Microstructure and Thermomechanical Properties of Atmospheric Plasma-Sprayed Yb<Subscript>2</Subscript>O<Subscript>3</Subscript> Coating

In this study, a Yb₂O₃ coating was fabricated by the atmospheric plasma spray technique. The phase composition, microstructure, and thermal stability of the coating were examined. The thermal conductivity and thermal expansion behavior were also investigated. Some of the mechanical properties (elastic modulus, hardness, fracture toughness, and flexural strength) were characterized. The results reveal that the Yb₂O₃ coating is predominantly composed of the cubic Yb₂O₃ phase, and it has a dense lamellar microstructure containing defects. No mass change and exothermic phenomena are observed in the thermogravimetry and differential thermal analysis curves. The high-temperature x-ray diffraction results indicate that no phase transformation occurs from room temperature to 1500 °C, revealing the good phase stability of the Yb₂O₃ coating. The coefficient of thermal expansion of the Yb₂O₃ coating is (7.50-8.67)?×?10^?6 K^?1 in the range of 200-1400 °C. The thermal conductivity is about 1.5 W m^?1 K^?1 at 1200 °C. The Yb₂O₃ coating has excellent mechanical properties and good damage tolerant. The unique combination of these properties implies that the Yb₂O₃ coating might be a promising candidate for T/EBCs applications.     

Microstructure and Thermophysical Properties of SrZrO<Subscript>3</Subscript> Thermal Barrier Coating Prepared by Solution Precursor Plasma Spray

Strontium zirconate (SrZrO₃) thermal barrier coatings were deposited by solution precursor plasma spray (SPPS) using an aqueous precursor solution. The phase transition of the SrZrO₃ coating and the influence of the aging time at 1400 °C on the microstructure, phase stability, thermal expansion coefficient, and thermal conductivity of the coating were investigated. The unique features of SPPS coatings, such as interpass boundary (IPB) structures, nano- and micrometer porosity, and through-thickness vertical cracks, were clearly observed evidently in the coatings. The vertical cracks of the coatings remained substantially unchanged while the IPB structures gradually diminished with prolonged heat treatment time. t-ZrO₂ developed in the coatings transformed completely to m-ZrO₂ phase after heat treatment for 100 h. Meanwhile, the SrZrO₃ phase in the coatings exhibited good phase stability upon heat treatment. Three phase transitions in the SrZrO₃ coatings were revealed by thermal expansion measurements. The thermal conductivity of the as-sprayed SrZrO₃ coating was ~1.25 W m^?1 K^?1 at 1000 °C and remained stable after heat treatment at 1400 °C for 360 h, revealing good sintering resistance.     

高分数GNFs/6061Al基复合材料的放电等离子体烧结制备及其显微组织与性能

采用放电等离子体烧结(SPS)工艺在610℃制备30％～50％(质量分数)纳米石墨片(GNF)/6061Al基复合材料,研究烧结压力及GNF含量对复合材料显微组织和力学、热学性能的影响.结果表明,SPS有效抑制GNFs/6061Al基复合材料中Al4C3等界面反应产物的生成.随着GNF含量的增加,GNFs团聚程度增加,...     

Preparation and properties of a MnCo2O4 for ceramic interconnect of solid oxide fuel cell via glycine nitrate process

MnCo₂O₄ powder was prepared by a wet chemistry method using metal nitrates and glycine in an aqueous solution. The phase stability, sintering behavior, thermal expansion and electrical conductivity were examined to characterize powder suitability as an interconnect material in solid oxide fuel cells (SOFCs). X-ray diffraction indicated that the MnCo₂O₄ spinel synthesized by the glycine nitrate process was stable until 1100 °C and it was possible to obtain a fully densified single phase spinel. On the other hand, the MnCo₂O₄ synthesized by a solid state reaction decomposed into a cubic spinel and CoO after being sintered at 1100 °C. This might be associated with the reduction of Co³⁺ in the octahedral site of the cubic spinel phase. MnCo₂O₄ showed a thermal expansion coefficient comparable to that of other SOFCs components, as well as good electrical conductivity. Therefore, MnCo₂O₄ is a potential candidate for the ceramic interconnects in SOFCs, provided the phase instability under reducing environments can be improved.     

