Thermal Shock Resistance Parameters Research Articles

Properties of an Infrared‐Transparent MgO:Y2O3 Nanocomposite

A 50:50 vol% MgO–Y2O3 nanocomposite with ~150 nm grain size was prepared in an attempt to make 3–5 μm infrared‐transmitting windows with increased durability and thermal shock resistance. Flexure strength of the composite at 21°C is 679 MPa for 0.88 cm2 under load. Hardness is consistent with that of the constituents with similar grain size. For 3‐mm‐thick material at 4.85 μm, the total scatter loss is 1.5%, forward scatter is 0.2%, and absorptance is 1.8%. Optical scatter below 2 μm is 100%. Variable intensity OH absorption (~6% absorptance) is observed near 3 μm. The refractive index is ~0.4% below the volume‐fraction‐weighted average of those of the constituents. Thermal expansion is equal to the volume‐fraction‐weighted average of expansion of the constituents. Specific heat capacity is equal to the mass‐fraction‐weighted average of heat capacities of the constituents. Thermal conductivity lies between those of the constituents up to 1200 K. Elastic constants lie between those of the constituents. The Hasselman mild thermal shock resistance parameter for the composite is twice as great as that of common 3–5 μm window materials, but half as great as that of c‐plane sapphire.

The Thermal Shock Resistance of Mg-PSZ/LaPO<sub>4 </sub>Ceramics

The mechanical properties and thermal shock behavior of Mg-PSZ/LaPO4ceramics was investigated. The thermal shock resistance of the materials was evaluated by water quenching and a subsequent three-point bending test to determine the flexural strength degradation. Mg-PSZ/15LaPO4composite showed a higher thermal shock resistance and behaved as a typical refractory. The calculation of thermal shock resistance parameters for the composites and the monolith had indicated possible explanations for the differences in thermal shock behavior.

Theoretical Research on Thermal Shock Resistance of Ultra-High Temperature Ceramics Focusing on the Adjustment of Stress Reduction Factor.

The thermal shock resistance of ceramics depends on not only the mechanical and thermal properties of materials, but also the external constraint and thermal condition. So, in order to study the actual situation in its service process, a temperature-dependent thermal shock resistance model for ultra-high temperature ceramics considering the effects of the thermal environment and external constraint was established based on the existing theory. The present work mainly focused on the adjustment of the stress reduction factor according to different thermal shock situations. The influences of external constraint on both critical rupture temperature difference and the second thermal shock resistance parameter in either case of rapid heating or cooling conditions had been studied based on this model. The results show the necessity of adjustment of the stress reduction factor in different thermal shock situations and the limitations of the applicable range of the second thermal shock resistance parameter. Furthermore, the model was validated by the finite element method.

Properties and appropriate conditions of stress reduction factor and thermal shock resistance parameters for ceramics

Through introducing the analytical solution of the transient heat conduction problem of the plate with convection into the thermal stress field model of the elastic plate, the stress reduction factor is presented explicitly in its dimensionless form. A new stress reduction factor is introduced for the purpose of comparison. The properties and appropriate conditions of the stress reduction factor, the first and second thermal shock resistance (TSR) parameters for the high and low Biot numbers, respectively, and the approximation formulas for the intermediate Biot number-interval are discussed. To investigate the TSR of ceramics more accurately, it is recommended to combine the heat transfer theory with the theory of thermoelasticity or fracture mechanics or use a numerical method. The critical rupture temperature difference and the critical rupture dimensionless time can be used to characterize the TSR of ceramics intuitively and legibly.

Impact of Biodiesel Impurities on the Performance and Durability of DOC, DPF and SCR Technologies

It is estimated that operating continuously on a B20 fuel containing the current allowable ASTM specification limits for metal impurities in biodiesel could result in a doubling of ash exposure relative to lube-oil derived ash. The purpose of this study was to determine if a fuel containing metals at the ASTM limits could cause adverse impacts on the performance and durability of diesel emission control systems. An accelerated durability test method was developed to determine the potential impact of these biodiesel impurities. The test program included engine testing with multiple DPF substrate types as well as DOC and SCR catalysts. The results showed no significant degradation in the thermo-mechanical properties of cordierite, aluminum titanate, or silicon carbide DPFs after exposure to 150,000 mile equivalent biodiesel ash and thermal aging. However, exposure of a cordierite DPF to 435,000 mile equivalent aging resulted in a 69% decrease in the thermal shock resistance parameter. It is estimated that the additional ash from 150,000 miles of biodiesel use would also result in a moderate increases in exhaust backpressure for a DPF. A decrease in DOC activity was seen after exposure to 150,000 mile equivalent aging, resulting in higher HC slip and a reduction inmore » NO{sub 2} formation. The metal-zeolite SCR catalyst experienced a slight loss in activity after exposure to 435,000 mile equivalent aging. This catalyst, placed downstream of the DPF, showed a 5% reduction in overall NOx conversion activity over the HDDT test cycle.« less

