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In this study, macroporous SiC ceramics were fabricated via in situ liquid-phase bonding using an Al2O3–Y2O3–SiO2 bonding additive at 1500 °C. The pore size of the ceramics was successfully varied from 7.8 to 45.5 μm by increasing the SiC particle size from 37.6 to 262.6 μm, without changing the porosity. The effects of the SiC particle size and pore size on the flexural strength, permeability, and electrical resistivity of the ceramics were investigated. The pore size and SiC particle size significantly affected the properties of the macroporous SiC. With an increase in the pore size and SiC particle size, the flexural strength and electrical resistivity of the ceramics decreased significantly, while the permeability and the thermal conductivity increased considerably. The porous SiC with a pore size of 45.5 μm and a porosity of 37.8% exhibited a high permeability of 21.2 × 10−12 m2, relatively high flexural strength of 20.6 MPa, moderate electrical resistivity of 3.8 × 105 Ω cm, and high thermal conductivity of 66.4 Wm−1K−1.

Macro-porous SiC was fabricated without using pore-forming agents by an in situ reaction bonding process. A bonding additive, Al2O3–Y2O3–SiO2, with a low melting temperature was mixed with SiC particles and sintered at 1500 °C and 1600 °C for 1 h in Ar. Macro-porous SiC with a porosity of 32.7–45.9%, a pore size of 3.4–4.2 μm, and a relatively narrow and uniform pore size distribution was fabricated by varying the amount of bonding additive. The flexural strength of macro-porous SiC prepared at 1500 °C increased from 47.2 MPa to 71.2 MPa while the porosity decreased from 45.9% to 42.8%, respectively. When the sintering temperature of the macro-porous SiC was increased to 1600 °C, the flexural strengths were significantly reduced to 32.6–35.6 MPa, along with a reduction in porosity and pore size. The permeability of macro-porous SiC prepared at 1500 °C varied from 1.59 × 10−12 m2 to 1.26 × 10−12 m2, depending on the porosity. As the sintering temperature increased from 1500 °C to 1600 °C, the permeability decreased to less than 1.00 × 10−12 m2 because of the reduced porosity and average pore size. The electrical resistivity of macro-porous SiC prepared at 1500 °C and 1600 °C varied from 2.7 × 108 Ω-cm to 1.4 × 109 Ω-cm and from 1.3 × 108 Ω-cm to 1.7 × 109 Ω-cm, respectively, with increasing volume percent of bonding additives. The relatively high electrical resistivity was apparently due to neck bonding phase between SiC particles formed by phases consisting of Y2Si2O7, YAG, and residual Al2O3.

Quarternay carbides of Ti3AlxSi1-xC2 (x=0.3, 0.5, and 0.7) were oxidized at 400, 600, and 800 °C for 0.5-6 months in order to study their long-time oxidation behavior in air. When they were oxidized at 400-600 oC for 0.5-3 months, oxidation proceeded relatively slowly with moderate weight gains. However, further oxidation at 400-600 oC for 6 months resulted in the oxidation-induced microcracking of oxide scales due to large volume expansion and large stress induced owing to the formation of Al2O3, SiO2. TiO3, and TiO2 in oxide scales. However, at 800 oC, microcracking of oxide scales, which could lead to pulverization of Ti3AlxSi1-xC2, did not occur due to stress relaxation in oxide scales. Instead, at 800 oC, Ti3AlxSi1-xC2 oxidized rapidly to form thick, somewhat porous oxide scales, which consisted primarily of an outer TiO2 layer with some Al2O3, an intermediate Al2O3 layer with some TiO2, and an inner TiO2 layer with some (SiO2+Al2O3). The overall longtime oxidation resistance of Ti3AlxSi1-xC2 at 400-800 °C was considered to be poor. Factors that determined the oxidation rates of Ti3AlxSi1-xC2 were; (1) How fast titanium oxidized to semi-protective titanium oxides, (2) How fast Al and Si oxidized to Al2O3 and SiO2 barrier oxides, and (3) Whether oxidation-induced microcracking occurred in oxide scales or not. The ratio of Al/Si in Ti3AlxSi1-xC2 and the matrix grain size were apparently not dominant factors, because the basic oxidation mode of Ti3Al0.3Si0.7C2, Ti3Al0.5Si0.5C2, and Ti3Al0.7Si0.3C2 was similar under the identical oxidation condition.

