Behavior of electrical resistance of SiC<Subscript>CVD</Subscript> fiber and development of micro-heater with SiC<Subscript>CVD</Subscript> fiber

Behavior of electrical resistance was examined in room temperature and elevated temperatures up to 1000 °C for two types of SiC_CVD fibers with diameters of 140 and 70 μm, respectively. The results showed that electrical resistance showed a good linear relationship with the length of fibers. Electrical resistance decreased as temperature increased, besides, temperature coefficient of electrical resistance was a minus constant, −5.2 × 10⁻⁴ °C⁻¹ except that in the first heating. In the first heating, electrical resistance and temperature coefficient increased and had a peak in the range of 550–700 °C owing to the burning of the carbon-rich layer on the fiber surface. It suggested that behavior of electrical resistance of the fibers depended mainly on the carbon core and the carbon-rich layer. It was confirmed that SiC_CVD fiber could be used as heating elements for micro-heater and finally a micro-heater using SiC_CVD fiber as heating elements was developed.     

Microstructure and mechanical properties of SiC<Subscript>P</Subscript>/SiC and SiC<Subscript>W</Subscript>/SiC composites by CVI

37.2 vol.% SiC_P/SiC and 25.0 vol.% SiC_W/SiC composites were prepared by chemical vapor infiltration (CVI) process through depositing SiC matrix in the porous particulate and whisker preforms, respectively. The particulate (or whisker) preforms has two types of pores; one is small pores of several micrometers at inter-particulates (or whiskers) and the other one is large pores of hundreds micrometers at inter-agglomerates. The microstructure and mechanical properties of 37.2 vol.% SiC_P/SiC and 25.0 vol.% SiC_W/SiC composites were studied. 37.2 vol.% SiC_P/SiC (or 25.0 vol.% SiC_W/SiC) consisted of the particulate (or whisker) reinforced SiC agglomerates, SiC matrix phase located inter-agglomerates and two types of pores located inter-particulates (or whiskers) and inter-agglomerates. The density, fracture toughness evaluated by SENB method, and flexural strength of 37.2 vol.% SiC_P/SiC and 25.0 vol.% SiC_W/SiC composites were 2.94 and 2.88 g/cm³, 6.18 and 8.34 MPa m^1/2, and 373 and 425 MPa, respectively. The main toughening mechanism was crack deflection and bridging.     

Preparation and mechanical properties of ZrB<Subscript>2</Subscript>-based ceramics using MoSi<Subscript>2</Subscript> as sintering aids

ZrB₂ (zirconium diboride)-based ceramics reinforced by 15vol.% SiC whiskers with high density were successfully prepared using MoSi₂ as sintering aids. The effects of sintering condition and MoSi₂ content on densification behavior, phase composition, and mechanical properties of SiC_w/ZrB₂ composites were studied. Nearly, fully dense materials (relative density >99%) were obtained by hot-pressing (HP) at 1700°C–1800°C in flow argon atmosphere. The grain size of ZrB₂ phase in the samples sintered by HP at 1700°C–1800°C were very fine, with mean size below 5 μm. Mechanical properties (such as flexural strength, fracture toughness, and Vickers hardness) of the sintered samples were measured. The sample with 15vol.% MoSi₂ addition sintered by HP at 1750°C displayed the best mechanical properties.     

A high-strength SiCw/SiC–Si composite derived from pyrolyzed rice husks by liquid silicon infiltration

A high-strength SiC composite with SiC whiskers (SiC_w) as reinforcement has been fabricated by liquid silicon infiltration (LSI) using pyrolyzed rice husks (RHs) as raw material. RHs were coked and pyrolyzed subsequently at high temperature to obtain a mixture containing SiC whiskers, particles, and amorphous carbon. The pyrolyzed RHs were then milled and modeled to preforms, which were then used to fabricate biomorphic SiC_w/SiC–Si composites by liquid silicon infiltration at 1,450, 1,550, and 1,600 °C, respectively. Dense composite with a density of 3.0 g cm⁻³ was obtained at the infiltration temperature of 1,550 °C, which possesses superior mechanical properties compared with commercial reaction-sintered SiC (RS-SiC). The Vickers hardness, flexure strength, elastic modulus, and fracture toughness of the biomorphic SiC_w/SiC–Si composite were 18.8 ± 0.6 GPa, 354 ± 2 GPa, 450 ± 40 MPa, and 3.5 ± 0.3 MPa m^1/2, respectively. Whereas the composites obtained at the other two infiltration temperatures contain unreacted carbon and show lower mechanical properties. The high flexure strength of the biomorphic composite infiltrated at 1,550 °C is attributed to the dense structure and the reinforcement of the SiC whiskers. In addition, the fracture mechanism of the composite is also discussed.     

