Superior Thermal Shock Resistance Research Articles

Silica/Epoxy Hybrid Encapsulation with High Heat-Resistance and Low Coefficient of Thermal Expansion

As a matrix resin of silica/epoxy hybrid encapsulation material with high heat resistance and low coefficient of thermal expansion, a tri-functional triglycidyl p-aminophenol (TGPAP) epoxy was successfully synthesized with the desired chemical structure by reacting epichlorohydrin with p-aminophenol using sodium hydroxide catalyst. In order to produce highly pure TGPAP with lower viscosity, a physical thin film vacuum distillation was carried out after synthesis, resulting in 97% purity with the viscosity of 700 cps at room temperature. Silica/TGPAP hybrid was fabricated using two kinds of ground fumed silica with different sizes of 20 μm and 9 μm. Glass transition temperature of the hybrid was found to be as high as 185 °C. Viscosity of the hybrid was 30,000 cps at room temperature before curing, implying good processability. When two different sizes of silica were used with 1 to 1 weight ratio, very low coefficient of thermal expansion of 22.79 ppm/°C was obtained. From thermal shock test, no cracks were observed even after 1,000 cycles of thermal shock between −40 °C and 125 °C when 67 wt% (45.8 vol%) of silica was involved in the hybrid. In conclusion, silica/TGPAP hybrid with improved processability, superior thermal shock resistance and low thermal expansion coefficient could be fabricated as electronic device encapsulation.

The roles of stress in the thermal shock failure of YSZ TBCs before and after laser remelting

In order to reveal the roles of stress in the thermal shock failure of thermal barrier coatings (TBCs), yttria-stabilised zirconia (YSZ) TBCs were prepared via atmospheric plasma spraying and then remelted by a laser remelting process. Subsequently, thermal cycling tests of the as-sprayed YSZ TBCs and remelted YSZ TBCs were carried out at 1100 °C. The thermal shock failure mechanism of the two TBCs was investigated comparatively from the perspective of stress, i.e., phase transformation stress of ceramic coating, growth stress and thermal mismatch stress generated by thermally grown oxide (TGO). The results showed that, compared with the as-sprayed YSZ TBCs, there was no m-ZrO2 in the remelted YSZ TBCs, which avoided the ceramic coating phase transformation stress produced by the transformation between m-ZrO2 and t-ZrO2 during thermal cycling. Therefore, the thermal shock failure mode of the as-sprayed YSZ TBCs was severe buckling driving delamination, while that of the remelted YSZ TBCs was slight edge delamination. In addition, the TGO growth stress and the thermal mismatch stress between the TGO and the bonding coating were reduced by the laser remelting treatment, leading to the spallation tendency of YSZ TBCs was decreased. Consequently, the remelted YSZ TBCs exhibited superior thermal shock resistance to the as-sprayed YSZ TBCs. Therefore, the phase transformation stress, growth stress and thermal mismatch stress determined the thermal shock failure mode and thermal shock life of these two YSZ TBCs.

Preparation of Cordierite-mullite Ceramics for Solar Thermal Storage

We developed cordierite-mullite composite ceramic materials to package and encapsulate PCM, and presented a preparation process from raw materials of kaolin, talc and alumina. The properties and microstructre of cordierite-mullite composite ceramic were studied. Due to the strengthening effects of mullite, the sample C2 (80 wt% of cordierite and 20 wt % of mullite) sintered at 1 420 °C possessed excellent physical properties. Determined by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) analysis, cuboid-shaped cordierite crystals and needle-like or long quadrilateral-prism mullite crystals with staggered patterns were found, which endowed the composites preferable mechanical strength. After 30 cycles of thermal shock (room temperature to about 1 100 °C, air-cooled), the sample presented superior thermal shock resistance, which is suitable to be applied as solar thermal storage materials.