SiO2–C precursors with various surface areas were derived from tetraethyl orthosilicate and phenolic resin as Si and C sources, respectively, by a modified sol–gel process using the in situ precipitation of phenol resin in a prepared wet gel. The surface area of the SiO2–C precursors was varied from 20 to 175 m2/g by changing the C/Si molar ratio in the preform. β-SiC powders were synthesized using carbothermal reduction in vacuum at the temperature range of 1200–1600 °C. The effects of the temperature and heat treatment time as well as that of the surface area of the preform on the formation of β-SiC powders were studied. It was determined that the formation of β-SiC started at 1200 °C and was considerably promoted as the heat treatment temperature and time further increased during the carbothermal reduction of SiO2–C preforms with high surface area. When high surface area SiO2–C preforms were used, highly crystalline SiC powders were synthesized at 1600 °C in vacuum with a high yield of 85%.

In this study, Si-SiC composites were fabricated using a Si melt infiltration method using β-SiC/C composite powders synthesized by the carbothermal reduction of SiO₂-C precursors made from a TEOS and a phenol resin. The purity of the synthesized SiC-C composite powders was higher than 99.9993 wt% and the average particle size varied from 4 to 6 μm with increasing carbon contents of the SiO₂-C precursors. It was found that the Si-SiC composites fabricated in this study consist of β-SiC and residual Si, without any trace of α-SiC. The 3-point bending strengths of the fabricated Si-SiC composites were measured and found to be higher than 550 MPa, although the density of the fabricated Si-SiC composite was less than 2.9 g/cm3. The bending strengths and the densities of the fabricated Si-SiC composites were found to decrease with increasing C/Si mole ratios in the SiC-C composite powders. The specific resistivities of the Si-SiC composites fabricated using the SiC-C composite powders were less than 0.018 Ωcm. With increasing C content in the SiC-C composite powders used for the fabrication of Si-SiC composites, the specific resistivity of the Si-SiC composites was found to slightly increase from 0.0157 to 0.018 Ωcm.

The objective of this study was to develop a synthesis process for β-SiC powders to reduce the synthesis temperature and to control the particle size and to prevent particle agglomeration of the synthesized β-SiC powders. A phenol resin and TEOS were used as the starting materials for the carbon and Si sources, respectively. SiO₂-C hybrid precursors with various C/Si mole ratios were fabricated using a conventional sol-gel process. β-SiC powders were synthesized by a carbothermal reduction process using SiO₂-C hybrid precursors with various C/Si mole ratios (1.6 ~ 2.5) fabricated using a sol-gel process. In this study, the effects of excess carbon and the addition of Si powders to the SiO₂-C hybrid precursor on the synthesis temperature and particle size of β-SiC were examined. It was found that the addition of metallic Si powders to the SiO₂/C hybrid precursor with excess carbon reduced the synthesis temperature of the β-SiC powders to as low as 1300℃. The synthesis temperature for β-SiC appeared to be reduced with an increase of the C/Si mole ratio in the SiO₂-C hybrid precursor by a direct carburization reaction between Si and excess carbon.

β-SiC powders were synthesized by a carbothermal reduction process using SiO₂-C precursors fabricated by a sol-gel process using phenol resin and TEOS as starting materials for carbon and Si sources, respectively. The C/Si molar ratio was selected as an important parameter for synthesizing SiC powders using a sol-gel process, and the effects of the C/Si molar ratio (1.4-3.0) on the particle size, particle size distribution, and yield of the synthesized β-SiC powders were investigated. It was found that (1) the particle size of the synthesized β-SiC powders decreased with an increase in the C/Si molar ratio in the SiO₂-C hybrid precursors, (2) the particle size distribution widened with an increase in the C/Si molar ratio, and (3) the yield of the β-SiC powder production increased with an increase in the C/Si molar ratio.

Nanolaminate Ti 3 SiC 2 was synthesized from a mixture of TiC x ( x = 0.67)/Si powder by hot pressing to increase machinability. Ti 3 SiC 2 was synthesized at temperatures of 1360 °C and 1420 °C for 90 min under a pressure of 25 MPa. The X-ray diffraction results showed that while mainly Ti 3 SiC 2 with some unreacted TiC x were detected in the synthesized samples at 1360 °C, no phases except Ti 3 SiC 2 phases remained in the synthesized samples at 1420 °C. The cutting resistance of Ti 3 SiC 2 was measured in terms of the principle, feed, and thrust forces and was compared with that of middle-carbon steel, SM45C. The values of the principal force of the synthesized Ti 3 SiC 2 were lower than those of SM45C. After machining, the roughness of the Ti 3 SiC 2 was lower than those of SM45C; however, the damage to the tool bit used for the machining of SM45C was less than the damage to those used for the machining of the Ti 3 SiC 2 .

