Thermal Shock Parameter Research Articles

Fabrication of yttrium oxide refractory with high strength and thermal shock stability for smelting TiAl alloy

Yttrium oxide materials have a low theoretical flexural strength, making it challenging to simultaneously achieve high strength and thermal shock stability. This limitation hinders their practical utility as refractory materials for the smelting of TiAl alloys. In this work, we have developed high strength and thermal shock resistant yttrium oxide ceramics by the control of pore size distribution. For initial short pore lengths, e.g. pore sizes are concentrated within the range of 0∼5 μm, high strength can be achieved, but thermal shock resistance is poor (low thermal shock parameter, R'''') due to the easy growth of a main crack until sample fracture. For initial long pore (crack) lengths (100–200 μm), the crack propagation mechanism follows a quasi-static pattern, in which crack propagation can lead to excellent thermal shock stability. Nevertheless, the diminished strength resulting from these elongated cracks can directly result in structural damage. If the pore size distribution is primarily controlled within the range of 10∼20 μm, it is possible to attain relatively high strength along with exceptional thermal shock performance (indicated by elevated R'''' and Rst values). Therefore, we have obtained a flexural strength of yttrium oxide of 80 MPa, while the flexural strength retention rate after cooling from 1100 °C in ambient conditions for 15 min, repeated for 5 cycles, reached as high as 95 %. Furthermore, we have successfully fabricated a large-sized ceramic crucible made of yttrium oxide was fabricated for smelting Ti–46Al(wt.%) alloy ingots. Even after 3 cycles, the crucible remains undamaged. The interfacial reaction between yttrium oxide and the Ti–46Al(wt.%) alloy is minimal, which suggests that the yttrium oxide refractory shows great potential as a suitable choice for smelting TiAl based alloys.

On the determination of the thermal shock parameter of MAX phases: A combined experimental-computational study

Thermal shock resistance is one of the performance-defining properties for applications where extreme temperature gradients are required. The thermal shock resistance of a material can be described by means of the thermal shock parameter RT. Here, the thermo-mechanical properties required for the calculation of RT are quantum-mechanically predicted, experimentally determined, and compared for Ti3AlC2 and Cr2AlC MAX phases. The coatings are synthesized utilizing direct current magnetron sputtering without additional heating, followed by vacuum annealing. It is shown that the RT of both Ti3AlC2 and Cr2AlC obtained via simulations are in good agreement with the experimentally obtained ones. Comparing the MAX phase coatings, both experiments and simulations indicate superior thermal shock behavior of Ti3AlC2 compared to Cr2AlC, attributed primarily to the larger linear coefficient of thermal expansion of Cr2AlC. The results presented herein underline the potential of ab initio calculations for predicting the thermal shock behavior of ionically-covalently bonded materials.

Boosting Nd3+ emission in Nd/Al/Y co-doped silica glass by mid-range localized environmental manipulation

Improving the repetition frequency of high-energy lasers is vital to promote the development and application of high-energy lasers, and its key lies in finding the laser gain media with excellent thermal shock resistance and high emission cross-section. Rare-earth ion (RE3+)-doped silica glass, whose thermal shock parameter is 1.5 times higher than that of yttrium aluminum garnet (YAG) crystals, is expected to satisfy this criterion after enhancing the emission intensity of RE3+. Nevertheless, modulating the spectral properties of RE3+ in rigid silica glass networks poses a significant challenge. Herein, we propose a mid-range localized environmental manipulation approach, which dramatically changes the microscopic symmetry of RE3+ from amorphous to the desired crystal structures for emission enhancement. As a representative, Nd/Y/Al co-doped silica glasses were prepared and heat-treated for different durations to form different mid-range localized environments, which were demonstrated by transmission electron microscopy, electron paramagnetic resonance tests, and Judd-Ofelt theory analysis. The results show that Nd3+ enters the YAG structure in favor of its emission enhancement. After optimizing the mid-range local environment of Nd3+ to balance fluorescence enhancement with fluorescence quenching and Rayleigh scattering, the emission intensity increases by 300%. Our work suggests that the mid-range localized environmental manipulation approach is extremely promising for promoting the development of high-energy and high-repetition-rate lasers. Furthermore, this approach can be generalized to adjust the emission characteristics of other RE3+-doped glasses and fibers.