Mg2SiO4 as a novel thermal barrier coating material for gas turbine applications

Forsterite-type Mg2SiO4 was investigated systematically for thermal barrier coating (TBC) applications. Results showed that Mg2SiO4 synthesized by solid-state reaction possessed the good phase stability up to 1573 K. The thermal conductivity of Mg2SiO4 at 1273 K was lower ˜20% than that of yttria stabilized zirconia (8YSZ). Mg2SiO4 also presented moderate thermal expansion coefficients, which increased from 8.6 × 10−6 K−1 to 11.3 × 10−6 K−1 (473˜1623 K). Mechanical properties including hardness, fracture toughness, and Young’s modulus of Mg2SiO4 were comparable to those of 8YSZ. The sintering results indicated a promising low-sintering activity of Mg2SiO4. Mg2SiO4 samples were subjected to water quenching test at 1573 K and showed a superior thermal shock resistance compared to 8YSZ. Mg2SiO4 coating with stoichiometric composition was produced by atmospheric plasma spraying. The thermal cycling test result showed that Mg2SiO4 coating had a lifetime more than 830 cycles at 1273 K, which is desirable for TBC applications.

Thermal and mechanical cyclic tests and fracture mechanics parameters as indicators of thermal shock resistance – case study on silica refractories

The paper discusses alternative methods to assess thermal shock in refractories. Thermal shock performance of two conventional silica bricks and two novice fused silica materials has been studied. Methods involving fracture mechanical tests of monotonic loading, repetitive thermal shock tests and cyclic strain controlled fatigue tests have been utilised. The amorphous fused silica is rather brittle. However low thermal expansion guarantees its superior thermal shock resistance. In-service crystallisation is to happen rather quickly. For crystallised fused silica of the studied morphology the tests coherently predict superior thermal shock resistance due to less brittle failure. Strain controlled fatigue test allows assessment of strain limits in the conditions of cyclic loading and thus combines the benefits of the thermal shock and monotonic fracture mechanics tests. Strain tolerance seems to be the property to correlate the results of the alternative test methods and those with the service loads.

Heightening mechanical properties and thermal shock resistance of low–carbon magnesia–graphite refractories through the catalytic formation of nanocarbons and ceramic bonding phases

The in-situ catalytic formation of nanocarbons and ceramic bonding phases in low–carbon magnesia–graphite refractories is one of the significant strategies for heightening their mechanical properties and thermal shock resistance. Here, effect of aluminum content and nickel–containing catalyst addition on microstructural evolution, mechanical and thermo-mechanical behavior of such refractories was explored. Under the function of the catalyst, addition of aluminum powders allowed the newly growth of plenty of nanocarbons (e.g., carbon nanotubes and carbon onions), and also accelerated the in–situ formation of more ceramic bonding phases (e.g., magnesia whiskers and spinel whiskers/particles, etc.) in samples. This occurrence optimized significantly the microstructure of samples, correspondingly giving rise to their superior mechanical properties and thermal shock resistance. This work might provide a path for exploring low–carbon magnesia–graphite refractories with high performances.

In situ synthesis and thermal shock resistance of a cordierite‐mullite composite for solar thermal storage

AbstractCordierite‐mullite composite ceramic was synthesized in situ by semidry pressing and pressureless sintering from andalusite, kaolin, γ‐Al2O3, talc, potassium feldspar, and albite in air. The effects of composition and sintering temperature on the density, bending strength, thermal shock stability, crystal phases, and microstructure of the specimens were studied. The results show that specimen B2 (the theoretical content of cordierite was 20 wt%) has excellent performance, that is, a bending strength of 104.59 MPa, 30 cycles of thermal shock resistance without cracking, and a loss rate of 13.12%. X‐ray diffractometer (XRD) analysis and scanning electron microscope (SEM) micrographs showed that spherical cordierite crystals were grown on the surface of the mullite, therefore, the specimen possessed a superior bending strength and thermal shock resistance, where a great number of granules combined to restrain crack initiation as well as propagation over time during the thermal shock test. The thermal conductivity of specimen B2 was determined to be 3.83 W/(m·K) (36°C), and the sensible heat storage density was 1136 kJ/kg, with the temperature difference (ΔT) ranging from 0 to 800°C. Consequently, the cordierite‐mullite composite is a potentially applicable material for solar thermal storage.