Cr2AlC compounds were synthesized via a powder metallurgical route and their long-term oxidation behavior studied. Oxidation at temperatures between 700 and 1,000 °C for up to 360 days in air resulted in formation of a thin, adherent Al2O3 surface layer and a narrow Cr7C3 sublayer, accompanied by evaporation of carbon from the Cr2AlC. Preferential oxidation of Al on the surface suppressed oxidation of the less mobile Cr in Cr2AlC. In the Al2O3 layer, (0.7–8.3) at.% Cr was incorporated. In the Cr7C3 sublayer, Al was either absent or incorporated. When Cr2AlC oxidized at 850 and 1,000 °C for 30–360 days, metastable θ-Al2O3 blades formed on the α-Al2O3 layer. However, such blades were scarcely visible when the oxidation was carried out above 1,100 °C, because of the fast θ → α-transition. Moreover, the θ-Al2O3 were not noticeable during oxidation at 700 °C for 30–360 days, due to a small extent of oxidation.

High-purity Ti 3 SiC 2 compounds have been fabricated by infiltration of molten Si into a precursor, a partially sintered TiC x ( x = 0.67) preform. The Si source and the TiC x preform were placed side by side on carbon cloth, and the system was heated to 1550 °C. Molten Si infiltrated the preform through the carbon cloth, and a direct reaction between TiC x and molten Si immediately occurred at the reaction temperature to yield pure Ti 3 SiC 2 . We could observe phase formation and the microstructure of the bulk products with time, which were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS). Pure Ti 3 SiC 2 compounds were formed on the exterior of the TiC x preform at 1550 °C when the sintered TiC x :Si ingot molar ratio was 3:1.4. At 1550 °C, no other minor phases were detected for any of the sintering time ranges.

Ti 3 AlC 2 was corroded between 800 and 1100 °C in an Ar–0.2% SO 2 gas atmosphere according to the equation: Ti 3 AlC 2 + O 2 → rutile-TiO 2 + α-Al 2 O 3 + (CO or CO 2 ). The scales that formed on the Ti 3 AlC 2 were thin and rich in α-Al 2 O 3 , whose growth rate was exceedingly slow. The TiO 2 was present either as the outermost surface scale or a mixture inside the α-Al 2 O 3 -rich scale. In the Ti 3 AlC 2 , the activity and diffusivity of Ti were low, whereas those of Al were high. This was the main reason for the superior corrosion resistance of Ti 3 AlC 2 over TiAl.

Nanopowders of Fe 0.54 Cr 0.18 Al 0.26 Si 0.016 and Al 2 O 3 were synthesized from Fe 2 O 3 , Cr, Si, and Al powders using high-energy ball milling. A high-density nanocrystalline 3.7Fe 0.54 Cr 0.18 Al 0.26 Si 0.016 –Al 2 O 3 composite was consolidated with mechanically synthesized powders of Al 2 O 3 and 3.7Fe 0.54 Cr 0.18 Al 0.26 Si 0.016 –Al 2 O 3 through a pulsed current activated sintering (PCAS) method within 1 min. The hardness of the composite and the average grain sizes of Al 2 O 3 and Fe 0.54 Cr 0.18 Al 0.26 Si 0.016 were investigated.

Nanopowders of 5.33Fe0.37Cr0.16Al0.4Si0.07 and Al2O3 were synthesized from Fe2O3, Cr, Si, and Al powders by high-energy ball milling. A high-density nanocrystalline 5.33Fe0.37Cr0.16Al0.4Si0.07–Al2O3 composite was consolidated by a high-frequency, induction-heated sintering (HFIHS) method within three minutes from mechanically synthesized powders of Al2O3 and 5.33Fe0.37Cr0.16Al0.4Si0.07. The advantage of this process is that it allows very quick densification to near theoretical density and prohibits grain growth in nano-structured materials. The average grain sizes of Al2O3 and 5.33Fe0.37Cr0.16Al0.4Si0.07 were 99 nm and 14 nm, respectively.

Nanopowder of Fe2O3, Al, Cr and Si was fabricated by high energy ball milling. A dense nanostuctured A2O3 and 6.06Fe0.33Cr0.16Al0.23Si0.29 composite was simultaneously synthesized and consolidated using high frequency induction heated sintering method within 1 minute from mechanically activated powders of Fe2O3, Al, Cr and Si. The grain sizes of Al2O3 and Fe0.33Cr0.16Al0.23Si0.29 in composite are 80 and 18 nm, respectively. (Received December 1, 2010)

Cr2AlC compounds were synthesized by a powder metallurgical route and corrosion tested at 900, 1000, 1100 and 1200 °C for up to 150 h under an Ar/1% SO2 gas atmosphere. The compounds were resistant to corrosion because a thin α-Al2O3 barrier layer quickly formed on the surface which suppressed sulfidation. Virtually no sulfur was detected inside the scale except during the initial corrosion stage. The superior corrosion resistance of Cr2AlC originated from the high affinity of Al for oxygen to form the thermodynamically stable Al2O3. Unlike Al, Cr was not active because Cr was strongly bound to carbon as Cr2C layers in Cr2AlC. The small amount of Cr2O3 that had formed was dissolved in the Al2O3 layer. The corrosion of Cr2AlC resulted in the formation of an α-Al2O3 layer and an underlying Cr7C3 layer.

