Ultra-high Temperature Oxidation Research Articles

Analysis of Test Specimen Temperature Gradients Incurred in Resistive Heating System Oxidation Studies of Ultra-High Temperature Ceramics

AbstractThe need for advanced materials that can meet application requirements at ultra-high temperatures in oxidizing environments is an area of active research. One challenge facing the high temperature materials community is the ability to conduct controlled ultra-high temperature oxidation tests with minimal to no contamination or reaction with the chamber. A unique resistive heating system (RHS) capable of achieving ultra-high temperatures (> 1700 °C) to enable such experimentation is described. A concern of such a system is the potential presence of thermal gradients in directions not reflective of actual material applications, e.g., the hottest region being in the center of the sample. Experimental results from the oxidation of ZrB2 specimens at nominal temperatures of 1500°, 1700° and 1800 °C in low pO2 (0.1–1% O2 in Ar) environments are presented. Specimen thermal gradients generated during oxidation were evaluated using finite element analysis models. Thermal gradients on the order of the uncertainty in temperature measurements were calculated, confirming the RHS suitability for conducting ultra-high temperature oxidation exposures on ultra-high temperature ceramics.

Elastic, antiflaming phenolic aerogels for advanced thermal protection at extreme environments

In practical applications, thermal protection demands materials with reliable structural stability under extreme conditions. Phenolic aerogels exhibit remarkable thermal protection capabilities; however, their inherent brittleness and porous structure make them susceptible to catastrophic failure when subjected to mechanical stresses or thermal oxidation. To address this, robust phenolic composite (PFR@SiO2) aerogels were synthesized through a three-step process involving sol-gel generation, ambient-pressure drying, and chemical vapor deposition. The resulting PFR@SiO2 aerogels display impressive mechanical properties, as well as a high degree of flame resistance. Furthermore, under fatigue stress/strain cycling tests or ultrahigh-temperature oxidation circumstance, such phenolic composite aerogels also exhibit exceptional structural stability. Overall, these combined properties position the PFR@SiO2 aerogels as a promising material system for robust thermal superinsulation under extreme conditions.

Enhancing fruity and sweet-associated aroma attributes of honeybush tea (Cyclopia spp.) by ultra-high temperature oxidation

Ultra-high temperature oxidation (UHTO) was explored as an alternative to the standard oxidation of honeybush tea, aiming to shorten the processing time and enhance the fruity and sweet-associated sensory characteristics. Different UHTO temperature × time regimes were applied to Cyclopia subternata, C. genistoides, C. intermedia and C. longifolia. Descriptive sensory analysis showed that UHTO treatment, regardless of species, increased the intensities of fruity and sweet-associated aromas while decreasing the intensities of floral aromas. The optimal UHTO regime for each species was subsequently applied to a larger number of plant material batches (n = 10 per species) to introduce a wider range of intra-species variation and confirm broad trends for the sensory profile, as well as for the colour and phenolic composition, of the infusions over species. The effect of treatment conditions on the retention of the phenolic compounds varied, depending on the compound and species. The substantial decrease in the mangiferin and isomangiferin concentrations of C. genistoides infusions, which was not observed for the other species, was notable. The study highlights the potential of UHTO to reduce oxidation time while enhancing the fruity and sweet-associated aroma attributes of honeybush tea, changes that may be beneficial in attracting new consumers and expanding the honeybush tea market.

Ultra-high temperature oxidation resistance of a MoSi2 composite coating with TaB2 diffusion barrier on tantalum alloy

High-temperature elemental inter-diffusion is an important cause of failure for high-temperature antioxidant coatings on tantalum substrate, yet little research has been reported on improving this phenomenon. A new MoSi2 composite coating with TaB2 diffusion barrier was prepared on a tantalum substrate surface by three steps of embedding/slurry sintering/embedding to solve the failure of silicide coating on tantalum substrate due to elemental diffusion under a high-temperature oxidation environment. The composite silicide coating showed excellent oxidation resistance at 1700 ℃, and the protection time was more than 14 h before failure. This may be due to the prevention of diffusion of Si atoms at the initial stage by the TaB2 with many B atoms in a solid solution. The Mo5SiB2 layer with a low diffusion coefficient will be formed with the slow release of B atoms. The diffusion of Si atoms is further hindered by the formation of a new diffusion barrier between the TaB2 phase and the main layer, and thus slowing down the depletion of Si elements in the main coating. In addition, the coating successfully passed 1021 thermal cycles from room temperature to 1700 ℃, which may be because of the alleviation of thermal mismatch between the ceramic coating body and the tantalum substrate by a diffusion layer thus reducing the cracking trend of coating.