Short silica fibre-reinforced polymethylsilsesquioxane–phenolic interpenetrating networks: exploration for use as ablative thermal protection system in aerospace

Short silica fibre-reinforced simultaneous interpenetrating polymer network (IPN) composites consisting of polymethylsilsesquioxane (PMSQ) and phenol–formaldehyde resin were prepared through thermal cross-linking. Such IPN composites combine the advantageous ablative features of individual systems and excellent thermal insulation aspects. IPNs were prepared with varying composition between PMSQ and PF, at constant fibre content (60% by weight). The influence of IPN composition on the ablation and out-gassing characteristics was explored in detail. Arc-jet evaluation brought out the superior thermal shock resistance of IPN composites that were richer in PMSQ content. Further, it was inferred that the mechanism of thermal protection shifted to ablative heat shielding with increase in PF content. Microstructural characterization of the arc plasma eroded specimens was carried out using FTIR and Raman spectroscopy to understand the chemical mechanism of erosion process. Higher PMSQ content facilitated the formation of SiOC glass, whereas higher PF content resulted in well-defined char layer comprised of predominantly pyrolytic carbon. Higher PMSQ content also favoured lower out-gassing properties. The study brought out the superior thermal protection capability of the IPN with respect to aerospace applications.

Preparation and Oxidation Behavior of Metallic Nickel Containing MgAlON Composite

AbstractTo survey the probability of the re-utilization of nickel containing slag corroded magnesia and alumina-graphite refractories, different amounts of nickel oxide (NiO) were used as one of raw materials to synthesize nickel containing magnesium aluminum oxynitride (MgAlON) composites, and the sintering and oxidation behavior have been explored in this work. The results reveal that with the increasing NiO additive, submicron metallic Ni grains are segregated from the (Ni,Mg)AlON spinel, and these grains are beneficial to improve thermal shock resistance of MgAlON. Though the oxidation reaction of (Ni,Mg)AlON starts at lower temperature than MgAlON, the metallic Ni grains in the plate can improve the oxidation resistance. Hence, the metallic nickel containing (Ni,Mg)AlON composite has not only superior thermal shock resistance but also excellent oxidation resistance, and it is of great potential to utilize in plant. So, the fabrication of metallic nickel containing (Ni,Mg)AlON composite should be a feasible way to reuse the nickel slag corroded magnesia and alumina graphite refractories.

What is the suitable segmentation crack density for atmospheric plasma sprayed thick thermal barrier coatings with the improved thermal shock resistance?

The optimization and control of the segmentation crack density (Ds) for the thick thermal barrier coatings (TTBCs) with the improved thermal shock resistance has been performed via finite element modeling. The simulation results based on the current property parameters of each layer of the TTBCs fabricated by atmospheric plasma spraying (APS) are well consistent with the experimental results of thermal shock test. The investigation results indicate that too large or too low Ds will be not beneficial to the improvement of the thermal shock resistance of the TTBCs. The Ds must be located at a suitable range, and this paper has revealed the objective law quantitatively. Based on our simulation and experimental results, the appropriate segmentation crack density is in the range of 2.38–4.76 cracks/mm which will be beneficial to improve the thermal shock resistance ability. It has been found that the as-sprayed TTBCs exhibited superior thermal shock resistance when the segmentation crack density is about 4mm−1. The stress intensity factor (KI) and energy release rate (J integration) will increase with the increasing of segmentation crack length. The existence of segmentation crack will improve the strain tolerance of TTBCs. The strain tolerance has been characterized by Ds and segmentation crack length quantitatively. The failure mechanism of APS-TTBCs can be attributed to the propagation of segmentation crack at the top-coat of the TTBCs, formation and propagation of the horizontal crack at the top-coat/TGO (thermally grown oxide) interface. The propagation rate of the main segmentation crack has been calculated and the life prediction model of the TTBCs during thermal cycle has been established. The possible methods which can prolong the service life of the TTBCs have also been proposed.

Improved thermal shock resistance of magnesia-graphite refractories by the addition of MgO-C pellets

Magnesia-graphite (MgO-C) refractories with 12–20wt% carbon contents are extensively used for the taphole sleeve bricks, bottom blowing elements, slag line bricks, etc. in steelmaking operations. To reduce the carbon content but to have the same or even superior thermal shock resistance with commercial available material compositions (14wt% flaky graphite), we report here a new approach based on the granulating treatment of flaky graphite to improve the thermal shock resistance of MgO-C refractories (10wt% flaky graphite). MgO-C pellets are firstly prepared by the crushing granulation method, and then introduced into such refractories. Addition of MgO-C pellets has no apparent influence on their flexural strength, but enhances their flexural strength after thermal shocks and residual strength ratio. Particularly, when 10wt% flaky graphite of the specimens is replaced totally by MgO-C pellets, their thermal shock resistance is superior to that of the specimens containing 14wt% flaky graphite. This new method opens up possibilities to obtain MgO-C refractories with improved thermal shock resistance.

