Effect of carbon content on electrical,thermal, and mechanical properties of porous SiC ceramics with B4C and C additives

The electrical, thermal, and mechanical properties of porous SiC ceramics with B₄C-C additives were investigated as functions of C content and sintering temperature. The electrical resistivity of porous SiC ceramics decreased with increases in C content and sintering temperature. A minimal electrical resistivity of 4.6 × 10^?2 Ω·cm was obtained in porous SiC ceramics with 1 wt% B₄C and 10 wt% C. The thermal conductivity and flexural strength increased with increasing sintering temperature and showed maxima at 4 wt% C addition when sintered at 2000 °C and 2100 °C. The thermal conductivity and flexural strength of porous SiC ceramics can be tuned independently from the porosity by controlling C content and sintering temperature. Typical electrical resistivity, thermal conductivity, and flexural strength of porous SiC ceramics with 1 wt% B₄C-4 wt% C sintered at 2100 °C were 1.3 × 10^?1 Ω·cm, 76.0 W/(m·K), and 110.3 MPa, respectively.     

Thermal and electrical properties of additive-free rapidly hot-pressed SiC ceramics

The thermal and electrical properties of newly developed additive free SiC ceramics processed at a temperature as low as 1850 °C (RHP0) and SiC ceramics with 0.79 vol.% Y₂O₃-Sc₂O₃ additives (RHP79) were investigated and compared with those of the chemically vapor-deposited SiC (CVD-SiC) reference material. The additive free RHP0 showed a very high thermal conductivity, as high as 164 Wm⁻¹ K⁻¹, and a low electrical resistivity of 1.2 × 10⁻¹ Ω cm at room temperature (RT), which are the highest thermal conductivity and the lowest electrical resistivity yet seen in sintered SiC ceramics processed at ≤1900 °C. The thermal conductivity and electrical resistivity values of RHP79 were 117 Wm⁻¹ K⁻¹ and 9.5 × 10⁻² Ω cm, respectively. The thermal and electrical conductivities of CVD-SiC parallel to the direction of growth were ∼324 Wm⁻¹ K⁻¹ and ∼5 × 10⁻⁴Ω⁻¹ cm⁻¹ at RT, respectively.     

Electrical and Thermal Properties of SiC Ceramics Sintered with Yttria and Nitrides

The electrical and thermal properties of SiC ceramics containing 1 vol% nitrides (BN, AlN or TiN) were investigated with 2 vol% Y₂O₃ addition as a sintering additive. The AlN‐added SiC specimen exhibited an electrical resistivity (3.8 × 10¹ Ω·cm) that is larger by a factor of ~10² compared to that (1.3 × 10^?1 Ω·cm) of a baseline specimen sintered with Y₂O₃ only. On the other hand, BN‐ or TiN‐added SiC specimens exhibited resistivity that is lower than that of the baseline specimen by a factor of 10^?1. The addition of 1 vol% BN or AlN led to a decrease in the thermal conductivity of SiC from 178 W/m·K (baseline) to 99 W/m·K or 133 W/m·K, respectively. The electrical resistivity and thermal conductivity of the TiN‐added SiC specimen were 1.6 × 10^?2 Ω·cm and 211 W/m·K at room temperature, respectively. The present results suggest that the electrical and thermal properties of SiC ceramics are controllable by adding a small amount of nitrides.     

Microstructural,electrical, and mechanical properties of conductive SiC ceramics fabricated by spark plasma sintering

Nitrogen (N)-doped conductive silicon carbide (SiC) of various electrical resistivity grades can satisfy diverse requirements in engineering applications. To understand the mechanisms that determine the electrical resistivity of N-doped conductive SiC ceramics during the fast spark plasma sintering (SPS) process, SiC ceramics were synthesized using SPS in an N₂ atmosphere with SiC powder and traditional Al₂O₃–Y₂O₃ additive as raw materials at a sintering temperature of 1850–2000°C for 1–10 min. The electrical resistivity was successfully varied over a wide range of 10⁻³–10¹ Ω cm by modifying the sintering conditions. The SPS-SiC ceramics consisted of mainly Y–Al–Si–O–C–N glass phase and N-doped SiC. The Y–Al–Si–O–C–N glass phase decomposed to an Si-rich phase and N-doped Y_xSi_yC_z at 2000°C. The Vickers hardness, elastic modulus, and fracture toughness of the SPS-SiC ceramics varied within the ranges of 14.35–25.12 GPa, 310.97–400.12 GPa, and 2.46–5.39 MPa m^1/2, respectively. The electrical resistivity of the obtained SPS-SiC ceramics was primarily determined by their carrier mobility.     

Mechanical and Thermal Properties of Pressureless Sintered Silicon Carbide Ceramics with Alumina–Yttria–Calcia

The effect of sintering temperature on the mechanical and thermal properties of SiC ceramics sintered with Al₂O₃–Y₂O₃–CaO without applied pressure was investigated. SiC ceramics containing A₂O₃–Y₂O₃–CaO as sintering additives can be sintered to >97% theoretical density at temperatures between 1750°C and 1900°C without applied pressure. A toughened microstructure, consisting of relatively large elongated grains and relatively small equiaxed grains, has been obtained when sintered at temperatures as low as 1800°C for 2 h in an argon atmosphere without applied pressure. The achievement of toughened microstructures under such mild conditions is the result of the additive composition. The thermal conductivity of the SiC ceramics increased with increasing sintering temperature because of the decrease in the lattice oxygen content of the SiC grains. Typical sintered density, flexural strength, fracture toughness, hardness, and thermal conductivity of the 1850°C‐sintered SiC, which consisted of 62.2% 4H, 35.7% 6H, and 2.1% 3C, were 99.0%, 628 MPa, 5.3 MPa·m^1/2, 29.1 GPa, and 80 W·(m·K)^?1, respectively.     