Characterization of Transparent Fluorapatite Ceramics Fabricated by Spark Plasma Sintering.

Highly optically transparent polycrystalline fluorapatite ceramics with hexagonal crystal structures were fabricated via a liquid-phase synthesis of fluorapatite powder, followed by spark plasma sintering (SPS). The effect of sintering temperature, as observed using a thermopile, on the optical transmittance and microstructure of the ceramics was investigated in order to determine suitable sintering conditions. As a result, high optical transmittance was obtained in the SPS temperature range of 950-1100 °C. The highest optical transmittance was obtained for the ceramic sample sintered at 1000 °C, and its average grain size was evaluated at only 134 nm. The grain size dramatically increased with temperature, and the ceramics became translucent at SPS temperatures above 1200 °C. The mechanical and thermal properties of the ceramics were measured to evaluate the thermal shock parameter, which was found to be comparable to or slightly smaller than that of single-crystal fluorapatite. This transparent polycrystalline fluorapatite ceramic material should prove useful in a wide range of applications, for example as a biomaterial or optical/laser material, in the future. Furthermore, the knowledge obtained in this study should help to promote the application of this ceramic material.

Microstructure and mechanical and thermal shock properties of hierarchically porous ceramics

Hierarchically porous Al2O3 ceramics with a porosity of 30–69% containing small intergranular pores and large pores created by a pore-forming agent were fabricated by gel-casting followed by partial sintering with starch addition. The influence of various pore types on the mechanical and thermal shock properties of the ceramics was subsequently investigated. A thermal shock parameter K was introduced to evaluate the dependence of strength degradation rate on the thermal shock temperature difference, and a new method for calculating the intergranular porosity and extrinsic porosity of the porous ceramics was developed. A low thermal shock strength degradation rate was obtained at a high ratio of fracture toughness (KIC) and elasticity modulus (E) of the material, which was achieved by decreasing the relative fraction of spheroidal extrinsic pores. However, the presence of these pores increased both the KIC and E values.

Experimental Investigations on Wavefront Distortion of LD-Pumped Neodymium-Doped Silica-Glass Rod with High Thermal Shock Parameter

The characterization of a laser diode (LD)-pumped neodymium-doped silica glass (NDSG) laser is here described. The gain performance and wavefront distortion were measured, and the thermal toughness and uniformity distribution of the material were experimentally observed. At a pumping frequency of 1 Hz and energy 7.79 J, a small-signal gain of 1.16 was measured, and the wavefront distortion reached 2.67 λ (wavelength λ = 1053 nm). At a pumping frequency of 25 Hz with 194 W power, the NDSG was still not cracked, which is consistent with its high thermal shock parameter. However, the material uniformity was relatively poor. These results indicate good prospects for the application of NDSG lasers at high energy and high repetition frequency, but the gain performance, uniformity, and other aspects affected by the manufacturing process need to be improved.

Thermal Shock and Thermo-Mechanical Behavior of Carbon-Reduced and Carbon-Free Refractories

The thermal shock behaviour of novel carbon-reduced refractories with maximum grain size of 1 mm was investigated. A wedge splitting test for small specimen geometries (max. 40 × 40 × 20 mm 3 ) was successfully implemented with different loading configurations to determine “work of fracture” and thermal shock parameters. Additionally, heating-up thermal shock tests were performed with an electron beam facility. The addition of 2.5 wt% ZrO 2 and TiO 2 to Al 2 O 3 refractories appears to improve their thermal shock resistance due to microstructural changes that reduce brittleness and inhibit critical crack growth. However, a phase transition of ZrO 2 affects the properties at elevated temperature. For another pure alumina refractory, no geometry-independent value for the work of fracture could be obtained for the sample geometry used, which is probably related to the formation of a large interaction zone of the fracture surfaces. Al 2 O 3 -C materials with addition of semi-conductive Si and nanoparticles revealed a strong effect of the pressing direction on the work of fracture. However, the thermal shock parameter R’’’’ was hardly affected by the different additives. Furthermore, thermal shock tests using the electron beam facility JUDITH 1 did not indicate any significant differences in the damage pattern of the different Al 2 O 3 -C materials.