Suspension HVOF Spraying of Reduced Temperature Solid Oxide Fuel Cell Electrolytes

Metal-supported solid oxide fuel cells (SOFCs) composed of a Ce_0.8Sm_0.2O_2?δ (SDC) electrolyte layer and Ni-Ce_0.8Sm_0.2O_2?δ (Ni-SDC) cermet anode were fabricated by suspension thermal spraying on Hastelloy × substrates. The cathode, a Sm_0.5Sr_0.5CoO₃ (SSCo)-SDC composite, was screen-printed and fired in situ. The anode was produced by suspension plasma spraying (SPS) using an axial injection plasma torch. The SDC electrolyte was produced by high-velocity oxy-fuel (HVOF) spraying of liquid suspension feedstock, using propylene fuel (DJ-2700). The emerging technology of HVOF suspension spraying was explored here to produce thin and low-porosity electrolytes in an effort to develop a cost-effective and scalable fabrication technique for high-performance, metal-supported SOFCs. In-flight particle temperature and velocity were measured for a number of different gun operating conditions and standoff distances and related to the resulting microstructures. At optimized conditions, this approach was found to limit material decomposition, enhance deposition efficiency, and reduce defect density in the resulting coating, as compared to previous results reported with SPS. Produced button cells showed highly promising performance with a maximum power density (MPD) of 0.5 W cm^?2 at 600 °C and above 0.9 W cm^?2 at 700 °C, with humidified hydrogen as fuel and air as oxidant. The potential of this deposition technique to scale-up the substrate size to 50 × 50 mm was demonstrated.     

Thermal, micro-structural, and electrical properties of a La1−x Sr x Mn0.85Fe0.05Co0.05Ni0.05O3+δ (x = 0–0.4 mole) cathode system

The oxygen stoichiometry, thermal expansion, morphology, and electrical conductivity of a co-doped perovskitetype cathode system, La_1?x Sr _x Mn_0.85Fe_0.05Co_0.05Ni_0.05O_3+ä (x = 0–0.4 mole), are studied for intermediate-temperature solid oxide fuel cell applications. Sr²⁺-doping led to a decrease in the unit cell volume, oxygen stoichiometry, particle size, and activation energy, and an increase in the coefficient of thermal expansion and electrical conductivity. The sample with x = 0.3 mole exhibited four to five fold weight loss with respect to La_0.75Sr_0.25MnO_3+δ at an intermediate temperature range and suggested the availability of a large number of oxygen vacancies due to a co-doping effect. This sample also showed sufficiently high electrical conductivity (～76 S cm^?1) at 650 °C, a low activation energy (～0.15 eV), and a coefficient of thermal expansion (～12.1 × 10^?6 °C^?1) comparable to those of the adjacent components and submicron sized particles. The experimental results are explained using defect models.     

Strontium Zirconate TBC Sprayed by a High Feed-Rate Water-Stabilized Plasma Torch

A novel thermal barrier coating (TBC) material, strontium zirconate SrZrO₃, was sprayed by a high feed-rate water-stabilized plasma torch WSP 500. Stainless steel coupons were used as substrates. Coatings with a thickness of about 1.2 mm were produced, whereas the substrates were preheated over 450 °C. The torch worked at 150 kW power and was able to spray SrZrO₃ with a high spray rate over 10 kg per hour. Microstructure and microhardness, phase composition, adhesion, thermal conductivity and thermal expansion were evaluated. The coating has low thermal conductivity under 1 W/m K in the interval from room temperature up to 1200 °C. Its crystallite size is slightly over 400 nm and thermal expansion 12.3 µm K^?1 in the similar temperature range.     