Influence of surface morphology and microstructure on performance of CVD tungsten coating under fusion transient thermal loads

Thick tungsten coatings have been deposited by chemical vapor deposition (CVD) at a rapid growth rate. A series of tungsten coatings with different thickness and surface morphology were prepared. The surface morphology, microstructure and preferred orientation of the CVD tungsten coatings were investigated. Thermal shock analyses were performed by using an electron beam facility to study the influence of the surface morphology and the microstructure on the thermal shock resistance of the CVD tungsten coatings. Repetitive (100 pulses) ELMs-like thermal shock loads were applied at various temperatures between room temperature and 600°C with pulse duration of 1ms and an absorbed power density of up to 1GW/m2. The results of the tests demonstrated that the specific surface morphology and columnar crystal structure of the CVD tungsten have significant influence on the surface cracking threshold and crack propagation of the materials. The CVD tungsten coatings with a polished surface show superior thermal shock resistance as compared with that of the as-deposited coatings with a rough surface.

Effect of Sm2O3 on microstructure, thermal shock resistance and thermal conductivity of cordierite-mullite-corundum composite ceramics for solar heat transmission pipeline

Cordierite-mullite-corundum composite ceramics for solar heat transmission pipeline were fabricated via pressureless sintering at a low sintering temperature with added Sm2O3. The effects of Sm2O3 on sintering behaviors, mechanical property, phase transformation, microstructure, thermal shock resistance and thermal conductivity of the composite ceramics were investigated. TEM analysis results demonstrated that Sm3+ located in glass and grain boundaries to facilitate the densification via the liquid-phase sintering mechanism and improve bending strength by grain refinement, respectively. Proper addition (3wt%) of Sm2O3 could promote the crystallization of cordierite, and improve thermal shock resistance of the composite ceramics with an increasing rate of 16.70% for bending strength after 30 thermal shock cycles (air cooling from 1100°C to RT). The composite ceramics possessed a superior thermal shock resistance, where a large amount of particles were formed to suppress crack initiation and propagation during thermal shock. Cordierite-mullite-corundum composite ceramics with proper Sm2O3 addition (3wt%) had a lower thermal conductivity than that of composite ceramics without Sm2O3 addition by strengthening the scattering of phonon, which could reduce the heat loss during solar heat transmission process.

Fabrication of Al2O3–Mullite–AlN Multiphase Ceramic Layer on W–Cu Substrates for Power Semiconductor Packaging

This paper proposes a novel method to manufacture ceramic insulated metal substrate for power semiconductor modules and high-temperature electronics. α-Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -mullite (Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sub> )-AlN multiphase ceramic layer was fabricated on W80Cu20/Cr substrate at 1050 °C by firing. The polycarbosilaneAlN green ceramic layer was synthesized by spin processor, and then was fired under a steady flow of wet nitrogen Al-Si-O nanocrystalline rods were formed and their diameters were about 100 nm. The α-Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -mullite-AlN multiphase ceramic layer had perfect interface bonding, and exhibited superior mechanical properties and thermal shock resistance.

Development of a Zirconia-Graphite Refractory for a SEN Application

Zirconia-graphite refractories are generally used in the slag-like zone of the subentry nozzle in continuous casting. A zirconia-graphite refractory has superior thermal shock resistance and corrosion resistance compared to MgO-C and Al2O3-C refractories. The effect of graphite on the zirconia-graphite refractory is not well documented. Therefore, in the present work an attempt has been made to evaluate the effect of graphite on the properties of the zirconia refractory. Graphite can be classified in terms of fixed carbon content and in this work three different types of graphite with varying fixed carbon content were used (99, 94, and 80%). The amount of zirconia was maintained at 82% and that of the graphite at 18%. Other processing parameters and ingredient concentrations were constant throughout the work. Refractory samples were fabricated using the cold isostatic method and refractory samples were coked at 1000°C in a reducing atmosphere. The effect of fixed carbon percentage on apparent porosity, bulk density, spalling resistance, and oxidation resistance were evaluated.

