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Studies on plasma spraying of zircon (ZrSiO4) have been carried out by the authors as one of the candidates for an environmental barrier coating (EBC) application, and had reported that substrate temperature is one of the most important factors to obtain crack-free and highly-adhesive coating. In this study, several amount of yttria were added to zircon powder, and the effect of the yttria addition on the structure and properties of the coatings were evaluated in order to improve the stability of the zircon coating structure at elevated temperature. The coatings obtained were composed of yttria stabilized zirconia (YSZ), glassy silica, while the one prepared from monolithic zircon powder composed of the metastable high temperature tetragonal phase of zirconia and glassy silica. After the heat treatment over 1473K, silica and zirconia formed zircon in all the coatings. However, the coatings with the higher amount of yttria had less amount of zircon formed. This resulted in the less open porosity of the coating at elevated temperature. These yttria added coatings also showed good adhesion even after the heat treatment, while monolithic zircon coating had peeled off.

Titanium dioxide TiO 2 can be used as a photo-anode to give generated electrons to the metal substrate under illumination. The transition metal oxide such as iron oxide Fe 2 O 3 can be used to store electrons generated by the photo-electric conversion function of TiO 2 under the illuminated situation while the electrons are discharged from the transition metal oxide to the metal substrate in the dark. In this paper, coatings of nano-sized composite of TiO 2 and Fe 2 O 3 were fabricated by the Warm Spray process, in which the feedstock powder is accelerated by a supersonic gas jet with speed above 1.0 km s - 1 and temperature between 800 and 2500 K, and then impacted onto the target substrate continuously to form coatings. The coatings of TiO 2 and Fe 2 O 3 nano-composite fabricated by Warm Spray showed no thermal deterioration such as phase transformation and particle growth of the feedstock during the spray process. The coatings fabricated by the Warm Spray had larger photo-current and the electron charge/discharge capacity than that by a conventional HVOF process. In addition, these characteristics were improved by decreasing the primary particle size of TiO 2 and Fe 2 O 3 .

By putting photo-electric conversion materials and electron storage materials together on one device, full-time cathodic protection system was expected to function and its coating technique is required to realize the system. So far, Warm Spray, impact deposition of supersonic particle with temperature controllable in a wide range, enabled nano-sized TiO2 and Fe2O3 to be deposited controlling thermal degradation of the particles and it was revealed that each of photo-electric conversion and electron storage were functioned. The rest potential of the coatings as an indicator of overall system performance was not negative sufficiently, which might be related to the less conductive nature. Therefore, metallic Zn was introduced in this paper for shifting the rest potential to more negative direction and increase of the conductivity. The coatings were successfully fabrication with control of thermal degradation of Zn, which worked as a binder to connect the oxide particles. Photo-cathodic protection behaviour indicated that the steady rest potential of the Warm Sprayed coatings of the TiO2-Fe2O3 system was not influenced on the introduction of metallic Zn, implying that it might not be limited by the conductive property of the coatings.

Thermoelectric modules composed of eight pairs of p-type Ca 2.7 Bi o3 Co 4 O 9 (Co-349) and n-type CaMn 0.98 Mo 0.02 O 3 (Mn-113) bulks were constructed using Ag electrodes and paste including powder of the n-type oxide. The former bulks were prepared by hot pressing. On the other hand, the latter were densified using a cold isostatic pressing technique and sintered in atmospheric pressure. Dimensions of both oxide legs were 5 mm wide and thick and 4.5 mm high. An alumina plate was used as a substrate, and there was no alumina plate on the other side of the modules. When the substrate side was heated, the module could generate up to 1.0 V and 0.17W of open circuit voltage (V O ) and maximum power (P max , respectively, at a hot-side temperature of 1273 K (furnace temperature as a heat source) and a cold-side temperature of 298 K (circulated water temperature) in air. But internal resistance R I reached a value of 1.5 Ω, which is about six times higher than the calculated one from resistivity of both p- and n-type bulks. When the substrate side was cooled, V O and P max reached 0.7V and 0.34 W of V O and P max , respectively, at a furnace temperature of 1273 K.

