detected various types of damages to the matrix and fibers caused by thermal cycling and analyzed the morphology of their types. On the basis of the obtained results, we proposed a mechanism of fracture processes in fiber glasses subjected to thermal cycling.Kharkov Engineering-Pedagogical Institute, Kharkov. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 5, pp. 40–46, September – October, 1995.     

Fracture toughness and thermal shock of tool and turbine ceramics

The behaviour of some commercial tool carbides and turbine ceramics has been investigated in regard to resistance to crack initiation, crack propagation and retained strength after thermal shock. New data are provided, particularly measurements of the fracture toughness of these materials at actual operating temperatures (up to 1200° C). Many of the materials did not follow the generally accepted Hasselman theory for thermal shock in ceramics, and instead of showing a discontinuity in retained strength at some critical quenching temperature difference, their residual strengths fell gradually at temperatures lower than their supposed critical quenching temperature. This behaviour is explicable when high temperature toughnesses, strengths and moduli are used in the damage resistance parameter (ER/gs _f ² ). It seems that materials not following the Hasselman model suffer cumulative damage with increasing number of shocks. Sub-critical crack growth occurs even if (K _IC/gs _f)² values are constant, and such damage, which reduces the room temperature retained strength, is enhanced by (K _IC/gs _f)² decreasing at temperatures below ΔT _c. In contrast, materials obeying Hasselman's model appear to have a constant (K _IC/gs _f)² below ΔT_c and for some temperature range above. Only then are “one-shock“ characterizations of materials possible, otherwise, the retained strength depends upon the number of prior shocks. Experiments are also reported which describe the effects of rate of testing on the unshocked and shocked mechanical properties of ceramics. Oxidation is shown to influence the results in a manner not obvious from single shock tests.     

Microstructural aspects of Thermal Stress Resistance of high-strength engineering ceramics Part II: Influence of microstructure on thermal shock resistance of high-strength engineering ceramics

In part I of this paper basic criteria for the improvement of thermal shock resistance of engineering ceramics were summarized. Moreover, data were presented about thermal shock resistance to fracture initiation and about thermal cycling behaviour of high-strength engineering ceramics. In this part the influence of microstructure on thermal shock resistance to fracture initiation is discussed. This subject also includes the discussion of the influence of various microstructural variables on the mechanical and thermal properties which mainly control the thermal shock resistance to fracture initiation. These relations are demonstrated with Al₂O₃, but in particular by the example of dense and porous reactionbonded Si₃N₄. Furthermore, some other factors affecting thermal shock resistance are outlined: the influence of temperature dependence of properties on the microstructural effects on thermal shock resistance, and the influence of data scatter of the initial strength on the strength behaviour after thermal shock.     

Effect of environmental factors on thermal shock behaviour of polycrystalline alumina ceramics

The effect of the environmental factors on thermal shock behaviour of polycrystalline alumina ceramics was studied by quenching the alumina specimens into various quenching media. The environment factors of quenching media were controlled by changing the temperature of water and changing the concentration of the propylene glycol/water solution. The convection heat transfer coefficient and thermal stress increased as the temperature of cooling water increased and decreased as the concentration of the propylene glycol in water increased. The critical thermal stress which makes the cracks grow catastrophically was found to be generated by the critical cooling rate, and the critical cooling rate of alumina ceramics was found to be a certain value (550 °C/s) and same for all cooling liquids. Therefore, cooling rate was found to be the most influential of the environmental factors in thermal shock.     

Toward a method of thermal shock testing of structural ceramics

Characteristics of the process of development of thermal stresses in a cooled rod with a varying heat transfer coefficient are theoretically studied. It is established that the dependence of stresses on initial temperature of specimen heating is almost linear with a smooth variation in slope in the range of temperatures corresponding to maximum steepness of the right slope of the temperature dependence of the heat transfer coefficient. The importance of standardizing the water temperature within a narrow range to facilitate reproducibility of the thermal loading rigidity is established. It is recommended that specimens be guarded against cooling during their furnace-to-water bath transfer.Translated from Problemy Prochnosti, No. 7, pp. 76–79, July, 1990.     