Preparation of Silicon Carbide Reticulated Porous Ceramics Sintered at Low Temperature with PCS as Sintering Additive

Silicon carbide(SiC) reticulated porous ceramics(RPCs) were fabricated by polymer sponge replicas method with PCS as sintering additive.Slurry with PCS as binder was recoated on the SiC reticulated porous performs by centrifuging process.The effects of sintering temperature and holding time on SiC RPCs microstructure and properties were investigated.On consideration of the strength and strut porosity of RPCs,the RPCs can be sintered at 1100℃ holding for 1h.With PCS slurry as recoating slurry,the strut thickness of SiC RPCs is very uniform.The optimal compressive strengths of the RPCs prepared by 10PPI sponge and 25PPI sponge are(1.08±0.21)MPa and(2.19±0.32)MPa,respectively.The refectory of them is as high as 1690℃.At the same time,the RPCs with PCS as sintering additive have good thermal shock resistance.After quenched at 1400℃,thermal shock resistance parameter of the RPCs prepared with 25PPI sponge is only 0.36.

High temperature thermo-physical properties and thermal shock behavior of metal–diborides-based composites

The thermo-mechanical properties and thermal shock resistance of two metal diborides composites, HfB2 with 20 vol.% SiC and HfB2 with 20 vol.% SiC and 10 vol.% AlN, were investigated. Results showed that the composite HfB2–SiC–AlN had a higher specific heat capacity than that of composite HfB2–SiC. However, the occurrence of a Si–Al–O phase in the grain boundaries increased the heat flow resistance at grain boundaries, resulting in decrease thermal diffusivity and thermal conductivity. The calculated thermal shock resistance parameters indicated that the addition of AlN increased the resistance against crack initiation and crack propagation of composite, corresponding to the increase in critical thermal shock temperature difference from 400 to 600 °C.

Transport and mechanical properties of Yb-filled skutterudites

A series of Yb-filled skutterudites were produced and powder samples consolidated using spark plasma sintering (SPS). The effect of different heating cycles on the resulting transport properties of the consolidated samples was explored. Specifically, the effect of sample uniformity on the electrical and thermal transport properties was explored. In addition to the optimal Yb-filling fraction, other factors, such as heating profiles and sintering conditions, were found to play a pivotal role in the performance of these materials. Large quantities of Yb-filled skutterudite material can be generated with high purity and uniformity. Resonant ultrasound spectroscopy was used to determine the elastic modulus and Poisson's ratio. Fracture strength was measured for 12 specimens and, taken with information obtained from resonant ultrasound spectroscopy and from thermal expansion and thermal transport characteristics, the thermal shock resistance parameter was evaluated. These parameters will be important for the engineering of thermoelectric modules based on skutterudite materials.

Thermal shock modeling of Ultra-High Temperature Ceramics under active cooling

Thermal shock resistance is one of the most important parameters in Ultra-High Temperature Ceramics (UHTCs) since it determines their performance in various applications. In this paper, due to the fact that the material parameters of UHTCs are very sensitive to the variations of temperature, the temperature-dependent thermal shock resistance parameters of UHTCS are studied using an analytical model. The effects of target temperature on the thermal shock behavior of UHTCs thermal protection system (TPS) under active cooling are examined. The calculated results show that thermal shock behavior of the UHTCs’ TPS is strongly affected by the target temperature.

Thermal shock resistance of dense zirconia matrix composites evaluated by indentation techniques

The thermal shock behavior of 3Y-TZP/LaPO 4 composites containing 0, 15, 30 vol.% LaPO 4 was evaluated with an indentation-quench method based on propagation of Vickers cracks. The effect of LaPO 4 particle size on the thermal shock behavior of 3Y-TZP/LaPO 4 composites was investigated preliminarily. Results showed that the composite containing 15 vol.% 900 °C sintered LaPO 4 exhibited better resistance to crack propagation and thermal shock under water quenching condition. Larger grain size or higher content of LaPO 4 would deteriorate the thermal shock resistance of 3Y-TZP/LaPO 4 composites. The calculation of thermal shock resistance parameter R st for the composites gives a possible explanation for the difference in thermal shock behavior.