Thermal, spectroscopic and laser characterization of monoclinic vanadate Nd:LaVO_4 crystal

The monoclinic vanadate Nd:LaVO(4) crystal was characterized, including the first investigation of thermal properties and anisotropic laser performance to our knowledge. The thermal properties behave anisotropically with the highest thermal conductivity of 3.27 W/m/K along the c* direction. With a laser diode as the pump source, the efficient continuous-wave (cw) and passively Q-switched monoclinic Nd:LaVO(4) crystal lasers were realized, which resulted in the nearly isotropic maximum output power and laser wavelength. The maximum cw output power is 3.56 W with a slope efficiency of 41.4%, and the passively Q-switched pulse width is 10.9 ns with pulse energy of 38.3 μJ. Based on the laser results, the thermal shock parameter anisotropy is calculated to be 1:1.28. The results show that, in contrast to other vanadate crystals, Nd:LaVO(4) is a novel laser material with low symmetry.

The effect of SnO2 on the improvement of mechanical properties of MgO–MgAl2O4 composites

Incorporation of SnO2 into MgO–spinel (M–S) increased mechanical properties significantly. Relationships between the parameters improving mechanical properties and microstructural variables were examined. Basic parameters improving the mechanical properties of M–S–SnO2 composites were identified as follows: (a) when microcracks come across with either Mg2SnO4 particles or pores; crack branching and deviation of interlinked microcracks or crack arresting occurred more effectively than those of spinel particles, (b) fracture type was converted to intergranular fracture with incorporation of spinel into MgO, and transgranular fracture with addition of SnO2 to M–S; additionally with the incorporation of additives, (c) critical defect size, (d) work of fracture values increased, and (e) MgO grain size decreased. Rst thermal shock parameter values of M–S–SnO2 composites were markedly higher than those of M–S materials, associated with low strength loss, high thermal shock damage resistance and thus longer service life of M–S–SnO2 composites for high-temperature industrial applications.

Controlled Crack Propagation Experiments with a Novel Alumina‐Based Refractory

AbstractInnovative, carbon‐reduced and carbon‐free refractory materials are currently under development within the framework of the DFG priority program “FIRE”. Among various novel material solutions an alumina‐based refractory with titania and zirconia additives (AZT) has gained special interest for application in high temperature processes under thermal shock conditions. The resistance against fracture of the AZT material and, for comparison, of a pure alumina refractory was examined by controlled crack propagation experiments. Wedge splitting and compact tension tests with in situ crack growth observation, partially on microstructural level, have been performed for both materials. Based on the measured room temperature values of dissipated energy, refractory stiffness and fracture stress, the Hasselman thermal shock parameter R″″ was determined. The results allow to predict that AZT is less prone to scale thermal shock damage than pure alumina. The microstructural observations reveal that growth and opening displacement of the main crack is accompanied in AZT by pronounced microcracking, branching and bridging processes. First efforts are also directed towards a mechanical quantification of this fracture behavior in terms of an R‐curve representation (fracture resistance as a function of apparent crack length). The specific problems of R‐curve evaluation that exist in AZT due to nonlinear deformation behavior are addressed and the influence of the observed crack growth mechanisms is discussed.

세라믹 모노리스 담체의 열충격 특성에 관한 연구

Technical ceramics, due to their unique physical properties, are excellent candidate materials for engineering applications involving extreme thermal and chemical environments. When ceramics are rapidly cooled, they receive thermal shock. The thermal shock parameter is defined as the critical temperature difference. The critical temperature difference for ceramic parts is influenced by its size, the convective heat transfer coefficient, etc. The thermal shock for a component is analyzed by using the transient thermal stress. If the transient thermal stress exceeds the modulus of rupture (MOR), cracking by thermal shock is initiated. The critical temperature difference for water is less than the critical temperature difference for air. The three-way catalyst substrate used in this study has an adequate performance against thermal shock because its radial and axial temperature differences existed below the critical temperature differences.