Effect of Suspension Plasma-Sprayed YSZ Columnar Microstructure and Bond Coat Surface Preparation on Thermal Barrier Coating Properties

Suspension plasma spraying (SPS) is identified as promising for the enhancement of thermal barrier coating (TBC) systems used in gas turbines. Particularly, the emerging columnar microstructure enabled by the SPS process is likely to bring about an interesting TBC lifetime. At the same time, the SPS process opens the way to a decrease in thermal conductivity, one of the main issues for the next generation of gas turbines, compared to the state-of-the-art deposition technique, so-called electron beam physical vapor deposition (EB-PVD). In this paper, yttria-stabilized zirconia (YSZ) coatings presenting columnar structures, performed using both SPS and EB-PVD processes, were studied. Depending on the columnar microstructure readily adaptable in the SPS process, low thermal conductivities can be obtained. At 1100 °C, a decrease from 1.3 W m^?1 K^?1 for EB-PVD YSZ coatings to about 0.7 W m^?1 K^?1 for SPS coatings was shown. The higher content of porosity in the case of SPS coatings increases the thermal resistance through the thickness and decreases thermal conductivity. The lifetime of SPS YSZ coatings was studied by isothermal cyclic tests, showing equivalent or even higher performances compared to EB-PVD ones. Tests were performed using classical bond coats used for EB-PVD TBC coatings. Thermal cyclic fatigue performance of the best SPS coating reached 1000 cycles to failure on AM1 substrates with a β-(Ni,Pt)Al bond coat. Tests were also performed on AM1 substrates with a Pt-diffused γ-Ni/γ′-Ni₃Al bond coat for which more than 2000 cycles to failure were observed for columnar SPS YSZ coatings. The high thermal compliance offered by both the columnar structure and the porosity allowed the reaching of a high lifetime, promising for a TBC application.     

Experimental and Modeling Studies on the Microstructures and Properties of Oxidized Aluminum Nitride Ceramic Substrates

Oxidation of aluminum nitride (AlN) ceramic substrates doped with 2 wt.% Y₂O₃ was performed in air at temperatures ranging from 1000 to 1300 °C for various lengths of time. Microstructure, bending strength, and thermal conductivity of the oxidized AlN substrates were studied experimentally and also via mathematical models. The results show that the oxide layer formed on the AlN substrates is composed of α-Al₂O₃ nanocrystallines and interconnected micropores. Longitudinal and transverse cracks are induced in the oxide layer under tensile and shear stresses, respectively. Intergranular oxidation of the AlN grains close to the oxide layer/AlN interface also occurs, leading to widening and cracking of the AlN grain boundaries. These processes result in the monotonous degradation of bending strength and thermal conductivity of the oxidized AlN substrates. Two mathematic models concerning these properties of the oxidized AlN substrates versus the oxide layer thickness were put forward. They fit well with the experimental results.     

The Microstructure and Thermoelectric Properties of Yb-Filled Skutterudite Yb<Subscript>0.1</Subscript>Co<Subscript>4</Subscript>Sb<Subscript>12</Subscript> Under Cyclic Thermal Loading

This paper is devoted to investigating the microstructure and thermoelectric properties of Yb-filled skutterudite Yb_0.1Co₄Sb₁₂ under a cyclic thermal loading from room temperature to 773 K. The results indicate after 1000 cycles, the surface morphology changes dramatically, and clear grain boundaries appear on the surface of the sample. The grain sizes of the sample change little after 1000 cycles, and the main phase is still skutterudite; however, a trace amount of YbSb also exists. In addition, the electrical conductivity and thermal conductivity decrease distinctly after 1000 cycles, but the absolute value of the Seebeck coefficient increases a little. Consequently, the ZT value decreases slightly from 0.75 at 800 K before cycling to 0.69 after 1000 cycles. It indicates that the effect of the cyclic thermal loading on the ZT of the Yb_0.1Co₄Sb₁₂ material is not distinct.     