Nanopowders of 3.3Fe 0.6 Cr 0.3 Al 0.1 and Al 2 O 3 were synthesized from Fe 2 O 3 , Cr, and Al powders by high-energy ball milling. A high density nanocrystalline 3.3Fe 0.6 Cr 0.3 Al 0.1 –Al 2 O 3 composite was consolidated by a high frequency induction heated sintering (HFIHS) method within 3 min from mechanically synthesized powders of Al 2 O 3 and 3.3Fe 0.6 Cr 0.3 Al 0.1 . The average grain sizes of Al 2 O 3 and 3.3Fe 0.6 Cr 0.3 Al 0.1 were 84 and 32 nm, respectively.

Nanopowders of 3NiAl and Al2O3were synthesized from 3NiO and 5Al powders by high-energy ball milling. Nanocrystalline Al2O3reinforced composite was consolidated by high-frequency induction-heated sintering within 3 minutes from mechanochemically synthesized powders of Al2O3and 3NiAl. The advantage of this process is that it allows very quick densification to near theoretical density and inhibition grain growth. Nanocrystalline materials have received much attention as advanced engineering materials with improved physical and mechanical properties. The relative density of the composite was 97%. The average Vickers hardness and fracture toughness values obtained were 804 kg/mm2and 7.5 MPa⋅m1/2, respectively.

Nano laminated bulk Ti₃SiC₂ was synthesized by hot press process using TiCx/Si powder mixture at the temperature range of 1300℃~1500℃. pure Ti₃SiC₂ was synthesized by a hot pressing above 1400℃, while unreacted TiCx were remained in bulk Ti₃SiC₂ which synthesized below 1400℃. The sintering density of bulk Ti₃SiC₂ were varied with the amount of TiCx. It was found that the mechanical properties and micro structures of bulk Ti₃SiC₂ were closely related to the amounts of TiCx which was controlled by the hot pressing temperature. The TiCx increase the flexural strength of bulk Ti₃SiC₂, while the fracture toughness and thermal shock resistance of bulk Ti₃SiC₂ were decreased with the content of TiCx. The plastic deformations of bulk Ti₃SiC₂ were appeared above 1000℃.

In this study, we carried out a sol-gel processing to synthesize two types of phenol resin and TEOS sol with fluoric acid as the catalysts, and fabricated the β-SiC powder by carbothermal reduction of the obtained precursor gels. The gels were observed to be cross linked between TEOS and phenol resins by FT-IR and TG-DTA. After pyrolyzed at 1000oC in Ar to obtain the precursor, to confirm the phase transition and morphologies by XRD, and SEM were measured. Finally, the light green colored SiC powder was obtained after carbothermal reduction process up to 1800oC. The residual condensed SiO(v) phase and free carbons as a role of impurities for final products using the solid type phenol resin was observed in the surface of powder by BSE.

Highly densified Silicon Carbide (SiC) ceramics with 10 vol.% sintering additives, RE2O3 (RE: Sc, Lu, Y) and AlN, were prepared via hot pressing in N2 atmosphere. The influence of sintering additive composition on the mechanical properties of the sintered specimens is discussed. The Sc2O3-AlN system consisted of equi-axed grain structure, while the Lu2O3-AlN and Y2O3-AlN systems rendered largely grown platelet grains. While fracture toughness was enhanced with regards to the contribution of crack deflection and crack bridging mechanisms for intergranular fracture, strength remained almost the same for all specimens.

High-purity, dense nano-laminated (Cr0.95Ti0.05)2AlC compounds were synthesized via a powder metallurgical route. Their oxidation characteristics were investigated by exposing them to temperatures between 900 and 1200 degrees C in air. The alloying Ti in the Cr layer of Cr2AlC did not significantly change the crystal structure and microstructure of Cr2AlC. But, it increased the oxidation rate of the Cr,AlC. The scale morphology of the (Cr0.95Ti0.05)2AlC was basically similar to that of the pure Cr2AlC. The main difference was that the (Cr0.95Ti0.05)2AlC contained oxide nodules. These formed because the titanium oxidized to TiO2, making the Cr2AlC susceptible to nodular oxidation. However, alpha-Al2O3 was still the major oxidation product in both the (Cr0.95Ti0.05)2AlC and pure Cr2AlC.