Alumina/zirconia nano-composite coating was fabricated by plasma spraying using agglomerated feedstock from fine powders of about 100 nm. The coating was consisted of fine γ-alumina and zirconia crystals with size of several nano meter and some amorphous boundary layers. The amorphous phase was crystallized and disappeared after heat treatment at 930°C. However, the crystallite size was kept under 50 nm even after 1500°C-100hr heating, so the alumina-zirconia nano-composite showed good thermal stability against the grain growth.

Polymer impregnation pyrolysis method (PIP method) has some advantages to fabricate SiC fiber /SiC composite with large or complex form. However polymer derived SiC contains some amount of excess carbon and porosity. In particular the excess carbon is easy to be oxidized in air at high temperature. Therefore reliability of the composite is not sufficient for long term use, because the oxidized matrix makes strong bonding between the fiber and the matrix. In this study, TiO2 as a reactive filler was added into polymer to consume the excess carbon by chemical reaction. Polycarbosilane (PCS) was used as a precursor polymer to synthesize a SiC matrix. It was confirmed that TiO2 was formed TiC on reaction with PCS over 1673 K by preliminary study. SiC fiber, Hi-Nicalon®, was used as reinforcement. Al2O3 powder was coated on surface of SiC fiber as interface layer of fiber/matrix. TiO2 added PCS/hexane solution was compounded with the fiber tow by filament winding. SiC fiber/SiC composite was synthesized at 1673 K for 3.6 ks in Ar. The composite was densified by reinfiltration of TiO2 added PCS solution and pyrolysis at 1673 K. Though densification behavior of the composite with TiO2 is slower than composite without TiO2, it was suggested that composition of synthesized SiC is close to stoichiometric one by apparent density. Fracture behavior of the composite indicated that addition of TiO2 was increase bonding strength between fiber and matrix. Bending strength of composite was increased after oxidation test at 1473 K for 360 ks in air atmosphere.

We grew single crystals of partially Bi-substituted Ca3Co4O9 phase in a solution consisting of K2CO3–KCl solvent. All the X-ray diffraction patterns of the single crystals with different Bi contents were attributable to the Ca3Co4O9 structure, although weak diffraction peaks from a secondary phase of Bi2Ca2Co2Ox were observed in the crystals grown from a starting composition molar ratio of Ca:Bi:Co=2.5:0.5:4.0 (BC-0.5 crystal). Thermoelectric properties of the crystals in the ab-plane were measured at various temperatures. Seebeck coefficient (S) was increased by the partial Bi-substitution at room temperature, whereas electrical resistivity (ρ) was decreased at room temperature except for BC-0.5 crystals. The simultaneous increase in S and decrease in ρ suggest an increase in carrier mobility. Although Bi atoms are heavier than the replaced Ca or Co atoms, the phonon part of thermal conductivity is increased by the Bi-substitution. We suggest that these effects of the Bi-substitution on thermoelectric properties are largely governed by changes in the peculiar crystal structure, such as the misfit relationship between the CoO2 and Ca2CoO3 layers, which constitute the layered structure of the Co3O4O9 phase.

Nano-dispersed α-Al2O3/YAG composite coatings were successfully obtained by atmospheric plasma spraying (APS) of the granulated Al2O3/Y2O3 powder and the post heat treatment. The as-sprayed coating was composed of an amorphous phase, metastable Y-Al-O solid solution. After the heat treatment about 1473 K, two crystalline phases, α-Al2O3 and YAG, precipitated. So that the fine dispersed α-Al2O3/YAG composite coatings were obtained. The sizes of the precipitates could be widely controlled by time and temperature of the post heat treatment. The diameter of both particles was varied from about 10 to 150 nm, depending on the heat treatment time. It was found that crystallization of α-Al2O3 and YAG occurred during the heat treatment about 1218 K with huge volume shrinkage. Many cracks were formed in the coatings because of this volume shrinkage during the heat treatment. This affected the wear mechanism in Ball on Disk Test, and caused a fracture in the coatings heat-treated for 1-10 hr. Plasma pre-heating of the substrate before and during the plasma spray process was carried out, in order to keep the substrate temperature about 1423 K for in-situ crystallization. The as-sprayed coating with plasma pre-heating was composed of crystallized α-Al2O3 and YAG particles without any cracks. Open porosity of this coating was maintained about 2-3% even after the heat treatment at 1473 K, while the open porosity of the APS coatings increased up to 8%. Anti wear property of the coatings with plasma pre-heating were highly superior to that of the APS coatings even after the heat treatments.