Effect of La-Substitution on the Electrical Conductivity of Sr1−x La x MoO4+δ (x = 0-0.3) Compounds

Various compositions of Sr_1?x La _x MoO_4+δ (x = 0.05, 0.1, 0.2, and 0.3) compounds were prepared by solid state reaction route. The samples were characterized by powder-XRD, TG-DTA, and SEM-EDAX techniques. Formation of single crystalline phases of Sr_1?x La _x MoO_4+δ was confirmed from powder-XRD patterns. The thermal stability of La-doped SrMoO₄ compounds was investigated by TG-DTA. Uniform grain distribution was observed in the SEM image of 10-20 mol.% La-substituted compositions. Needle-shaped structures were observed in the SEM image of Sr_0.3La_0.1MoO_4+δ and were confirmed to be La₂Mo₂O₉ by XRD examination. The electrical conductivity of these compounds was measured by AC-impedance technique in the temperature range of 373-1073 K in air ambience and compared with that of pristine SrMoO₄. The electrical conductivity was found to decrease for La-substituted SrMoO₄ compared to pristine SrMoO₄. The diffusion coefficient calculated from the electrical conductivity was found to be in the range of 1.94 ± 0.02 × 10^?13 to 1.15 ± 0.01 × 10^?11 cm²/S at 873-1173 K for substituted composition and 3.47 ± 0.02 × 10^?13 to 2.48 ± 0.01 × 10^?10 cm²/S for pristine SrMoO₄ at 673-1073 K temperature range.     

High-Temperature Electrical Insulation Behavior of Alumina Films Prepared at Room Temperature by Aerosol Deposition and Influence of Annealing Process and Powder Impurities

Alumina (Al₂O₃) is a widely used material for highly insulating films due to its very low electrical conductivity, even at high temperatures. Typically, alumina films have to be sintered far above 1200 °C, which precludes the coating of lower melting substrates. The aerosol deposition method (ADM), however, is a promising method to manufacture ceramic films at room temperature directly from the ceramic raw powder. In this work, alumina films were deposited by ADM on a three-electrode setup with guard ring and the electrical conductivity was measured between 400 and 900 °C by direct current measurements according to ASTM D257 or IEC 60093. The effects of film annealing and of zirconia impurities in the powder on the electrical conductivity were investigated. The conductivity values of the ADM films correlate well with literature data and can even be improved by annealing at 900 °C from 4.5?×?10^?12 S/cm before annealing up to 5.6?×?10^?13 S/cm after annealing (measured at 400 °C). The influence of zirconia impurities is very low as the conductivity is only slightly elevated. The ADM-processed films show a very good insulation behavior represented by an even lower electrical conductivity than conventional alumina substrates as they are commercially available for thick-film technology.     

Electrical Transport Properties of Single-Crystalline β-Zn<Subscript>4</Subscript>Sb<Subscript>3</Subscript> Prepared Through the Zn-Sn Mixed-Flux Method

β-Zn₄Sb₃ is a promising p-type thermoelectric material for utilization in moderate temperatures. This study prepares a group of single-crystalline β-Zn₄Sb₃ samples using the Zn-Sn mixed-flux method based on the stoichiometric ratios of Zn_4+x Sb₃Sn _y . The effect of Zn-to-Sn proportion in the flux on the structure and electrical transport properties is investigated. All samples are strip-shaped single crystals of different sizes. The actual Zn content of the present samples is improved (>3.9) compared with that of the samples prepared through the Sn flux method. Larger lattice parameters are also obtained. The carrier concentration of all the samples is in the order of over 10¹⁹ cm^?3. With increasing Sn rate in the flux, this carrier concentration decreases, whereas mobility is significantly enhanced. The electrical conductivity and Seebeck coefficients of all the samples exhibit a behavior that of a degenerate semiconductor transport. Electrical conductivity initially increases and then decreases as the Sn ratio in the flux increases. The electrical conductivity of the x:y = 5:1 sample reaches 6.45 × 10⁴ S m^?1 at 300 K. Benefitting from the electrical conductivity and Seebeck coefficient, the flux proportion of the x:y = 7:1 sample finally achieves the highest power factor value of 1.4 × 10^?3 W m^?1 K^?2 at 598 K.     