Ultra-high temperature oxidation resistant refractory high entropy alloys fabricated by laser melting deposition: Al concentration regulation and oxidation mechanism

Poor oxidation resistance is a critical factor limiting the application of metalllic materials at ultra-high temperature environments. In this contribution, Alx(CrMoTaTi)1−x (x = 0.05, 0.1, 0.15) refractory high entropy alloy (RHEA) system was fabricated by laser melting deposition. The as-deposited RHEA was composed of two body-centered-cubic phases (BCC1 and BCC2). It was found that the volume fraction of BCC2 was increased with increasing the Al concentration from 0.05 to 0.15 and, surprisingly Al0.15(CrMoTaTi)0.85 could thermally grow an external Al2O3 scale during oxidation in air at 1300 °C. The insights into the microstructure-based oxidation mechanism of the RHEA were illustrated.

Insights into service-derived Zr-Ta-O film: Morphology evolution, temperature resistance, and atomic bonding

Entropy-stabilized ceramics for ultra-high temperature oxidation protection are springing up, accompanied by complex morphology characteristics and elusive protective mechanisms. Herein, how the Ta/Zr molar ratio change affected the morphology evolution, temperature resistance, and atomic bonding of Zr-Ta-O film was revealed, which was expected to deepen the understanding on A-B-O protective film (A = Hf, Zr; B = Ta, Nb). The Zr-Ta-O film was constructed on the thermally sprayed ZrC-TaC coating utilizing oxyacetylene ablation. Morphological characteristics of the Zr-Ta-O film were found highly related to Ta/Zr molar ratios. The long-term temperature endurance limit of the service-derived Zr-Ta-O film was considered below 2000 ℃, affected by the consumption of micro-scale Ta-rich regions under high-temperature gas scouring (∼2800 ℃, 5–8 m/s). The phenomena were well explained by the binding energy difference (Ta-doped t-ZrO2 and Zr-doped α-Ta2O5) and fluctuation law of Zr-O bond length with Ta/Zr molar ratio increasing (from local instability to whole instability).

Significant improvement of ultra-high temperature oxidation resistance of C/SiC composites upon matrix modification by SiHfBCN ceramics

Advanced C/SiC-SiHfBCN composites were prepared by modification of the matrix of C/SiC composites with single-source-precursor derived SiHfBCN ceramics. Ultra-high temperature oxidation behavior of the C/SiC-SiHfBCN composites under laser heating were systematically studied. The influence of temperature, preform structure and precursor on oxidation resistance of the composites were analyzed. The results revealed that the preform structure and precursor can influence the oxidation behavior of C/SiC-SiHfBCN composites by effecting its thermal conductivity in the thickness direction. The Hf-containing oxide layer formed during ultra-high temperature oxidation test can effectively protect the internal carbon fibers from being oxidized. Linear ablation rate of C/SiC-SiHfBCN composites was more than an order of magnitude lower than that of other reported C/SiC composites, indicating an enhanced ablation resistance of the C/SiC-SiHfBCN composites.

Dispersant-assisted preparation of ultra-fine HfB2 powder

Hafnium boride (HfB2), as an ultra-high temperature ceramic, has been extensively employed in the aerospace field due to its high melting point and excellent ultrahigh-temperature oxidation resistance. Despite its outstanding performance, the development of the HfB2 for practical applications remained limited by its low-quality production including high oxygen content, large particle size and high agglomeration. In this work, a series of dispersants are adopted for reducing the particle size and suppressing the agglomeration of HfB2 powder during the liquid phase co-precipitation method combined with carbothermal reduction. Based on various characterizations, it is discovered that the presence of dispersants can greatly reduce the particle size and restrain the agglomeration of the HfB2. Specifically, the HfB2 prepared using polyethylene glycol (PEG) demonstrates an ultra-fine particle size down to hundreds of nanometers. To better understand the function of the dispersant, the formation of the HfB2 is unveiled via a combination of multiple characterizations. In detail, the dispersants can cap on the formed HfB2 and restrict their further growth and agglomeration during the reaction through the electrostatic repulsion and steric hindrance of high molecular polymer, thereby greatly improving the quality of HfB2 powder.

A novel niobium based oxidation protective coating with three lines of defense at ultra-high temperature

To improve the ultra-high temperature oxidation and thermal shock resistance of Nb alloys, we prepared a novel niobium-based oxide protective coating that forms three lines of defense through multi-gradient design. In particular, Nb3B2-NbB2 prevents Si diffusion. WSi2 stores rich Si to generate SiO2. The composite oxide film can heal oxidation defects. The finite element and first-principles calculations results show that multi-gradient coating can radiate heat, relieve interlayer stress and CTE mismatch. The coating forcefully protects the substrate from O2 erosion, so that its oxidation life exceeds 13 h at 1850 ℃, and the thermal shock reaches 600 cycles.