Modification and validation of the thermal shock parameter for ceramic matrix composites under water quenching condition

The critical thermal shock temperature difference to represent the thermal shock resistance of ceramic matrix composites was commonly predicted by the thermal shock parameter R TSF = σ ( 1 - υ ) E α . It can be usually measured experimentally by quenching specimens from various elevated temperatures and determining the quenching temperature difference that results in a reduction of strength for a given specimen geometry. However, the large difference between the measured (Δ T c) and calculated values always exists. In this study, a modified thermal shock parameter ( R m) of ceramic matrix composites was proposed based on theoretical derivation and experimental verification, although the new coefficient k was introduced. The value of coefficient k in the R m was validated by our present and previous experiment data. Furthermore, the R m was verified using the literature data by comparing its predictions with the Δ T c reported in the literatures. The results showed that the calculated R m was very close to the Δ T c measured in the literatures.

Manipulation of air plasma spraying parameters for the production of ceramic coatings

Thermal barrier coatings (TBCs) were sprayed on stainless steel coupons by an air plasma thermal spray (APS) technique. The porosity of the topcoat was varied by controlling the different spraying parameters. Three types of thermal shock tests were designed to determine the TBCs life. Effect of different spraying parameters on the thermal shock life was observed. The spraying distance was found to be an important parameter that controls the thermal shock life. It was observed that the thermal shock properties could be related with an empirical parameter called the critical thermal shock parameter (CTSP). Fracture toughness ( K IC) was determined by Vicker's indentation technique and it was observed that for a certain range of the porosity the toughness increased with increase of porosity. The possible increase in thermal shock life for a certain range of CTSP may be attributed to residual stresses and increase in fracture toughness of the topcoat.

40 J class laser oscillation of Nd-doped silica glass with high thermal shock parameter

The authors demonstrated a 40J class laser oscillation from a bulk-type Nd-doped silica glass with a high thermal shock parameter. A maximum output energy of 37.3J was obtained from a laser oscillator that consisted of a 30% output coupler with a 30mm diameter and 300mm length laser medium. The thermal shock parameter, a figure of merit for the thermal toughness of the materials, was estimated to be 12.0W∕cm, which is 1.5 times larger than neodymium-doped yttrium aluminum garnet (7.9W∕cm). These results show that the Nd-doped silica glass has the potential to be installed in high-average-power or high-peak-power lasers.

Laser Oscillation of Nd-Doped Silica Glass with High Thermal Shock Parameter

Laser oscillation with the input/output characteristics of bulk-type Nd-doped silica glass with a high thermal shock parameter was demonstrated for the first time. The thermal shock parameter was estimated to be 12.0 W/cm, which is 1.5 times larger than that of Nd:YAG crystal (7.9 W/cm). The maximum output energy of 154 mJ was obtained with an output mirror of 20% transmittance. Although the output energy was 154 mJ, if without loss, output energy increases up to 1.69 J, eleven times larger than the present state. These results mean that Nd-doped silica glass can work as a high-average or a high-peak-power-laser.

Thermal Shock Resistance of the RE-Oxide and Oxynitride Glasses

Thermal shock resistance of the RE-Si-Mg-O-N glasses (RE = La, Nd, Yb, Lu) with 0 and 20 eq.% of nitrogen was investigated by the indentation-quench method based on propagation of Vickers cracks. Crack growth was measured on the same sample for a test series of different quenching temperatures. Thermal shock resistance of the studied materials was determined as a temperature difference resulting in 10 % growth of the initial cracks (∆T10) and by the thermal shock parameter R calculated from the material properties. Although the comparison of ∆T10 and R values as a function of glass composition revealed some differences between these two approaches, also a common trend was observed. Thermal shock resistance increased with the fractional glass compactness resulting from RE type and N content increase.