Fabrication of SiCf/SiC composites by chemical vapor infiltration and vapor silicon infiltration

Three-dimensional (3D) silicon carbide fiber reinforced silicon carbide matrix (SiC_f/SiC) composites, employing KD-1 SiC fibers (from National University of Defense Technology, China) as reinforcements, were fabricated by a combining chemical vapor infiltration (CVI) and vapor silicon infiltration (VSI) process. The microstructure and properties of the as prepared SiC_f/SiC composites were studied. The results show that the density and open porosity of the as prepared SiC_f/SiC composites are 2.1 g/cm³ and 7.7%, respectively. The SiC fibers are not severely damaged during the VSI process. And the SiC fibers adhere to the matrix with a weak interface, therefore the SiC_f/SiC composites exhibit non-catastrophic failure behavior with the flexural strength of 270 MPa, fracture toughness of 11.4 MPa·m^1/2 and shear strength of 25.7 MPa at ambient conditions. Moreover, the flexural strength decreases sharply at the temperature higher than 1200 °C. In addition, the thermal conductivity is 10.6 W/mk at room temperature.     

Interfacial control of uni-directional SiCf/SiC composites based on electrophoretic deposition and their mechanical properties

Interfacial control of uni-directional SiC_f/SiC composites were performed by EPD, and their mechanical properties at room temperature were evaluated. The effect of the thickness of carbon interphase on SiC fibers by EPD on mechanical properties of uni-directional SiC_f/SiC composites was also investigated. The average thickness of carbon coating on SiC fibers increased from 42 nm to 164 nm with an increase in the concentration of colloidal graphite suspension for EPD. Dense SiC_f/SiC composites were achieved and their fiber volume fraction was 47–51%. The SiC_f/SiC composites had a bending strength of 210–240 MPa. As the thickness of carbon coating was below 100 nm, the SiC_f/SiC composites (SC01 and SC02) fractured in almost brittle manner. In contrast, the SiC_f/SiC composites (SC03) showed a pseudo-ductile fracture behavior with a large number of fiber pullout as the thickness of carbon coating was above 100 nm. The fracture energy of SC03 was 3–4 times as high as those of SC01 and SC02 and the value was about 1.7 kJ/m². In consideration of the results of mechanical properties, the thickness of carbon coating on SiC fibers should be at least 100 nm to obtain high-performance SiC_f/SiC composites. The fabrication process based on EPD method is expected to be an effective way to control the interfaces of SiC_f/SiC composites and to obtain high-performance SiC_f/SiC composites.     

Microstructure, mechanical properties and reaction mechanism of KD-1 SiCf/SiC composites fabricated by chemical vapor infiltration and vapor silicon infiltration

Three-dimensional (3D) KD-1 silicon carbide fiber reinforced silicon carbide matrix (KD-1 SiC_f/SiC) composites were fabricated by a combining chemical vapor infiltration (CVI) and vapor silicon infiltration (VSI) process. The microstructure and mechanical properties of the resulting KD-1 SiC_f/SiC composites were studied. The results show that the resulting SiC_f/SiC composites have high bulk density and low open porosity (<6%). The mechanical properties of the resulting SiC_f/SiC composites firstly increase and then decrease with decreasing the open porosity of the SiC_f/C composites. The KD-1 SiC fibers were not severely deformed and adhered to the matrix with a weak interface during the VSI process. As a result, the composites exhibit non-catastrophic failure behavior. Additionally, the diffusion mechanism for the VSI process was also investigated in our work.     

Synthesis and microstructure of SrTiO<Subscript>3</Subscript> and BaTiO<Subscript>3</Subscript> ceramics by a reaction-sintering process

Strontium titanate and barium titanate ceramics prepared by a reaction-sintering process were investigated. The mixture of raw materials of stoichiometric SrTiO₃ and BaTiO₃ was pressed and sintered into ceramics without any calcination stage involved. A density 4.99 g/cm³ (97.5% of the theoretic value) was found in SrTiO₃ after 6 h sintering at 1,370 °C. Grains less than 1.5 μm were formed at 1,300–1,330 °C and became 2.2–3.3 μm at 1,350–1,370 °C SrTiO₃. A density 5.89 g/cm³ (97.9% of the theoretic value) was found in BaTiO₃ after 6 h sintering at 1,400 °C. Merged grains were observed in BaTiO₃ and were less than 10 μm after sintered at 1,400 °C.     