Zircon is widely used in industrial field as a refractory material, because of its excellent mechanical and chemical properties at high temperature. Authors have been studied on the effect of the heat treatment on the structure and properties of plasma sprayed zircon coating as a candidate for an environmental barrier coating (EBC). In this study, very dense coating with excellent adhesive strength was successfully obtained by optimizing the spray process. Substrate temperature is one of the dominant factors to control porosity and crack formation in the coating. The higher substrate temperature, obtained by plasma pre-heating, resulted in lower porosity and less cracks. Open porosity of the coating was quite low, about 2%, in the coatings obtained especially above 1473 K of the substrate temperature.

The process dependence of Ir-based-alloy-coated Ni-based single crystal superalloys was investigated. An Ir-4at%Ta alloy, 10 μm thick, was coated on a Ni-based single crystal superalloy TMS-82+, by means of magnetron sputtering deposition (SD) and electron beam physical vapor deposition (EB-PVD). X-ray analysis revealed that the grain size of the as-SD-coated layer (∼20 nm) has five times smaller than that of the as-EB-PVD coated layer (∼100 nm), and the SD-coated specimens showed more rapid outer diffusion of Ni and Al from the substrate after annealing, than the EB-PVD-coated ones. On the other hand, the aluminizing treatment reduces the differences between the SD coated and EB-PVD coated specimens in terms of compositional distribution and oxidation resistance. This study also revealed that even without aluminizing, the simple annealing treatment enhances the oxidation resistance of the Ir-4%Ta-coated TMS-82+.

Heat-resistant coatings prepared by two different spraying methods: atmospheric pressure plasma spraying (APS) and high-pressure plasma spraying (HPPS), were tested using tungsten carbide indenters of different diameters, for the purpose of proposing the best suited method of indentation testing. It was found that with the APS method, the indentation load–depth curve gave the indentation depth and the residual depth smaller than and the yield stress greater than those with the HPPS. On the basis of fracture morphology in the cross-section, it has been conjectured that the APS coating has greater elastic modulus than the HPPS coating, and exhibits high strength exceeding debonding force between bond coat and top coat.

The chemical state of implanted Mg atoms has been studied in 2.48 ×1017 Mg+ ions cm-2-implanted silica glass by X-ray photoelectron spectroscopy. It was found that Mg atoms were coordinated with oxygen atoms, irrespective of the heat treatment at 900°C in an argon gas atmosphere. Mg ion implantation caused oxygen deficient species (SiOx) and increased the refractive index by about 5%. The oxygen deficient species and the refractive index change were stable against the heat treatment.

The pyrochlore ruthenates Ln 2− x Bi x Ru 2 O 7 (Ln=Nd and Yb, x =1 and 0.6) have been synthesized and their thermoelectric properties have been studied from room temperature to 1000 K. These samples show the metallic-like resistivity ( ρ )–temperature ( T ) behaviors (except Yb 2− x Bi x Ru 2 O 7 ) and their ρ increases with the x values, while their Seebeck coefficients ( S ) are not sensitive to the x values. Although the S of the Nd-system samples is almost the same as that of the Yb-system ones, the ρ of the former is less than that of the latter. It indicates that the Nd-system samples have higher thermoelectric power factors than Yb-system ones. The ρ , S and power factor of NdBiRu 2 O 7 at 1000 K is 1.5 mΩ cm, 35 μV/K and 0.8×10 −4 W/mK 2 , respectively.