Reaction Bonded Silicon Carbide(RBSC) has been used for engineering ceramics due to low-temperature fabrication and near-net shape products with excellent structural properties such as thermal shock resistance, corrosion resistance and mechanical strength. Recently, attempts have been made to develop hot gas filter with gradient pore structure by RBSC to overcome weakness of commercial clay-bonded SiC filter such as low fracture toughness and low reliability. In this study a fabrication process of porous RBSC with multi-layer pore structure with gradient pore size was developed. The support layer of the RBSC with multi-layer pore structure was fabricated by conventional Si infiltration process. The intermediate and filter layers consisted of phenolic resin and fine SiC powder were prepared by dip-coating of the support RBSC in slurry of SiC and phenol resin. The temperature of 1550℃ to make Si left in RBSC support layer infiltrate into dip-coated layer to produce SiC by reacting with pyro-carbon from phenol resin.

High purity β-SiC powders were synthesized using sol-gel processing. TEOS and phenol resin were used as the starting material for the silicon source and carbon source, respectively. The process turned out to be capable of producing high purity SiC powder purity degree with 99.98 %. However, it was difficult to control the shape and size of β-SiC powders synthesized by sol-gel process. In this study, β-SiC powder with size of 1~5 ㎛ an 30 ㎚ were used as the seeds for β-SiC to control the β-SiC powder morphology. It was found that β-SiC powder seeds was effective to increase the powder average size of synthesized β-SiC using sol-gel process by acting as the preferred growing sites for β-SiC.

Nanoporous gold thin films were fabricated on Si substrates by dealloying Au–Ag alloy films, created by elevated temperature annealing of deposited Au and Ag multilayers. The effects of various chemical compositions, heat treatment, and dealloying methods and conditions (dissolution rate) were examined. Nanoporous gold films showing significantly less cracking in tens of millimeters and reduced ligament size under 10 nm were fabricated from properly alloyed Au–Ag films using a well-controlled electrochemical dealloying process with diluted aqueous HClO 4 as an electrolyte.

Silicon carbide is a potential material for various structural applications because of its unique combination of properties such as high hardness, high temperature strength, and excellent resistance to wear and corrosion [1–4]. In order to improve the fracture toughness, several attempts have been made to fabricate SiC ceramics consisting of heterogeneous microstructure with weak grain boundary phases [5–7]. The liquid-phase sintering using metal oxides, Al–B–C, AlN– metal oxides as sintering additives is recommended for obtaining SiC ceramics with tailored microstructure [8, 9]. Among various additive systems, AlN and RE2O3 (RE: rareearth elements) systems are preferred due to relatively less weight loss [10], easy control of oxynitride decomposition, and absence of gas-phase reactions below 2000 C [6]. The use of RE2O3 renders SiC ceramics with excellent high temperature strength and toughness properties [6, 11]. The platelet-reinforced microstructures exhibit enhanced toughness due to combination of intergranular crack mode, introduced by the glassy grain boundary phase, and energy dissipating processes in the crack wake [12]. Though most of the studies reported amorphous grain boundary or ternary junction phase, it has been recently identified that sintering additive composition plays a dominant role in affecting the crystallinity of grain boundary phase. Amorphous intergranular phase was obtained using AlN–Y2O3 [13, 14] and AlN–Er2O3 [9, 11] additives systems, whereas AlN– Sc2O3 render clean grain boundary [15, 16] and AlN–Lu2O3 systems render clean [17] or crystallized grain boundary [18]. It is to note that most of the published research has been conducted toward characterization and property evaluation of SiC ceramics prepared with relatively large amounts of additives (C5 wt%), while its performance with small additive contents is poorly understood. Therefore, an attempt has been made in this study, for the first time, to sinter SiC ceramics using small amount (3 wt%) of AlN–RE2O3 (RE: Sc, Lu and Y) additives. The major purpose is to investigate the influence of additive composition on microstructural and mechanical characteristics of SiC ceramics sintered with small additive amounts. Commercially available b-SiC (Ultrafine grade, Betarundum, Ibiden Co. Ltd., Ogaki, Japan), AlN (Grade F, Tokuyama Soda Co., Tokyo, Japan), and metal oxides (Sc2O3, Lu2O3, and Y2O3, 99.9% pure, Shin-Etsu Chemical Co., Tokyo, Japan) were used as the starting powders. The mean particle size and the specific surface area of the b-SiC powders were 0.27 lm and 17.5 m/g, respectively. A powder mixture of SiC and 3 wt% additives AlN and RE2O3 (RE: Sc, Lu, Y) was prepared. The relative amount of RE2O3 in the total content of additives was 20 mol.%. The respective powder batches were milled in ethanol for 24 h using SiC grinding balls. The milled slurry was dried, sieved, and hot-pressed at 2050 C for 6 h under a pressure of 25 MPa in N2 atmosphere. The details of specimen designation, batch composition, and additive systems are given in Table 1. The designation SCRE represents silicon carbide specimen prepared using 3 wt% RE–AlN (RE: Sc, Lu, Y) additive. The bulk density of the sintered specimen was measured using the Archimedes method in water. The theoretical B. V. Manoj Kumar M.-H. Roh Y.-W. Kim (&) W. Kim Department of Materials Science and Engineering, The University of Seoul, Seoul 130-743, Korea e-mail: ywkim@uos.ac.kr