Wettability of Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript> by Ag–Cu and Ag–Cu–Ti melts

Recently, the ternary carbide Ti₃SiC₂ has gained much attention due to its unique characteristics combining the properties of metals and ceramics (i.e., a low density, decent thermal and electrical conductivities, an excellent thermal shock resistance, a good machinability, damage tolerance, low friction, and so on). This study describes an investigation of the wettability in high vacuum of bulk Ti₃SiC₂ by a classical braze alloy based on the Ag–Cu–Ti system. Two techniques, i.e., the sessile drop and dispensed drop methods, were utilized. The results indicated that spreading kinetics is controlled by deoxidation kinetics of Ti₃SiC₂ surface under vacuum. The final contact angle on clean Ti₃SiC₂ is very small (~10°), testifying the development of strong, metallic interactions across the liquid–solid interface. The reactivity between the ternary carbide and the liquid phase during isothermal heating at 800 °C was also considered.     

High-temperature behavior and degradation mechanism of SiC fibers annealed in Ar and N<Subscript>2</Subscript> atmospheres

The thermal and mechanical stability of SiC fibers at elevated temperature is an important property for the practical application of SiC fiber-reinforced ceramic matrix composites and is related to the heat-treating atmosphere. In this study, the high-temperature behavior of KD SiC fibers with low oxygen content was investigated in both Ar and N₂ at temperatures from 1400 to 1800 °C through scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Auger electron spectroscopy, resistivity measurements, and tensile tests in order to understand the effects of atmospheres on the degradation of the fibers. The results show that high-temperature treatment caused more severe strength degradation in Ar than in N₂. In particular, the fibers heat treated in N₂ at 1700 °C retained a relatively high strength of 1.52 GPa, 60 % of their original strength, while the fiber strength was completely lost after heat treatment in Ar. Fiber strength degradation was mainly caused by a combination of crystal growth and surface flaws. The formation of huge grains and porosity in the fiber surfaces, owing to the thermal decomposition of the SiC _x O _y N _z and SiC _x O _y phases, significantly degraded the strength for fibers heat treated in Ar. However, the suppressing effect of N₂ on the decomposition of the SiC _x O _y N _z phase in the fiber surfaces and nitrided case on the decomposition of the SiC _x O _y phase in the fiber cores, led to higher SiC fiber temperature stability in N₂ rather than Ar.     

Fabrication and mechanical properties of oriented SiC short-fiber-reinforced SiC composite by tape casting

SiC fiber-reinforced SiC composites with nearly unidirectionally and randomly aligned SiC short fiber were prepared by tape-casting and hot-pressing (HP). Volume fractions of fibers were 10 and 20 vol.%. Three-point bending test was carried out at room temperature. The SiC short-fiber-reinforced SiC composites showed completely brittle fracture for any fiber volume fraction and orientation. The maximum strength increased with increasing sintering temperature regardless of orientation of short fiber. In the unidirectionally and randomly aligned composites sintered at 1700 °C containing 20 vol.% fiber, the maximum bending strength was about 390 and 280 MPa, respectively.     

High-temperature oxidation behaviour of Ti<Subscript>3</Subscript>Si<Subscript>(1−<Emphasis Type="Italic">x</Emphasis>)</Subscript>Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>C<Subscript>2</Subscript> in air

The oxidation behaviours of bulk Ti₃Si_(1−x)Al _x C₂ prepared by hot pressing were investigated. The results showed that the isothermal oxidation behaviour of Ti₃SiC₂ obeyed a parabolic law between 900 and 1100°C, and followed a two-step parabolic rate law between 1200°C and 1300°C. The cyclic oxidation behaviour of material is assumed to obey a three-step parabolic rate law at 1100°C and 1200°C. The calculated activation energy of isothermal oxidation is 101·43 kJ·mol⁻¹. The oxide layers consisted of a mass of α-Al₂O₃ and little TiO₂ and SiO₂ are observed on Ti₃SiC₂ as a dense and adhesive protect scale. The oxidation mechanism varies with the additive aluminum that greatly improves the oxidation resistance of Ti₃SiC₂.     