Improved Thermal Performance of Diamond-Copper Composites with Boron Carbide Coating

B₄C-coated diamond (diamond@B₄C) particles are used to improve the interfacial bonding and thermal properties of diamond/Cu composites. Scanning electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy were applied to characterize the formed B₄C coating on diamond particles. It is found that the B₄C coating strongly improves the interfacial bonding between the Cu matrix and diamond particles. The resulting diamond@B₄C/Cu composites show high thermal conductivity of 665 W/mK and low coefficient of thermal expansion of 7.5 × 10^?6/K at 60% diamond volume fraction, which are significantly superior to those of the composites with uncoated diamond particles. The experimental thermal conductivity is also theoretically analyzed to account for the thermal resistance at the diamond@B₄C-Cu interface boundary.     

Electrical properties,sintering and thermal expansion behavior of composite ceramic interconnecting materials,La0.7Ca0.3CrO3−δ/Y0.2Ce0.8O1.9 for SOFCs

This paper presents a high-performance interconnect ceramic for solid oxide fuel cells (SOFCs), based on a modification of La_0.7Ca_0.3CrO_3−δ (LCC). It was found that addition of a small amount of YDC (Y_0.2Ce_0.8O_1.9) into LCC dramatically increased the electrical conductivity. For the best system, LCC + 3 wt.% YDC, the electrical conductivity reached 104.8 S cm⁻¹ at 800 °C in air. The electrical conductivity of the specimen with 2 wt.% YDC in H₂ at 800 °C was 5.9 S cm⁻¹. With the increase of YDC content, the relative density increased, reaching 97.6% when the YDC content was 10 wt.%. The average coefficient of thermal expansion (CTE) at 30–1000 °C in air increased with YDC content, ranging from 11.12 × 10⁻⁶ K⁻¹ to 15.34 × 10⁻⁶ K⁻¹. The oxygen permeation measurement illustrated a negligible oxygen ionic conduction, indicating that it is still an electronically conducting ceramic. Therefore, it is a very promising interconnecting ceramic for SOFCs.     

Preparation and Thermophysical Properties of La2Zr2O7 Coatings by Thermal Spraying of an Amorphous Precursor

Free-standing La₂Zr₂O₇ coatings were obtained by plasma spraying, using an amorphous La-O-Zr precursor as the feedstock. The La-O-Zr precursor powder was prepared by coprecipitation. During thermal spraying, the formation of coatings can be regarded as a joint process of melting-solidification, thermal decomposition, and crystallization. The time required for crystal growth was significantly shortened during spraying. Consequently, the average grain size of coatings was approximately 200 nm, with a narrow distribution (100-500 nm). Coatings prepared by this method show better thermophysical properties than those prepared with crystalline La₂Zr₂O₇ powder as the feedstock. The thermal conductivity of the as-sprayed coating was approximately 0.36-0.47 W/m K and the average coefficient of thermal expansion (CTE) is 11.1 × 10^?6/K.     

Fabrication and Characterization of Nickel Ferrite Based Inert Anodes for Aluminum Electrolysis

Nickel ferrite-based cermets and their relevant composites have been widely used as inert anodes for aluminum electrolysis due to the good combination of chemical resistance, thermal, and mechanical stability. In this study, various NiO/NiFe₂O₄ composites consisting 5, 10, and 15% NiO in conjunction with Cu/NiFe₂O₄ cermets containing 5, 10, and 15% Cu have been prepared by powder metallurgy method. The degradation resistance of developed inert composites has been evaluated under hot corrosion conditions by plunging the samples in the molten electrolyte at 1,000 °C for various holding times. The strength, toughness, hardness, relative density, microstructural observation, phase analysis, and electrical resistivity have been investigated in details by the 3-points bending test, Vickers hardness test, Archimedes method, scanning electron microscope, x-ray diffraction, and conventional direct current four-probe technique, respectively. The experimental results for NiO/NiFe₂O₄ composites show that a significant improvement of toughness and degradation resistance occurred in conjunction with a moderate decrease in strength by adding NiO content from 5 to 15%, while the relative density has been increased only up to 5%NiO content and then decreased. Moreover, increasing of Cu content from 5 to 15% in the cermet samples, all of the mentioned engineering properties such as strength, toughness and electrical conductivity have been improved considerably, but the degradation resistance has been decreased.     