Development of Nd-doped Optical Gain Material Based on Silica Glass with High Thermal Shock Parameter for High-Average-Power Laser

In this paper, we report a demonstration of amplification at 1.06 µm in a bulk-type Nd-doped silica glass (Nd2O3: 1.25 wt%, Al2O3: 2.72 wt%, SiO2: 96.03 wt%) with a high thermal shock parameter for the first time. The optical gain of 1.75 was obtained in a 93-mm-long sample with a flash lamp pumping at 1.9 kJ, corresponding to a gain coefficient of 0.06 cm-1. The thermal shock parameter of Nd-doped silica glass was measured to be 12.0 W/cm, which is thirty times greater than that of phosphate glass and one and a half times greater that of Nd:YAG. We carried out a quick quenching experiment to determine the thermal shock toughness of Nd-doped silica glass, and the experiment demonstrated that the thermal shock toughness of Nd-doped silica glass was superior to that of the phosphate laser glass. We also calculated the thermal stress distribution in Nd-doped silica glass with a simple model for comparison with the experimental results; the model well explains the experimental results. These results indicate that Nd-doped silica glass can produce a compact, high-energy density, and high-average-power laser.

Work of fracture and fracture surface energy of magnesia-spinel composites

This study used a model system of fully dense high purity magnesia, incorporating fine grain spinel prepared by hot-pressing. Work of fracture ( γ WOF) and fracture surface energy ( γ i ) were determined to be a function of spinel particle size and volume fraction. Predictions made from the thermal shock parameter, R⁗, were tested by measurements made on γ WOF γ i ratios of magnesia and magnesia-spinel composites. The results obtained from γ WOF γ i ratios matched the predictions. On the basis of γ WOF γ i ratios, the optimum spinel content and particle size were determined. Thermal shock data confirmed that higher values of the γ WOF γ i ratios were considered to be a good indicator for thermal shock resistance. Higher values of γ WOF were associated with the occurrence of more intergranular fracture with increasing spinel additions. Crack propagation was found to be much more difficult in magnesia-spinel composites, than the initiation of cracks.

Young’s modulus measurements of magnesia–spinel composites using load–deflection curves, sonic modulus, strain gauges and Rayleigh waves

Abstract The extent of interlinking of the microcracking and a decrease in strength and modulus values were determined to be a function of both spinel particle size and volume fraction to allow calculation of thermal shock parameter, R‴. Measurements of Young’s modulus were carried out both at room temperature and after thermal shock testing by using the load–deflection curves (defined as “mechanical” modulus) and by the sonic modulus technique. The values obtained from these methods were significantly different for quenched/unquenched samples. To understand the basis for these differences, strain gauge and Rayleigh wave methods were also used to determine Young’s modulus of the composites. Modulus values obtained from these methods confirmed the differences measured, and provided a guide to the values to be used in calculating thermal shock parameters. The mechanical modulus technique was considered the most meaningful indicator of Young’s modulus for a situation in which large mechanical strains were to be applied to the materials during thermal shock.

A study on the optimal monolith combination for improving flow uniformity and warm-up performance of an auto-catalyst

The optimal design of an auto-catalyst needs a good compromise between the pressure drop and flow uniformity in the monolith. One of the effective methods to achieve this goal is to use the concept of a radially variable cell density. But this method has not been examined with respect to its usefulness in terms of chemical behavior and light-off performance. In this study, a multi-dimensional performance prediction of catalyst coupled with a turbulent reacting flow simulation has been used to evaluate the benefits of this method from the simultaneous viewpoint of fluid dynamics and chemical response during warm-up. The results showed that the combined monolith with 93/73 and 93/62 was very prospective for improved light-off performance and reduced the pressure loss due to both the balanced space velocity and the efficient usage of the geometric surface area of the channels. It was also found that the air distribution between the different cell densities greatly affects not only pressure loss and flow uniformity but also the light-off pattern. In this study, thermal durability was also examined by using simple correlation governed by mechanical and thermal properties. It was found that based on the thermal shock parameter, the cell combined monolith shows weaker thermal resistance than the conventional monolith.