Dense, monolithic Ti3Al0.7Si0.3C2 compounds were cyclically oxidized between 900 and 1100 °C in air for 100 h. Ti3Al0.7Si0.3C2 oxidized into rutile-TiO2, α-Al2O3, and amorphous SiO2, accompanied by the evaporation of carbon. Ti3Al0.7Si0.3C2 had good thermal shock resistance, so that adherent oxide scales formed, regardless of their thickness. The cyclic oxidation resistance of Ti3Al0.7Si0.3C2 was poorer than that of Ti3SiC2, Ti3AlC2, and Cr2AlC. The thermal cycling did not affect the scale morphology or the oxidation mechanism that was identified in the isothermal oxidation tests, due mainly to the formation of the adherent oxide scales.

The layered, machinable quaternary compound, Ti 3 Al 0.5 Si 0.5 C 2 , was synthesized via the powder metallurgical route, and oxidized between 900 and 1200 °C in air for up to 80 h. The oxidation of Ti 3 Al 0.5 Si 0.5 C 2 produced rutile-TiO 2 , α-Al 2 O 3 , and amorphous SiO 2 , together with the evolution of CO or CO 2 gases. The oxide scale consisted primarily of an outer TiO 2 -rich layer, an intermediate Al 2 O 3 layer, and an inner TiO 2 -rich layer. Relatively thick scales formed because of the formation of the semi-protective TiO 2 and the gaseous escape of carbon.

Pressureless infiltration of molten Al−Mg alloys into particulate Al2O3 preforms has been known to occur only in a nitrogen atmosphere. In order to understand the pressureless infiltration mechanism of Al−Mg alloys into particulate Al2O3 preforms, nitrogen was injected at 25°C, 300°C, and 600°C. The higher the injection temperature of the nitrogen gas was, the lower the infiltration temperature of the molten Al−Mg alloys into the particulate Al2O3 preform was. Pressureless infiltration of the Al−6Mg alloy occurred at 700°C when the nitrogen gas was injected at 600°C. The formation of an Mg−N compound (Mg3N2) on Al2O3 particles, which improves wettability by decreasing the interfacial energy between the Al−Mg alloys and the Al2O3 particles, enabled the formation of the Al−Mg alloy/Al2O3 composite via pressureless infiltration. Increasing the injection temperature close to the melting point of the Al−Mg alloys appeared to enhance the formation of Mg3N2 on the surface of the Al2O3 particles.

A study has been performed to synthesize polycrystalline bulk Ti 3 AlC 2 by hot pressing from TiC x ( x = 0.6) and Al powder mixture at the temperature range of 800–1600 °C under 25 MPa for various times. Relatively pure Ti 3 AlC 2 was successfully synthesized above 900 °C. Small amount of Ti 2 AlC as a secondary phase was observed along with Ti 3 AlC 2 below 1400 °C. With increasing hot pressing time to 1250 °C, the amount of un-reacted TiC x was gradually decreased while Ti 3 AlC 2 was appeared to be a dominant phase. The densification of Ti 3 AlC 2 was examined as a function of hot pressing temperature up to 1600 °C as well as it was investigated with hot pressing time at 1250 °C. Near fully dense Ti 3 AlC 2 was synthesized as the hot pressing time was increased at 1250 °C. Fully dense and pure Ti 3 AlC 2 could be synthesized by hot pressing above 1400 °C. The resulting Ti 3 AlC 2 above 1400 °C showed the typical laminated and platelet grain structure of ternary carbides. With increasing processing temperature to 1600 °C, the typical platelet grain structure of Ti 3 AlC 2 was started to be destroyed and significant grain coarsening was occurred, which might be caused by partial melting of Ti 3 AlC 2 . The Vickers hardness of bulk Ti 3 AlC 2 was about 3.5–6 GPa under the loading of 10 N. The maximum flexural strength of synthesized bulk Ti 3 AlC 2 was over 900 MPa. The new process using TiC x /Al powder mixture as a starting material made it possible to shorten the synthesis time, to reduce processing temperature, as well as to improve the flexural strength of Ti 3 AlC 2 .