Spark plasma sintering (SPS) was applied for preparing dense Ca2.75Gd0.25Co4O9 ceramics, and their thermoelectric properties were investigated from room temperature to 700 °C in air. The SPS process was effective in obtaining dense Ca2.75Gd0.25Co4O9 ceramics, typically 98% of the theoretical x-ray density, with lower resistivity, without any degradation of the thermoelectric power. The power factor and figure of merit of the spark plasma sintered samples attain 4.8×10−4 W m−1 K−2 and ZT=0.23 at 700 °C, respectively.

The effects of SiC and B4C additives in the MoSi2 matrix on the microstructures and mechanical properties at room temperature were investigated. Their coefficients of thermal expansion (CTE) were also evaluated up to 1200°C by a thermal mechanical analysis (TMA). The experimental results show that the Mo2B5 reinforced phase was formed in situ in the hot-pressed MoSi2/SiC/B4C composites. Both the Mo2B5 phase and the SiC phase significantly improved the mechanical behavior of MoSi2. Besides, the SiC with a high content up to 40 vol% could be added into the MoSi2 composite with the B4C additive. As a result, a dense and homogenous MoSi2/SiC/B4C composite was obtained, which possessed a relatively high bending strength and fracture toughness. Meanwhile, the CTE of the MoSi2/SiC/B4C composites linearly decreased with the increasing SiC content, which dropped to 21% at 1200°C in comparison with the pure MoSi2 when adding 40 vol% SiC. This MoSi2/SiC/B4C composite system is very important for developing new applications at elevated temperature, particularly for high-temperature coating applications.

A new series of oxides Ca3−xSrxCo4O9+δ, (x=0.0–1.0) were synthesized, and their structures, electrical properties and Seebeck coefficients were measured from room temperature to 700°C. All the oxides have Ca2Co2O5 type structure. For all the oxides, both the electrical conductivity and the Seebeck coefficients increase with increasing temperature. The Seebeck coefficients are all positive. The values of Seebeck coefficient decrease with increasing Sr content for the Sr substituted samples. The power factor decreases with increasing Sr content for the samples with x 0.5. The Sr substituted sample (x=1.0) has lower thermal conductivity than Ca3Co4O9+δ due to its lower electrical conductivity. At 600°C, the figure of merit for Ca2SrCo4O9+δ is 0.9×10−4 K−1, and the value increases toward higher temperature.

Fibrous Al2O3 ceramics with a mixture of SiC and Al2O3 as the cell boundaries were fabricated by extrusion-molding and hot-pressing techniques. The effects of the cell boundary composition on the mechanical properties and fracture behavior are investigated. It is shown that a 65:35 mixture of SiC:Al2O3 can act as a suitable cell boundary for Al2O3 cells. In bending tests, such a ceramic displays a non-catastrophic fracture behavior with reasonable load-carrying capability, and its fracture energy and apparent toughness are up to 1349 J/m2 and 6.0 MPa m1/2, respectively.

Fibrous, multilayered and hybrid Al2O3 laminates were fabricated by extrusion and hot-pressing techniques. The effects of different architectures on the mechanical properties and fracture behavior are investigated. It is shown that hybrid laminates may provide a considerable flexibility and tailorability in mechanical behavior. An alternative arrangement of the fibrous and monolithic layers in hybrid laminates gives an average strength of up to 450 MPa, an apparent toughness of ∼11 MPa m1/2 and a fracture energy in excess of 2600 J/m2, with a non-catastrophic fracture behavior.

A new series of oxides NaCo2−xMxO4 (M = Mn, 0.0 ≤ x ≤ 1.0; M = Ru, 0.0 ≤ x ≤ 0.5) were synthesized, and their crystal structure, electrical conductivity, thermoelectric power, and magnetic properties were studied. All the samples crystallized in hexagonal systems. The electrical resistivity and the Seebeck coefficient of NaCo2−xMxO4 (M = Mn, Ru) increase with increasing Mn content. The magnetic properties of NaCo2−xMnxO4 change with increasing Mn content.