Ultra-high temperature oxidation resistance of a novel (Mo, Hf, W, Ti)Si2 ceramic coating with Nb interlayer on Ta substrate

In order to solve the challenge of recyclability of tantalum substrates in high temperature oxidation environments, a novel MoSi2-WSi2-HfSi2-TiSi2 composite ceramic coating containing an Nb interlayer was prepared on the surface of tantalum substrate by a three-step method. The mix ceramic silicide coating exhibited superior performance and effective protection for 10.2 h at 1800 °C, possibly due to the formation of an outer SiO2-HfO2-HfSiO4 composite oxide film with low oxygen permeability, moderate viscosity and thermal expansion coefficient, as well as good self-healing ability. Furthermore, the coating successfully passed 537 thermal cycles from room temperature to 1800 °C. The presence of Nb interlayer significantly mitigated the thermal mismatch between the ceramic coating and the tantalum substrate, and the bidirectional diffusion of Nb element during the high temperature oxidation and thermal shock process further reduced the tendency of the coating to crack.

Ultra-high temperature oxidation behavior of ZrB2/YSZ modified Si-Mo-W coating with a diffusion barrier on niobium alloy

To develop an ultra-high temperature resistant coating for niobium alloy, a new type of 10%ZrB2 + 5%YSZ-modified Si-Mo-18%W coating with a boride diffusion barrier was prepared by a novel three-step method. The coating with NbB2-Nb3B2 diffusion barrier can effectively protect Nb521 alloy at 1850 °C for more than 8 h, while the life of coating without diffusion barrier was only 3.5 h. The excellent performance of the coating was attributed to the present of diffusion barrier and formation of self-healing SiO2-B2O3-ZrSiO4-ZrO2 oxide scale. The diffusion barrier can effectively hinder mutual diffusion of coating and substrate, and reduce inward consumption of silicon element.

Microstructure and ultra-high temperature isothermal oxidation behaviour of YSZ-particle-modified WSi2 coating

To improve the ultra-high temperature oxidation resistant performance of WSi2 coating in air, a novel dispersed Yttria (Y2O3) stabilised zirconia (ZrO2) (YSZ)-particle-modified WSi2 coating was prepared using a two-step method, including, slurry spray sintering and pack siliconising. The microstructure, high-temperature oxidation-resistant performance, and oxidation failure mechanism of the coating were investigated. The coating is metallurgically bonded to the substrate and demonstrates outstanding performance during the oxidation test at an ultra-high temperature of up to 1800 °C in atmosphere. The coating stably survived for 40 h without being destroyed. During the oxidation process, a protective thin film formed on the surface of the coating, which mainly consisted of SiO2, W5Si3, YSZ, ZrSiO4, and Y2Si2O7. This film greatly improved the oxidation resistance of the coating. Under the protection of this film, the mass gain per unit area is maintained below 30 mg/cm2. Most of the YSZ particles existing in the coating body serve in strengthening it while the remaining particles in the SiO2 film act as flow barriers. The excellent performance of the coating demonstrates high potential for use in ultra-high temperature applications.

Ultrahigh-temperature oxidation of 4H-SiC (0 0 0 1) and gate oxide reliability dependence on oxidation temperature

The paper confirmed the SiC/SiO2 interface state density obtained from the ultrahigh-temperature dry oxidation process on 4H-SiC Si-face (0 0 0 1) at up to 1550 °C without any other passivating techniques. Our results were consistent with those of previous reports. Furthermore, we also considered the reliability of SiO2, which is important for its practical application, by TDDB measurements. The optimal interface state density was obtained for the gate oxide formed at 1450 °C at EC − E = 0.2–0.6 eV, whereas the gate oxide was relatively the most reliable for the oxidation at 1250 °C. It suggests that the effects of oxidation temperature of 4H-SiC (0 0 0 1) had a trade-off between gate oxide reliability and SiC/SiO2 interface properties.

Ultra-high temperature oxidation resistance of ZrB2-20SiC coating with TaSi2 addition on siliconized graphite

Abstract ZrB2-20SiC and ZrB2-20SiC-10TaSi2 (volume fraction) coatings were prepared on siliconized graphite by slurry method. The as-prepared coatings were oxidized at 1700 °C in air for 30 min by induction heating method. The oxidation resistance of the coatings was evaluated based on the consumption of the coating. The addition of TaSi2 significantly improved the ultra-high temperature oxidation resistance of ZrB2-20SiC coating, due to the formation of a Ta dissolved oxide layer with much lower oxygen permeability.

Designing oxidation resistant ultra-high temperature ceramics through the development of an adherent native thermal barrier

We present a design concept for developing ZrB2-SiC-AlN composites with enhanced oxidative stability at ultra-high temperatures (∼2000 °C) and low pressures (100 Torr). The oxidative stability of these materials arises from a protective silica based scale. However, active oxidation of SiC above 1700 °C presents a challenge, which we circumvent through the in-situ growth of a zirconia layer that serves as a thermal barrier, ensuring that the effective temperature at the zirconia/Si rich subscale is less than the active oxidation temperature. The design concept is validated by a series of ultra-high temperature oxidation experiments under static as well as cyclic conditions.