Thermally Sprayed Large Tubular Solid Oxide Fuel Cells and Its Stack: Geometry Optimization,Preparation, and Performance

In this study, we develop a large tubular solid oxide fuel cells design with several cells in series on a porous cermet support, which has many characteristics such as self-sealing, low Ohmic loss, high strength, and good thermal expansion coefficient matching. Here, we investigate aspects of the cell design, manufacture, performance, and application. Firstly, the cell length and number of cells in series are optimized by theoretical analysis. Then, thermal spraying is applied as a cost-effective method to prepare all the cell components. Finally, the performance of different types of cells and two types of stacks is characterized. The maximum output power of one tube, which had 20 cells in series, reaches 31 and 40.5 W at 800 and 900 °C, respectively. Moreover, the output power of a stack assembled with 56 tubes, each with ten cells in series, reaches 800 W at 830 °C. The excellent single tube and cell stack performance suggest that thermally sprayed tubular SOFCs have significant potential for commercialized application.     

Effects of Silver Addition on Properties and Performance of Plasma Sprayed La0.6Sr0.4Co0.2Fe0.8O3−δ Interconnect Layer

Interconnect layers on stainless steel substrates (STS430) for solid oxide fuel cells (SOFC) were built up by atmospheric plasma spraying (APS) using spray dried La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) and blended LSCF/Ag composites. The microstructure and phase of each coating were analyzed using scanning electron microscopy (SEM) and x-ray diffraction (XRD) studies, respectively. Furthermore, bond strength, microhardness, performance in a thermal cycle test and in an oxidation test, and electrical conductivity were measured and compared. The coatings prepared from spray dried LSCF have higher porosity and more cracks within the splats and at intersplat boundaries. In contrast, the coatings prepared from LSCF/Ag had fewer cracks and less porosity due to the relatively high ductility of silver. After oxidation testing at 800 °C for 200 h, the weight change of the STS430 substrate and the LSCF and LSCF/Ag-coated alloys were found to be 0.06833, 0.01950, and 0.01656 mg/cm², respectively. Also the electrical conductivity of LSCF and LSCF/Ag coatings were higher than that of STS430 by two orders.     

Wear and thermal stability of nano-TiB2 modified TiCN-WC-Co-Cr3C2 cermet

This work discusses the friction, wear and thermal response of TiCN-WC-Co-Cr₃C₂ cermet modified by the addition of TiB₂ nanoparticles. The specimens were prepared by spark plasma sintering by adding different weight percentages (5, 10 and 15) of TiB₂ nanoparticles in TiCN-WC-Co-Cr₃C₂ cermets. The sintered pieces were subjected to sliding wear under an unlubricated condition and a constant load of 20 N at sliding velocities of 0.23 m/s, 0.35 m/s and 0.47 m/s against a steel disc of 48 mm diameter. Friction was the maximum at a sliding velocity of 0.23 m/s. The wear loss decreased with increasing speed of sliding and was between 10⁻⁵ to 10⁻⁶ mm³. The wear rates and coefficient of friction were highly dependent on microstructure and mechanical properties of the cermets. Abrasion and adhesion were the active modes of wear. Heating of the samples was carried out at 600 °C, 800 °C and 1000 °C for 4 h. A substantial mass gain and decrease in hardness were observed in the specimen annealed at 1000 °C, for which Scanning Electron Microscopy revealed considerable grain growth and XRD showed oxide phases. A cermet containing 15% TiB₂ nanoparticles displayed the lowest volumetric wear at room temperature, but had rather low thermal stability. Response surface methodology was used to develop models and regression equations for wear and thermal stability.