A new series of oxides Ca3-xBixCo4O9+δ, (x = 0.0−0.75) with Ca2Co2O5-type structures were synthesized, and their structures, electrical properties, Seebeck coefficients, and thermal conductivities were measured. The values of Seebeck coefficients of the new oxides are all positive, showing that they are p-type conductors. Both the electrical conductivity and Seebeck coefficients increase with the increasing Bi contents which can be attributed to the increase of carrier mobility due to the larger size of Bi ion. The electrical conductivity, Seebeck coefficient, and the calculated value of the power factor of Ca3-xBixCo4O9+δ (x = 0.5) are 105 S cm-1, 160 μV K-1, and 2.7 × 10-4 W K-2 m-1 at 700 °C, respectively. The thermal conductivity of Ca3-xBixCo4O9+δ (x = 0.5) at room temperature is 1.14 W m-1 K-1 and increase slightly with the increasing temperature. At 700 °C, the figure of merit of Ca3-xBixCo4O9+δ (x = 0.5) is 2.0 × 10-4 K-1.

Large sized n-and p-type FeSi2 thermoelectric devices were spray-formed by high-velocity oxy-fuel thermal spraying (HVOF) method. One is a plate 100mm square with the thickness of 6mm and another is a tube-type device which was formed on a stainless tube (25 mm in diameter, 500 mm long) with the thickness of 5 mm. Their microstructure and physical properties were compared to those of hot-pressed material. The bulk density of HVOF-formed FeSi2 was lower than that of hot-pressed materials due to higher amount of pore. For n-type FeSi2, Seebeck coefficient and thermal conductivity of HVOF-formed material were comparable to those of hot-pressed material. While electrical conductivities of both n-and p-type HVOF-FeSi2 were quite lower than that of hot-pressed n-type material, resulted in low performance as thermo electric device.

The effect of heat treatment on the structure and properties of a plasma-sprayed zircon coating was evaluated. The as-sprayed coating, with open porosity of 10%, is composed of t-ZrO2 and amorphous SiO2 After heat treatment at 1473 K, ZrO2 grows in columnar shape perpendicular to the coating surface, while SiO2 crystallizes to cristobalite. The open porosity decreases to about 5% after the heat treatment. This can be attributed to the sintering of SiO2 and the phase transformation of zirconia. When heat-treated at 1673 K, the coating is composed of ZrSiO4 with dispersed fine m-ZrO2 The open porosity increases again up to 10%. The thermal conductivity of the zircon coating increases with increasing heat treatment temperature.

Si3N4 matrix composite reinforced with 5vol% TiN particles was fabricated by the in-situ reaction: 4TiO2+4AlN→4TiN+2Al2O3+O2, followed by hot-pressing. The internal TiN formation reaction was studied using TiO2 powders with three different particle sizes (0.28, 0.35 and 1.19μm). The reaction is likely to start from nearly 1150°C and finishes at 1450°C, before the Si3N4 matrix is sintered, and TiN formation is confirmed from the unit cell dimension. TiN particle sizes in the composites sintered at 1600°C and 1850°C were measured. When the TiO2 particle size was large (1.19μm), the particle crumbled into fragments and underwent nitridation to TiN with a smaller size than that of the original TiO2. Smaller TiO2(0.28μm) was converted into TiN particles of nearly the same size. These results suggest that the in situ TiN formation proceeds through nitridation of TiO2 particles.

Plasma spraying of yttria-stabilized zirconia was carried out under chamber pressures ranging from low (30 kPa) to high pressure (300 kPa) to investigate pressure effects on the plasma jet and to clarify the potential of high-pressure plasma spraying (HPPS) as a high performance coating tool. Plasma flame length and velocity of the particles were measured in situ, and the coating characteristics including its microstructure, density, and hardness were studied. A condensed plasma flame under high pressure facilitated sufficient melting of zirconia particles, resulting in high deposition efficiency and a dense coating with improved hardness, in spite of reduced particle velocity. High-pressure plasma spraying was found to be suitable for thermal spraying of high-melting-point materials such as zirconia.

An oxide thermoelectric device was fabricated using Gd-doped Ca3Co4O9 p-type legs and La-doped CaMnO3 n-type legs on a fin. The power factors of p legs and n legs were 4.8×10−4 Wm−1 K−2 and 2.2×10−4 Wm−1 K−2 at 700 °C in air, respectively. With eight p–n couples the device generated an output power of 63.5 mW under the thermal condition of hot side temperature Th=773 °C and a temperature difference ΔT=390 °C. This device proved to be operable for more than two weeks in air showing high durability.