Zirconium-Doped Hybrid Composite Systems for Ultrahigh-Temperature Oxidation Applications: A Review

The re-entry phase of a space vehicle demands thermal shielding against the heat elicited by an aerodynamic heating process. The quantity of heat spawned in such process is close to 11.35 × 105 kJ m–2 and temperature reaching 8000 °C at the stagnation point on the surface of the re-entry vehicle. Ablative materials are effectual for heat shielding re-entry vehicles, and their performance is improved by various physicochemical techniques. Utilization of ultrahigh-temperature ceramics to enhance the performance of ablative materials has set a new milestone in heat shielding applications. The article briefly introduces the conditions that prevail during a re-entry phase, discusses thermophysical properties that are to be adorned by ablative materials other mere thermal insulation. This review article uniquely discusses the effect of the selection of ceramic filler system on the formation of an oxidation layer, the emissivity of the oxidation layer and thermophysical behavior of the oxidation layer.

Key issues of MoSi2-UHTC ceramics for ultra high temperature heating element applications: Mechanical, electrical, oxidation and thermal shock behaviors

MoSi2 based ceramics have great potential for high temperature heating element applications. In this paper, MoSi2-UHTC ceramics were produced by pressureless sintering by introducing ZrB2-20 vol% SiC ultra high temperature ceramic as additives. The key issues of the MoSi2-UHTC ceramics for ultra high temperature heating element applications, including mechanical, electrical, oxidation and thermal shock behaviors, were investigated. The aim of the study was to evaluate the possibility and ability of MoSi2-UHTC ceramics for high temperature heating element applications.

Ultra-high temperature oxidation behavior of micro-laminated ZrC/MoSi2 coating on C/C composite

A protective coating with transition inner layer of SiC and micro-laminated outer layer of ZrC/MoSi2 was prepared by packaging siliconizing and magnetron sputtering on C/C composite. The total thickness of the micro-laminated outer layer was 60 μm, and the thicknesses of ZrC and MoSi2 sublayers were 4 μm and 1 μm, respectively. Ultra-high temperature oxidation test was conducted at 1800 °C for 30 min in static air. The oxidation kinetics of coated C/C composite followed parabolic law. Such attractive ultra-high temperature oxidation resistance was attributed to the formation of a dense and spallation-free oxide scale and low oxygen diffusion coefficient of ZrCxOy.

Simulation of hydrogen generated from fuel bundles and verification of a semi-empirical cladding oxidation model by PWR type CORA tests

It is important to predict the hydrogen generated from cladding oxidation under severe accident conditions. The processes of high and ultrahigh temperature oxidation with cladding damaged are complicated and the accurate simulation models need to be developed. In this work, a non-parabolic and semi-empirical cladding oxidation model was discussed. The different damage regimes and surface increase effect had been considered by this model. Four past out-of-pile integrated experiments CORA13 (ISP31), CORA7, CORA10, and CORA29 were employed as verification cases. Then, the hydrogen generation rates predicted by different oxidation models were compared and analyzed. This work was based on the best-estimated temperatures of CORA tests. It is found that a modified Cathcart-Pawel model works well at temperatures below 1,773 K, and the semi-empirical model works better than the above modified model by making a further correction for the early cladding failure since 1,473 K. The suitable values of coefficients in the semi-empirical model may roughly represent the oxidation and damage status of the whole bundle. The rough axial nodalization can result in some unrealistic peaks for hydrogen generation rate. A time-delay exists in the process of hydrogen generated and released, but this phenomenon has not been taken into account by the existing oxidation models.

Ultrahigh-Temperature Oxidation of 4H-SiC(0001) and an Impact of Cooling Process on SiO<sub>2</sub>/SiC Interface Properties

This paper reviews our recent work on ultrahigh-temperature oxidation of 4H-SiC(0001) surfaces. Our rapid thermal oxidation experiments demonstrated the reaction-limited linear growth at temperatures ranging from 1200 to 1600°C. The Arrhenius plot of linear growth rate of thermal oxidation can be fitted by a linear line, and the activation energy of oxide growth in dry O2 oxidation was estimated to be 2.9 eV. We also found that unintentional oxidation during the cooling down process severely degrades SiO2/SiC interface properties, resulting in positive flatband voltage shift (VFB) and hysteresis in capacitance-voltage (C-V) characteristics regardless of oxidation temperature. By effectively suppressing oxide growth during the cooling process, we have clarified that SiO2/SiC interface properties depend on oxidation temperature and the lowest interface state density was obtained for the oxide formed at 1450°C.