The immobilization of high level radioactive wastes using ceramics and glasses

An overview is given of the immobilization of high level radioactive waste (HLW) and surplus materials from a variety of commercial and defence sources employing glass and ceramic hosts. A number of specific host materials are reviewed, including borosilicate and phosphate glasses, glass-ceramics and crystalline ceramics. Topics covered include wasteform processing and manufacture, in addition to wasteform stability, durability and mechanical behaviour. Although, at the present time, borosilicate glass is the generally accepted first generation wasteform for the immobilization of HLW, the emergence of new sources of radioactive materials requiring immobilization has renewed interest in many of the alternative candidates. These include, in particular, titanate, zirconate and phosphate based ceramics, together with iron phosphate based glasses and basaltic glass-ceramics. The relative merits and limitations of each host material are compared and discussed, with particular reference to processing considerations and to current and likely future requirements. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of bath and specimen temperature on the thermal stress resistance of brittle ceramics subjected to thermal quenching

The effect of specimen and bath temperature on the failure of brittle ceramics in a thermal quench experiment was studied by quenching glass and alumina rods in water and silicon oil baths at different temperatures. The results were discussed in terms of the variation of heat transfer coefficient of the quenching media and the change in material properties as a function of temperature. It was found that the usual assumption of constant heat transfer coefficient and material properties may lead to considerable errors in the quantitative interpretation of the results of thermal quench experiments. Effective values for the film coefficient of heat transfer for water and oil baths were estimated as a function of film temperature from thermal quench data. Recommendations were made for the selection of quenching media and for the procedure to be followed in reporting the results.     

超导线圈的等效热膨胀系数的理论与实验研究

给出了计算超导线圈等效热膨胀率的理论模型和计算方法,对比了各种测量材料线膨胀系数的实验方法,并且给出了低温应变片测量热膨胀系数的技术原理.采用低温应变片法测量了矩形截面导线缠绕成型的超导线圈由77 K到300 K的线膨胀系数,并将实验测试结果与理论计算结果进行了对比分析.     

Theoretical and experimental considerations for a single-mode fiber-optic bend-type sensor

A novel single-mode bend fiber-optic sensing principle is presented. The design makes use of the translucent protective sheath that encases a typical fiber as a means of locating the position of a small bend present on an otherwise straight fiber. We can simultaneously determine bend magnitude by measuring the reduction in the fiber's core light. The theoretical model presented and the experimental results obtained are in excellent agreement, suggesting that a single-point sensor system is feasible with the proposed technique.     

Characteristics of crack patterns controlling the retained strength of ceramics after thermal shock

The physical mechanisms that determine the retained strength of ceramics after thermal shock are studied by measuring experimentally and statistically the density and depth of cracks produced in the interior of the ceramics. The analysis indicates that the key factor controlling the retained strength is the maximum depth of cracks rather than the density of cracks in the ceramics. The result presented here forms a basis to further understand the thermal shock behavior of ceramics.     

Observations on the characteristics of a fluidized bed for the thermal shock testing of brittle ceramics

The heat transfer characteristics of a fluidized bed used as a quenching medium for the thermal stress testing of brittle ceramics, were determined by measurements of the thermal shock behaviour of rods of a soda-lime—silica glass. The heat transfer coefficient was found to be strongly dependent on the mean particle size of the powder and air flow rate, and was relatively independent of the position within the bed. The results indicate that the heat transfer coefficient during thermal shock fracture may have a value lower than that obtained under heat transfer conditions which more closely resemble steady state. The heat transfer data inferred from the quenching experiments with the glass gave excellent agreement between calculated and measured values for the thermal shock behaviour for rods of a polycrystalline aluminium oxide. It is concluded that fluidized beds are excellent inert quenching media with variable heat transfer coefficient controlled by particle size and flow rate.