The influence of sintering temperature and time on mechanical properties was studied for a 20wt% SiC whisker/Si3N4 composite. α to β phase transformation of Si3N4 was retarded by the whisker incorporation compared to monolithic Si3N4, but the delay became smaller with increasing sintering temperature. The bending strength and the fracture toughness increased with increasing sintering temperature from 1700° to 1900°C when sintered for 30min. On the other hand, when the sintering time was extended from 30min to 90min at 1800°C, both the bending strength and the toughness decreased significantly. The behavior of the composite properties can be explained on the basis of the contribution of the matrix microstructure including the content and the size of elongated β grain and the bonding strength between the Si3N4 grains and SiC whiskers.

Bi2Sr2Co2O9 (BC-2202) polycrystalline materials with a layered structure have been prepared by partial melting. The chemical compositions of the samples are Bi2Sr2Co2Ox (2202), Bi1.8Sr2Co2Ox (Bi-1.8), and Bi2Sr1.8Co2Ox (Sr-1.8). All three samples are p-type conductors. The electric properties, namely, the Seebeck coefficient (S) and electric resistivity (ρ), of the samples are dependent on chemical composition. The S values increase with temperature at T>673 K and, at 973 K, reach 100, 110, and 150 μV K−1 for the 2202, the Bi-1.8, and the Sr-1.8 samples, respectively. Thermal conductivity (κ) for all samples is lower than for ordinary conducting oxides. The figure of merit (Z) increases with temperature for all samples. Z values at 973 K are 0.77×10−4, 0.61×10−4, and 2.0×10−4 K−1 for the 2202, Bi-1.8, and Sr-1.8 samples, respectively. The thermoelectric properties depend on the chemical composition of the BC-2202 phase. The BC-2202 material thus appears to be a promising thermoelectric material due to its...

To obtain a micro gas turbine with high turbine inlet temperature and efficiency, a series of running tests has been carried out. J-850 jet engine (Sophia Precision Co., Ltd.) was chosen as a base line machine. The turbine nozzle and the rotor are replaced to ceramic type. The observed problems occurring during the running test taught us various measures to improve heat tolerance of the engine. Especially, the ceramic nozzle vanes can be installed on a metal disk very simply. The disk structure enables us to replace blades with various shape and attack angles. We tried to operate several sets of ceramic components and metal components, such as a set of a ceramic turbine nozzle and an Inconel rotor, etc.Copyright © 2007 by ASME

In order to develop a micro gas turbine with high turbine inlet temperature and thermal efficiency, a series of running tests has been carried out. J-850 jet engine (Sophia Precision Co., Ltd.) was chosen as a baseline machine. The turbine nozzle and the rotor are replaced by type SN-01 (Otsuka Ceramics Co., Ltd.) and type SN-235 (Kyocera Corporation) ceramic elements, respectively. By using type 3a engine, we succeeded one-hour running test of the engine without cooling and severe damages. The turbine inlet temperature was higher than 1000 °C. The rotating speed was about 120,000 rpm. Performances of the type 3a engine (with ceramic nozzle and rotor) and the type 1 (with Inconel alloy nozzle and ceramic rotor) were compared as follows: At the same rotation speed, turbine inlet temperature of the type 3a became higher than that of the type 1. Simultaneously, fuel consumption of type 3a was larger than that of the type 1. Thrust of the type 3a was slightly larger than that of the type 1. Those results imply that the thermal efficiency of type 3a is slightly, 2%, lower than that of the type 1. The present sealing configurations between ceramic nozzle-vanes and their holder plate and ceramic rotor-housing and metal combustion chamber were found to work well.Copyright © 2006 by ASME