EFFECTS OF THERMAL ENVIRONMENTS ON THE THERMAL SHOCK RESISTANCE OF ULTRA-HIGH TEMPERATURE CERAMICS

In the present study, the temperature-dependent thermal shock resistance parameter of Ultra-High Temperature Ceramics (UHTCs) was measured based on the current evaluation theories of thermal shock resistance parameters, since the material parameters of UHTCs are very sensitive to the changes of temperature. The influence of some important thermal environment parameters on the thermal shock resistance and critical temperature difference of rupture of UHTCs were studied. By establishing the relation between the temperature and the thermal or mechanical properties of the UHTCs, we found that thermal shock behavior of UHTCs is strongly affected by the surface heat transfer coefficient, heat transfer condition and initial temperature of the thermal shock.

Microstructure, mechanical properties and thermal shock resistance of nano-TiN modified TiC-based cermets with different binders

Abstract Microstructure, mechanical properties and thermal shock resistance of TiC-based cermets modified by nano-TiN were studied. The highest transverse rupture strength, fracture toughness and hardness were obtained for the cermets with the compositions 14Mo–20Ni (cermet A), 4Mo–10Co–10Ni (cermet B) and 4Mo–20Co (cermet C), respectively. The quenching-strength method and indentation-quench method were used for studying the thermal shock resistance of nano-TiN modified TiC-based cermets. The results of quenching-strength test denote that the critical temperature differences for sharp decline of residual strength are about 410, 370 and 330 °C for cermets A, B and C, respectively. The results of indentation-quench test show that the micro crack length increases and the micro cracks propagate fast with increasing thermal shock temperature and cycles. The propagation rate of the crack is controlled by the stress intensity of crack tip. Both tests show that cermets with high transverse rupture strength have excellent thermal shock resistance. Thermal shock resistance parameter R and R st calculated are in good agreement with the experimental results.

Thermal Shock Resistance of Ultra-High Temperature Ceramics

Thermal shock resistance of Ultra-High Temperature Ceramics is one of the most important parameters in UHTCs characterization since it determines their performance in many applications. In order to reflect practical cases, the temperature-dependent thermal shock resistance parameter of UHTCS was measured since the material parameters of UHTCs are very sensitive to the changes of temperature. The influence of some important thermal environment parameters and the size of the material on the thermal shock resistance and critical temperature difference of rupture of UHTCs at different stages in the thermal-shock process were investigated. The results show that thermal shock behaviour of the UHTCs is strongly affected by the size of the material and the thermal environments parameters, such as the surface heat transfer coefficient, heat transfer condition and initial temperature of the thermal shock.

Thermal shock behaviour of mullite–cordierite refractory materials

The characterisation of thermal shock damage in cordierite–mullite refractory plates used as substrates in fast firing of porcelain whiteware has been investigated. Two different refractory compositions (termed REFO and CONC), characterised by different silica to alumina ratios, were studied. Thermal shock damage was induced in as received samples by water quenching tests from 1250°C. Thermal and mechanical properties were measured at room temperature by means of standard techniques and then the thermal shock resistance parameter R was calculated. The fracture toughness of selected samples was measured before and after thermal shock by the chevron notched specimen technique. The reliability of this technique for evaluation of small differences in fracture toughness after a given number of thermal shock cycles was investigated. The suitability of KIc measurements by the chevron notched specimen technique to characterise the development of thermal shock damage in refractory materials was proved in this investigation.

Thermal shock resistance of functionally gradient solid cylinders

In this paper, an analysis of the transient thermo-mechanical behavior of a solid cylinder of functionally gradient material (FGM) under the convective boundary condition is presented theoretically. The analytical formula of the unsteady temperature distribution is derived by using the separation-of-variables method and hence the maximum thermal stress attained at the surface of the FGM solid cylinder as well as its time of occurrence can be calculated. Based on a local tensile stress criterion, the expression of critical temperature change Δ T c leading to the local tensile strength at the surface, which is designated as the thermal shock resistance parameter for FGM solid cylinder, is obtained. The effects of the radial distributions of thermo-physical properties on the thermal shock resistance of the FGM solid cylinder are investigated via numerical calculations in contrast to homogeneous solid cylinder, from which some suggestions on design of FGM solid cylinders with high thermal shock resistance are put forward.