Effect of hot pressing time and temperature on the microstructure and mechanical properties of ZrB<Subscript>2</Subscript>–SiC

Structure–property relations were examined for ZrB₂ containing 30 volume percent SiC particulates. Two grades of ZrB₂ with initial particle sizes of 2 and 6 μm were used. Billets of ZrB₂–SiC were produced by hot pressing at 1850, 1950 or 2050 °C for 45 min. In addition, the material prepared from ZrB₂ with an initial particle size of 2 μm was hot pressed at 2050 °C for 90 and 180 min. Microstructures and mechanical properties were characterized to determine the effects of the initial particle size, hot pressing time, and hot pressing temperature on the final grain size and morphology. The average grain size of the ZrB₂ phase ranged from 2.2 to 4.7 μm. Similarly, the average grain size of the SiC phase ranged from 1.2 to 2.7 μm. Hardness and modulus of elasticity were not affected by the processing conditions with average values of 22 and 505 GPa, respectively. However, flexural strength decreased as grain size increased from a maximum of ∼1050 MPa for the finest grain sizes to ∼700 MPa for the largest grain sizes. Analysis suggested that the strength of ZrB₂–SiC was limited by the size of the SiC inclusions in the ZrB₂ matrix.     

Effect of replacement of MgO by CaO on sintering,crystallization and properties of MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> system glass-ceramics

The effects of replacement of MgO by CaO on the sintering and crystallization behavior of MgO–Al₂O₃–SiO₂ system glass-ceramics were investigated. The results show that with increasing CaO content, the glass transition temperature firstly increased and then decreased, the melting temperature was lowered and the crystallization temperature of the glass-ceramics shifted clearly towards higher temperatures. With the replacement of MgO by less than 3 wt.% CaO, the predominant crystalline phase in the glass-ceramics fired at 900 °C was found to be α-cordierite and the secondary crystalline phase to be μ-cordierite. When the replacement was increased to 10 wt.%, the predominant crystalline phase was found to be anorthite and the secondary phase to be α-cordierite. Both thermal expansion coefficient (TCE) and dielectric constant of samples increases with the replacement of MgO by CaO. The dielectric loss of sample with 5 wt.% CaO fired at 900 °C has the lowest value of 0.08%. Only the sample containing 5 wt.% and10 wt.% CaO (abbreviated as sample C5 and C10) can be fully sintered before 900 °C. Therefore, a dense and low dielectric loss glass-ceramic with predominant crystal phase of α-cordierite and some amount of anorthite was achieved by using fine glass powders (D₅₀ = 3 μm) fired at 875–900 °C. The as-sintered density approaches 98% theoretical density. The flexural strength of sample C5 firstly increases and then decreases with sintering temperature, which closely corresponds to its relative density. The TCE of sample C5 increases with increasing temperature. The dielectric property of sample C5 sintered at different temperatures depends on not only its relative density but also its crystalline phases. The dense and crystallized glass-ceramic C5 exhibits a low sintering temperature (≤900 °C), a fairly low dielectric constant (5.2–5.3), a low dielectric loss (≤10⁻³) at 1 MHz, a low TCE (4.0–4.25 × 10⁻⁶ K⁻¹), very close to that of Si (∼3.5 × 10⁻⁶ K⁻¹), and a higher flexural strength (≥134 MPa), suggesting that it would be a promising material in the electronic packaging field.     

Microwave synthesis and sintering characteristics of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript>

CaCu₃Ti₄O₁₂ (CCTO) was synthesized and sintered by microwave processing at 2·45 GHz, 1·1 kW. The optimum calcination temperature using microwave heating was determined to be 950°C for 20 min to obtain cubic CCTO powders. The microwave processed powders were sintered to 94% density at 1000°C/60 min. The microstructural studies carried out on these ceramics revealed the grain size to be in the range 1–7 μm. The dielectric constants for the microwave sintered (1000°C/60 min) ceramics were found to vary from 11000–7700 in the 100 Hz–00 kHz frequency range. Interestingly the dielectric loss had lower values than those sintered by conventional sintering routes and decreases with increase in frequency.     