A series of operation tests of a ceramic micro gas turbine has been successfully carried out. The baseline machine is a small single-shaft turbojet engine (J-850, Sophia Precision Corp.) with a centrifugal compressor, an annular type combustor, and a radial turbine. As a first step, an Inconel 713C alloy turbine rotor of 55 mm in diameter was replaced with a ceramic rotor (SN-235, Kyocera Corporation). A running test was conducted at rotational speeds of up to 140,000 rpm in atmospheric air. At this rotor speed, the compression pressure ratio and the thrust were 3 and 100 N, respectively. The total energy level (enthalpy and kinetic energy) of the exhaust gas jet was 240 kW. If, for example, it is assumed that 10% of the total power of the exhaust jet gas was converted into electricity, the present system would correspond to a generator with 24 kW output power. The measured turbine outlet temperature was 950°C (1,740°F) and the turbine inlet temperature was estimated to be 1,280°C (2,340°F). Although the ceramic rotor showed no evidence of degradation, the Inconel nozzle immediately in front of the turbine rotor partially melted in this rotor condition. As a second step, the Inconel turbine nozzle and casing were replaced with ceramic parts (SN-01, Ohtsuka Ceramics Inc.). The ceramic nozzle and case were supported by metal parts. Through tests with the ceramic nozzle, it became evident that one of the key technologies for the development of ceramic gas turbines is the design of the interface between the ceramic components and the metallic components, because the difference between the coefficients of linear thermal expansion of the ceramic and metal produces large thermal stress at their interface in the high-temperature condition. A buffer material made of alumina fiber was therefore introduced at the interface between the ceramic and metal.Copyright © 2005 by ASME

This study concerns the method for examining the load conditions and evolving stress related to debonding at the coat-substrate interface to identify the responsible factors, by indenting a ceramic coated specimen with a spherical indenter either through impact or quasi-statically under static loading. The morphology of cracks induced by the impact of a foreign object on to specimens of inconel substrate with thermo-resistive coating of zirconia sprayed by two different methods was studied in comparison with the indentation test results by a spherical indenter under static loading. Consequently, the load conditions for coat fracture and debonding at the coat-substrate interface were successfully identified in a plasma spray specimen.

In recent years, more and more attention has been put on oxides as thermoelectric materials. In this work, we synthesized the oxide Ca/sub 9/Co/sub 12/O/sub 28/, analyzed its structure, and measured its electrical properties and Seebeck coefficient from room temperature to 700/spl deg/C. Both the electrical conductivity and the Seebeck coefficient increase with the increasing of temperatures. The electrical conductivity is 83.4 S/cm at 700/spl deg/C. The Seebeck coefficient is negative, and the value is 117.9 /spl mu/V/K at 700/spl deg/C. The calculated values of the power factor increase with increasing temperature, and reach 1.2/spl times/10/sup -4/ W/K/sup 2/.m at 700/spl deg/C. The thermal conductivity at room temperature is 1.93 W/m.K and decreases slightly with increasing temperature. At 700/spl deg/C, the figure of merit is 0.7/spl times/10/sup -4/ K/sup -1/. The substitution of Ca by Bi results in a great improvement of the thermoelectric performance with Z=2.0/spl times/10/sup -4/ K/sup -1/ at 700/spl deg/C.

A new series of oxides Ca3-xBixCo4O9+δ, (x = 0.0−0.75) with Ca2Co2O5-type structures were synthesized, and their structures, electrical properties, Seebeck coefficients, and thermal conductivities were measured. The values of Seebeck coefficients of the new oxides are all positive, showing that they are p-type conductors. Both the electrical conductivity and Seebeck coefficients increase with the increasing Bi contents which can be attributed to the increase of carrier mobility due to the larger size of Bi ion. The electrical conductivity, Seebeck coefficient, and the calculated value of the power factor of Ca3-xBixCo4O9+δ (x = 0.5) are 105 S cm-1, 160 μV K-1, and 2.7 × 10-4 W K-2 m-1 at 700 °C, respectively. The thermal conductivity of Ca3-xBixCo4O9+δ (x = 0.5) at room temperature is 1.14 W m-1 K-1 and increase slightly with the increasing temperature. At 700 °C, the figure of merit of Ca3-xBixCo4O9+δ (x = 0.5) is 2.0 × 10-4 K-1.