Comments on “Thermal shock resistance of yttria-stabilized zirconia with Palmqvist indentation cracks” by G. Fargas, D. Casellas, L. Llanes, M. Anglada [J. Eur. Ceram. Soc. 23 (2003) 107–114

In a recent paper, Fargas et al. [Fargas, G., Casellas, D., Llanes, L. and Anglada, M., Thermal shock resistance of yttria-stabilized zirconia with Palmqvist indentation cracks. J. Eur. Ceram. Soc., 2003, 23, 107–114] make use of the Vickers indentation technique to characterise the thermal shock resistance of brittle materials exhibiting the Palmqvist indentation crack system. They claim that their approach can provide a measurement of Hasselman's R⁗ thermal shock resistance parameter. It is here demonstrated that the obtained parameter is in fact different from R⁗. In parallel, it is shown that a previously developed approach, which is both consistent with the concepts of R⁗ and of the critical quenching temperature difference, Δ T C, can be used in the case of the Y-TZP ceramics of Fargas et al. This approach also allows an estimation of the maximum thermal stresses and of the coefficient of heat transfer at the fluid–solid interface as a function of the quenching temperature difference.

A model of the thermal shock resistance parameter for functionally gradient ceramics

Abstract The thermo-elastic response of functionally gradient ceramics is investigated by means of the calculations of unsteady temperature fields and unsteady thermal stresses for an infinite functionally gradient ceramic plate with symmetrical structure under the convective boundary condition. A new strength-based fracture criterion for thermal shock of functionally gradient materials (FGM) ceramic plate is put foreword and consequently a new expression of critical temperature difference ΔTc, leading to the local fracture strength at the surface as the thermal shock resistance parameter for infinite FGM ceramic plate with symmetrical structure is obtained. The effects of the distributions for thermo-physical properties on the thermal shock resistance of the functionally gradient ceramics are analyzed via numerical calculations in contrast to homogeneous ceramics, from which some suggestions on design of functionally gradient ceramics with high thermal shock resistance are made.

Thermal shock behaviour of magnesia–spinel composites

The incorporation of up to 30% of spinel powder of median particle sizes 3, 11 and 22 μm into dense magnesia gives improved resistance to thermal shock as indicated both by the R‴ thermal shock resistance parameters calculated from measured mechanical properties, and by a standard quench test. The loss of strength of the composite materials in the quench test matched satisfactorily that predicted by the R‴ parameter. The maximum benefit from spinel additions was obtained with spinel of the largest particle size, at a loading of 20%. It is postulated that the pre-formed cracks in the composites, resulting from the thermal expansion mismatch between magnesia and spinel, inhibit the accumulation of strain energy during thermal shock.

Thermal shock behavior of SiC whisker reinforced Si 3N 4/BN fibrous monolithic ceramics

SiC whisker reinforced Si 3N 4/BN fibrous monolithic ceramics were fabricated by in situ synthesizing. The specimens were heated up to the test temperature and then quenched in a water bath of 25 °C. The thermal shock resistance parameters were calculated according to the elastic modulus, Poisson ratio, thermal expansion coefficient, strength and work of fracture. The thermal shock resistance critical temperature, Δ T, was 700 °C, which was improved more than SiC whisker reinforced Si 3N 4 ceramic. BN soft cell boundaries had an effect on the thermal shock behavior of the Si 3N 4/BN fibrous monolithic ceramics. The microstructures of the composites were observed by SEM.

Influence of tungsten carbide particles on resistance of alumina matrix ceramics to thermal shock

The mechanical properties and thermal shock behavior of the hot pressed alumina matrix ceramics, with added 6 vol.% tungsten carbide particles, was investigated. The thermal shock resistance of the materials was evaluated by water quenching and subsequent three-point bend testing of flexural strength diminution. The hot-pressed composite exhibited 70°C higher temperature differential of thermal shock resistance than the monolith, as well as increased flexural strength and fracture toughness. The calculation of thermal shock resistance parameters for the composite and the monolith gives a possible explanation for the differences in thermal shock behaviour.