High purity, dense Cr2AlC compounds were synthesized via a powder metallurgical route, and their oxidation behavior was investigated at 1300 °C in air for up to 336 h. A thin external oxide layer formed, which consisted primarily of not Cr2O3 but Al2O3. Since Al was consumed to produce the Al2O3, Al-depletion and Cr-enrichment occurred underneath the Al2O3 layer. This led to the formation of a Cr7C3 layer containing voids. These grew during oxidation, eventually destroying the Cr7C3 layer formed on the unoxidized Cr2AlC matrix.

High-purity, dense Cr2AlC compounds were synthesized by hot pressing a mixture of CrC X (x = 0.5) and Al powders. Oxidation at temperatures between 900 and 1200 °C in air for up to 480 h resulted in the formation of a thin Al2O3 layer. The consumption of Al to make the Al2O3 layer led to the enrichment of Cr immediately below the Al2O3 layer, resulting in the formation of an underlying Cr7C3 layer. At the same time, carbon escaped from the Cr2AlC into the air. During the initial stage of oxidation, oxygen diffused inward to form the Al2O3 layer, which vastly improved the oxidation resistance of Cr2AlC from the initial stage of oxidation.

A porous reaction bonded silicon carbide (RBSC) was fabricated by a molten Si infiltration method. The porosity and flexural strength of porous RBSC fabricated in this study were dependent upon the amount of carbon source used in the SiC/carbon preform as well as the amount of Si infiltrated into the SiC/carbon preform. The porosity and flexural strength of porous RBSC were in the range of 20 vol.~49 vol.% and 38~61㎫, respectively. With increase of carbon contents and molten Si for infiltration, volume fraction of the pores was gradually decreased, and flexural strength was increased. The porous RBSCs fabricated with the same amount of molten Si show less residual Si around neck with increase of carbon source, as well as a new SiC was formed around neck which resulted in the decreased porosity and improvement of the flexural strength. In addition, decrease of the porosity and increase of the flexural strength were also obtained by increase of the amount of molten Si with the same amount of carbon source. However, it was found that the flexural strength of porous RBSC depends on the porosity rather than the amount of the newly formed SiC in neck phase between SiC particles used as a starting material.

In this paper, we present results of an investigation on the physical, mechanical, chemical properties and processing ability of the Cr2AlC ternary carbide bulk material synthesized by hot pressing technique. The combination of excellent properties indicated that Cr2AlC ternary carbide alloy could be potential candidate materials as bipolar plates in polymer membrane fuel cell (PEMFC).

The Ti3SiC2 materials were synthesized by hot pressing TiCx and Si powder mixtures. The matrix grains were lamellar, having a small amount of TiCx. The high-temperature stability was investigated by subjecting Ti3SiC2 to high-temperature oxidation up to 1200oC in air. Ti3SiC2 began to oxidize appreciably above 850oC. The oxidation resulted in the formation of the oxide layer that consisted of TiO2 and SiO2. The scales formed were adherent.

Boron carbide is a very hard material with high abrasive wear resistance. It requires a very high sintering temperature of above 2200 oC to fabricate a monolithic B4C close to the theoretical density. However, the mechanical property of monolithic B4C is not good enough to use it directly to industrial applications. In this investigation, B4C based ceramic composites were fabricated by in-situ reaction hot pressing using B4C, TiC and SiC powder as starting materials. The reaction synthesized composites by hot pressing at 1950 oC was found to posses very high relative density. The reaction synthesized B4C composites comprise B4C, TiB2, SiC and graphite by the reaction between TiC and B4C. The newly formed TiB2 and graphite was embedded both inside grain and at grain boundary of B4C. The mechanical properties of reaction synthesized B4C-TiB2- SiC-graphite composites were more enhanced compared to those of monolithic B4C. The flexural strength and fracture toughness of these in-situ B4C synthesized composites were 400-570 MPa and 6-9.5 MPam1/2, respectively.

Cr2AlC, one of the ternary carbide families, was synthesized by a reactive hot pressing of CrCx (x=0.5) and Al powder mixture used as starting materials at the temperature range of 1200 oC~1400 oC under 25 MPa in Ar atmosphere. Fully dense Cr2AlC with high purity was synthesized by hot pressing CrCx and Al powder mixture at the temperature as low as 1200 oC. After synthesizing the bulk Cr2AlC, it was found that it has a typical layered structure as found in other ternary carbides such as Ti3AlC2 and Ti3SiC2. The average grain size of synthesized bulk Cr2AlC was varied in the range of 10-100 ㎛ depending on hot pressing temperatures. The Vickers hardness of bulk Cr2AlC was less than 4 GPa under the loading of 10N and this decreased with increasing indentation load. The maximum flexural strength of synthesized bulk Cr2AlC exceeded 600 MPa. Also, it was found that synthesized Cr2AlC was readily machinable by a conventional WC tool bit due to its low hardness as well as damage tolerance properties upon fracture.