To develop the spraying parameters for effective and safe operation of high pressure plasma spraying, the influence of nozzle bore diameter and plasma gas flow on the characteristics of plasma torch have been investigated under the chamber pressure ranging from 5 to 300 kPa. Large bore size allows high plasma gas flow rates without excessive increasing electric voltage, which is apt to damage the cathode. High flow rates (Ar: 1.3x10-3, H2: 1.3x10-4, He: 2.7x10-4 m3/s) is effective in imbuing more power into the plasma without nozzle damage under higher chamber pressures. The relationships among the parameters, such as the effective plasma power, the electric efficiency, the chamber pressure, the plasma gas flow and so on, were expressed qualitatively by a multiple regression analysis.

Reactive Plasma Spraying (RPS) is one of several new plasma spray processes. Some findings have been published on this technology, hi these reports, only metal powders, such as Ti, Cr and Si, were used for the fabrication of coatings composed of metal and non-oxide ceramics. When an oxide powder is used for starting materials to fabricate metal or non-ceramic coating, it is important to understand the reduction process of the oxide during spraying as well as the reaction among the powder, plasma and atmospheric gas. In this study, the effects of amounts and kinds of plasma and chamber gas, chamber pressure and the addition of carbon powder on the reduction of plasma sprayed TiO2 were investigated.

An oxide single-crystalline whisker with high thermoelectric properties at temperatures (T) higher than 600 K in air has been discovered. This whisker is assigned to Ca2Co2O5 phase (abbreviated to Co-225 whiskers) and has a layered structure in which Co–O layers of two different kinds alternate in the direction of the c-axis. Seebeck coefficient of the whiskers is higher than 100 µV·K-1 at 100 K and increases with temperature up to 210 µV·K-1. Temperature dependence of electric resistivity shows a semiconducting-like behavior. These results indicate that the electric carriers are transported via hopping conduction. Using thermal conductivity of a Co-225 polycrystalline sample, figure of merit (Z T) of the Co-225 whiskers is estimated 1.2–2.7 at T≥873 K. This compound is characterized with regard to low mobility and high density of carriers, which contradicts the conventional materials with high thermoelectric properties.

Silicon carbide containing 5-15wt% graphite was fabricated by hot-pressing at 2000°-2300°C using carbon black as a starting carbon material. B4C of 1wt% was doped in SiC as a sintering aid before mixing with carbon black. X-ray diffractometry and SEM revealed that the grain growth of SiC was suppressed by the carbon addition, and that carbon black changed to graphite microcrystals. Carbon addition increased the flexural strength of the composite appreciably. The composite with 10% graphite had the average strength of 820MPa and the maximum value of 1000MPa. The fracture toughness of 4.5MPa √m for the 10% graphite composite was obtained, as compared with 3.3MPa √m for monolithic SiC.

Titanium carbide containing 5-15 wt% graphite was fabricated by hot-pressing at 2000°-2200°C using carbon black as a starting carbon material. B4C of 1wt% was doped in TiC as sintering additive before mixing with carbon black. X-ray diffractometry and SEM revealed that the grain growth of TiC was suppressed significantly by the carbon addition, and that the carbon black changed to flaky graphite microcrystals 3-5μm across and 0.5-2μm thick. The graphite microcrystals were located at the TiC grain boundaries and even in TiC grains. Carbon addition increased the flexural strength of the composite appreciably. The composite with 10% carbon had the strength of 767MPa, nearly double for monolithic TiC, 427MPa. The fracture toughness of 6.72MPa√m the 10% carbon composite was obtained, as compared with 5.19 and 3.76MPa√m for monolithic TiC doped with and without B4C as a sintering additive, respectively.

Si3N4-SiC whisker composites with whisker content up to 30wt% were prepared by heating 1700° to 2000°C for 60min in 1MPa N2. Fully dense composites were obtained at 2000°C. The optimum content of sintering aids, Y2O3 plus La2O3, was 20 and 30mol% for the whisker content of 10 and 20wt% respectively. The room temperature bending strength were 596 and 560MPa for 10 and 20wt% additions of whisker, respectively. Moreover, these composites had strengths more than 80% of the room temperature value at 1300°C. Improvement in the strength was achieved by using sieved whisker.