Effects of amount and type of cation species on the phase formation of single and codoped α-SiAlONs

The phase formation of single phase α-SiAlON ceramics based on α-Si₃N₄ was investigated using different rare earth (RE) or metallic cations to stabilise the α-phase. The effects of sintering temperature, size and type of stabilising cation and the addition of extra liquid forming additives on the phase transformation of α-Si₃N₄ to α-SiAlON were considered. The influence of the chosen parameters on properties such as density and hardness is furthermore discussed for Ca-codoped silicon aluminium oxynitride (SiAlON) ceramics (50% RE cations combined with 50% calcium cations). For smaller and larger RE systems different intermediate phases occur and the dissolution of sintering additives takes place at different temperatures, resulting in different amounts of SiAlON at a given sintering temperature. For smaller cations, the SiAlON formation is favoured below 1,750 °C compared to larger cations. The addition of extra liquid to the starting composition supports the SiAlON formation above the eutectic temperature. Contrary to the RE systems the calcium-doped samples did not show an intermediate phase during sintering. The cation solubility for sintering additives is higher and therefore the amount of SiAlON created below 1,750 °C increased. The final amount of SiAlON at 1,850 °C was about the same for all systems. Mechanical properties are also influenced by the composition of the starting powder. In general, increasing temperature leads to higher density and hardness.     

Production and mechanical properties of SiC<Subscript>p</Subscript> particle-reinforced 2618 aluminum alloy composites

In this study, 2618 aluminum alloy metal matrix composites (MMCs) reinforced with two different sizes and weight fractions of SiC_p particles upto 10% weight were fabricated by stir cast method and subsequent forging operation. The effects of SiC_p particle content and size of the particles on the mechanical properties of the composites such as hardness, tensile strength, hot tensile strength (at 120 °C), and impact strength were investigated. The density measurements showed that the samples contained little porosity with increasing weight fraction. Optical microscopic observations of the microstructures revealed uniform distribution of particles and at some locations agglomeration of particles and porosity. The results show that hardness and tensile strength of the composites increased, with decreasing size and increasing weight fraction of the particles. The hardness and tensile strength of the forged composites were higher than those of the cast samples.     

In situ boron carbide–titanium diboride composites prepared by mechanical milling and subsequent Spark Plasma Sintering

Boron carbide–titanium diboride composites were synthesized and consolidated by Spark Plasma Sintering (SPS) of mechanically milled elemental powder mixtures. The phase and microstructure evolution of the composites during sintering in the 1,200–1,700 °C temperature range was studied. With increasing sintering temperature, the phase formation of the samples was completed well before full density was achieved. The distribution of titanium diboride in the sintered samples was significantly improved with increasing milling time of the Ti–B–C powder mixtures. A bulk composite material of nearly full density, fine uniform microstructure, and increased fracture toughness was obtained by SPS at 1,700 °C. The grain size of boron carbide and titanium diboride in this material was 5–7 and 1–2 μm, respectively.     

Optimization of the Carbosilicothermic Synthesis of the Ti<Subscript>4</Subscript>SiC<Subscript>3</Subscript> MAX Phase

We have studied the formation of the Ti4SiC3 MAX phase during the vacuum carbosilicothermic reduction of TiO₂ with a combined reducing agent consisting of SiC and elemental Si and analyzed the effects of the synthesis temperature, heat treatment time, and percentage of elemental silicon in the starting mixture on the Ti₄SiC₃ yield. Optimal Ti₄SiC₃ synthesis conditions are as follows: temperature from 1550 to 1650°C, isothermal holding time of 360 min, and the starting-mixture composition TiO₂ + 1.2SiC + 0.6Si. The Ti₄SiC₃ yield then reaches 92 wt %.     

Low-temperature sintering of SiC reticulated porous ceramics with MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> additives as sintering aids

SiC reticulated porous ceramics (SiC RPCs) was fabricated with polymer replicas method by using MgO–Al₂O₃–SiO₂ additives as sintering aids at 1,000∼1,450 °C. The MgO–Al₂O₃–SiO₂ additives were from alumina, kaolin and Talc powders. By employing various experimental techniques, zeta potential, viscosity and rheological measurements, the dispersion of mixed powders (SiC, Al₂O₃, talc and kaolin) in aqueous media using silica sol as a binder was studied. The pH value of the optimum dispersion was found to be around pH 10 for the mixtures. The optimum condition of the slurry suitable for impregnating the polymeric sponge was obtained. At the same time, the influence of the sintering temperature and holding time on the properties of SiC RPCs was investigated. According to the properties of SiC RPCs, the optimal sintering temperature was chosen at 1,300 °C, which was lower than that with Al₂O₃–SiO₂ additives as sintering aids.