Ti3SiC2 materials were synthesized by hot pressing using a new starting-material system consisting of a TiCx(x=0.6)/Si powder mixture. The oxidation of Ti3SiC2 at temperatures between 900 and 1200 °C in air for up to 100 h resulted in the formation of an outer TiO2 layer, an intermediate SiO2-rich layer and an inner (TiO2 + SiO2) mixed layer. During oxidation, Ti diffused outwards to form the outer TiO2 layer, and oxygen transported inwards to form the inner (TiO2 + SiO2) mixed layer. At the same time, the carbon in Ti3SiC2 escaped into the air. Below the scale, there was a narrow oxygen-affected zone, The oxidation at the scale-matrix interface proceeded by the disintegration of the lamellar Ti3SiC2 grains to form crystallites with a size of a few tens of nanometers containing oxygen. The detailed scale characteristics and oxidation mechanism are described.

Using the hot pressing method, the Ti3SiC2 materials having fine and coarse grains were synthesized from TiC0.6 and Si powders, and the Ti3AlC2 materials having fine and coarse grains were also synthesized from TiC0.6 and Al powders. The cyclic oxidation between 900 and 1200oC for 40 hr in air resulted in the formation of an outer TiO2 layer and an inner (TiO2+ amorphous SiO2) mixed layer for Ti3SiC2, and the formation of an outer TiO2 layer and an inner (TiO2+ Al2O3) mixed layer for Ti3AlC2. The effect of the grain size on the cyclic oxidation resistance was not significant.

Nanosized ITO(indium tin oxide) powders were prepared in ethylene glycol solution under mild temperature and pressure conditions by precipitation from metal nitrates with aqueous potassium hydroxide. The average size and distribution of the synthesized ITO powders were about 20 ㎚ and broad, respectively. The phase of synthesized particles was crystalline reacted at 230 0C for 6 h.. The optical properties of the synthesized ITO powders were transparent.

Fine-grained and coarse-grained Ti3SiC2 materials were synthesized by hot pressing using TiC0.67 and Si powders as starting materials. The oxidation between 900 and 1100oC in air resulted in the formation of an outer TiO2 (rutile) layer and an inner mixed layer of TiO2 and amorphous SiO2, accompanying the liberation of carbon into the air. Fine-grained Ti3SiC2 displayed better oxidation resistance than coarse-grained one, with activation energies of 376.5~380 kJ/mol for oxidation.

In this study, the reaction-bonded SiC (RBSC) hot gas filter was newly developed using Si melt infiltration technique. Si melt was infiltrated into the green body through the carbon source coated surface of SiC powder forming strong neck phases between SiC powders. The maximum flexural strength of porous RBSC support at room temperature and at 900 oC were about 80 MPa and 63 MPa, respectively, which were much higher than those of commercially available porous clay-bonded SiC with similar pore size and porosity.

The contact fracture behaviors of fine-grained Ti3SiC2 and coarse-grained high purity Ti3SiC2 are examined by the Hertzian indentation and Vickers indentation technique. The Vickers hardness of bulk Ti3SiC2 is as low as 5.3~6.3 Gpa, and the Hertzian contact stress-strain curves for Ti3SiC2 deviate much from linearity, which resembles the fracture behavior of a ductile metal rather than a brittle ceramic. The contact damages by both Vickers indentation and Hertzian indentation reveal a fairly good plastic deformation nature of Ti3SiC2. Un-reacted TiCx in fine-grained Ti3SiC2 may impede the plastic deformation by slip along basal plan inside Ti3SiC2 grain, making Ti3SiC2 less plastic under loading.

The variation in fracture strength of a brazed Si3N4/Cu/steel joint was compared with the change in residual stress as a function of the Cu-interlayer thickness that was used. The higher residual stress and the lower measured fracture strength for the joint, using a 0.1 mm thick Cu-interlayer, were ascribed to the entire dissolution of the Cu-interlayer into the brazing alloy. The finite element analysis of residual stress, which considered the microstructure at the interface region, could explain the fracture behavior for the brazed joints, which is dependent on the thickness of the Cu-interlayer.

Ag/SiO2 composite particles have been synthesized within reverse micelle via metal alkoxide hydrolysis and condensation. The size of the particles and the thickness of the coating can be controlled by manipulating the relative rates of the hydrolysis and condensation reactions of tetraethoxysilane (TEOS) within the microemulsion. The average size of the synthesized particles was in the size range of 20–40 nm. The effects of synthesis parameters, such as the concentration of the Ag solution, the molar ratio of water to TEOS, and the molar ratio of water to surfactant, are discussed.