Influence of Zirconia Interfacial Coating on Alumina Fiber‐reinforced Alumina Matrix Composites

The present study demonstrates an approach for fabricating fiber‐reinforced ceramic matrix composites (CMCs) involving the coating of 2‐dimensional woven alumina fibers with zirconia layer by sol gel, followed by impregnation of these coated fibers with alumina matrix and pressureless sintering. To emphasize the benefits of the zirconia coating on these CMCs, a reference sample without interfacial coating layer was prepared. The zirconia‐coated CMCs showed superior flexural strength and thermal shock resistance compared with their uncoated counterparts. Foreign object damage tests carried out on the ZrO2 coated CMCs at high impact speed showed localized damage without any shattering.

Effect of LaB6 on the thermal shock property of MoSi2-SiC coating for carbon/carbon composites

LaB6-MoSi2-SiC and MoSi2-SiC coatings were prepared on the surface of carbon/carbon composites by pack cementation method. The crystal structures of the coatings were measured by X-ray diffraction. The morphologies and element distributions were also analyzed by scanning electron microscopy and energy dispersive spectroscopy, respectively. The effect of LaB6 on the microstructure and thermal shock resistance of MoSi2-SiC coating was investigated. The results indicated that the LaB6-MoSi2-SiC coating possessed a denser structure and superior thermal shock resistance. After 25 times of thermal cycling oxidation between 1773K and room temperature, the weight losses of the LaB6-MoSi2-SiC and MoSi2-SiC coated samples were 0.627% and 2.019%, respectively.

Microstructure evolution of 1 mol % calcia-doped yttria

The microstructure of 1 mol % calcia-doped yttria under different sintering condition was investigated, and the thermal shock resistance of yttria with and without additive was compared in present work. The results showed that by adding 1 mol % calcia, the sinterability of yttria is greatly improved and the sample can be sintered as low as 1300°C. Increasing heating temperature and prolonging soaking time are beneficial to form coarse grains with lots of closed pores in calcia-added sample at temperatures higher than 1500°C. By theoretical analysis, it is known that because of the solid solution of Ca2+, defects are generated in the yttria lattice, and the activation energy for diffusion is decreased. Therefore, the grain boundary mobility of yttria is accelerated and closed pores are left inside coarse grains when pores cannot move with yttria grain boundaries anymore. The water quench test reveals that the non-additive sample heated at 1500°C has the highest thermal shock resistance and it cannot be utilized under high temperature in industry. On the contrary, the calcia-doped sample has superior thermal shock resistance by the effect of closed pores, and it might be utilized to melt and cast high purity titanium and its alloys through vacuum induction melting (VIM) process.

Mass‐Produced Mullite Crucibles in Medieval Europe: Manufacture and Material Properties

Crucibles from the German region of Hesse have been famous since the Middle Ages due to their exceptional quality, regarded by many as a mystery. We analyzed 50 Hessian and non‐Hessian archeological crucibles using SEM‐EDS, FESEM, and XRD to investigate their technology and material properties. It was revealed that Hessian crucibles were systematically made of kaolinitic clay with a low flux content, mixed with quartz sand, and fired to temperatures in excess of 1300°C. Primary mullite developed in most of the glass matrix, with secondary mullite in some regions of clay–feldspar relict mixtures. Consequently, the vessels showed superior creep and thermal shock resistance, high‐temperature strength, and thermal and chemical refractoriness. These crucibles represent the earliest industrial exploitation of mullite in Europe, which explains their historical success.

使用済み炭化ケイ素を用いる新規耐火物

Reuse/recycling of silicate bonded silicon carbide plates materials after removal from ceramic industrial service was evaluated for ceramic and steel industry. Experimental batches from spent slabs with different contents of bauxite and silica fume were prepared and fired up to 1400°C. Their densification properties in terms of bulk density and apparent porosity were measured and correlated with objecting properties. Mineralogy and microstructure of the sintered batches were studied and correlated with the ratio of their cold crushing strengths after 30 subsequent thermal shock cycles and before subjecting the batches to the thermal shock. The formation of mullite, replacement of silicon carbide by mullite and formation of pseudo silicon carbide, tight structure and well distribution of silicon carbide were recorded. The ratio of strengths after and before shock enhanced with increasing mullite content up to about 40%. All batches gave a superior thermal shock resistance where no spalling was observed up to 30 cycles of heating up to 950°C and water quenching.