Effects of porosity on electrical and thermal conductivities of porous SiC ceramics

The effects of porosity on the electrical and thermal conductivities of porous SiC ceramics, containing Y₂O₃–AlN additives, were investigated. The porosity of the porous SiC ceramic could be controlled in the range of 28–64 % by adjusting the sacrificial template (polymer microbead) content (0–30 wt%) and sintering temperature (1800–2000 °C). Both electrical and thermal conductivities of the porous SiC ceramics decreased, from 7.7 to 1.7 Ω⁻¹ cm⁻¹ and from 37.9 to 5.8 W/(m·K), respectively, with the increase in porosity from 30 to 63 %. The porous SiC ceramic with a coarser microstructure exhibited higher electrical and thermal conductivities than those of the ceramic with a finer microstructure at the equivalent porosity because of the smaller number of grain boundaries per unit volume. The decoupling of the electrical conductivity from the thermal conductivity was possible to some extent by adjusting the sintering temperature, i.e., microstructure, of the porous SiC ceramic.     

Effect of Ni content and its particle size on electrical resistivity and flexural strength of porous SiC ceramic sintered at low-temperature using clay additive

A low-temperature sintered porous SiC-based clay-Ni system with controlled electrical resistivity (2.54 × 10¹⁰ Ω cm to 2 Ω cm), and thermal conductivity (3.5 W/m. K to 12.6 W/m. K) was successfully designed. Clay (20 wt% kaolin) was used as a sintering additive in all the compositions. The electrical resistivity, and thermal conductivity was controlled by varying the Ni content (0–25 wt%) in the samples. The electrical resistivity was recorded as low as 2 Ω cm with 25 wt% Ni that was sintered at 1400 °C in argon. The interface reaction between Ni and SiC formed conductive nickel silicide (Ni₂Si), while the transformation of kaolin to mullite strengthened the mechanical properties. Submicron-sized Ni (0.3 μm) was more effective than micron-sized Ni (3.5 μm) in reducing the electrical resistivity, and increasing the thermal conductivity along with flexural strength. A comparative study of sintering temperatures showed that 1400 °C resulted in the lowest electrical resistivity (2 Ω cm) and the highest thermal conductivity of 12.6 W/m. K with flexural strength of 54 MPa at 32% porosity in the SiC-kaolin-Ni system.     

Direct bonding of silicon carbide ceramics sintered with yttria

SiC ceramics sintered with yttria were successfully joined without an interlayer by conventional hot pressing at lower temperatures (2000–2050 °C) compared to those of the sintering temperatures (2050–2200 °C). The joined SiC ceramics sintered with 2000 ppm Y₂O₃ showed almost the same thermal conductivity (˜198 Wm⁻¹ K⁻¹), fracture toughness (3.7 ± 0.2 MPa m^1/2), and hardness (23.4 ± 0.8 GPa) as those of the base material, as well as excellent flexural strength (449 MPa). In contrast, the joined SiC ceramics sintered with 4 wt% Y₂O₃ showed very high thermal conductivity (˜204 Wm⁻¹ K⁻¹) and excellent flexural strength (˜505 MPa). Approximately 16–22% decreases in strength compared to those of the base SC materials were observed in both joined ceramics, due to the segregation of liquid phase at the interface. This issue might be overcome by preparing well-polished and highly flat surfaces before joining.     

Microstructure,Ferroelectric, Piezoelectric,and Ferromagnetic Properties of Sc‐Modified BiFeO3–BaTiO3 Multiferroic Ceramics with MnO2 Addition

0.725BiFe_1?xSc_xO₃–0.275BaTiO₃ + y mol% MnO₂ multiferroic ceramics were fabricated by a conventional ceramic technique and the effects of Sc doping and sintering temperature on microstructure, multiferroic, and piezoelectric properties of the ceramics were studied. The ceramics can be well sintered at the wide low sintering temperature range 930°C–990°C and possess a pure perovskite structure. The ceramics with x/y = 0.01–0.02/1.0 sintered at 960°C possess high resistivity (~2 × 10⁹ Ω·cm), strong ferroelectricity (P_r = 19.1–20.4 μm/cm²), good piezoelectric properties (d₃₃ = 127–128 pC/N, k_p = 36.6%–36.9%), and very high Curie temperature (618°C–636°C). The increase in sintering temperature improves the densification, electric insulation, ferroelectric, and piezoelectric properties of the ceramics. A small amount of Sc doping (x ≤ 0.04) and the increase in the sintering temperature significantly enhance the ferromagnetic properties of the ceramics. Improved ferromagnetism with remnant magnetization M_r of 0.059 and 0.10 emu/g and coercive field H_c of 2.51 and 2.76 kOe are obtained in the ceramics with x/y = 0.04/1.0 (sintered at 960°C) and 0.02/1.0 (sintered at 1050°C), respectively. Because of the high T_C (636°C), the ceramic with x/y = 0.02/1.0 shows good temperature stability of piezoelectric properties. Our results also show that the addition of MnO₂ is essential to obtain the ceramics with good electrical properties and electric insulation.     

Development of spark plasma sintered conductive SiC–TiB2 composites for electrical discharge machining applications

Highly dense electrically conductive silicon carbide (SiC)–(0, 10, 20, and 30 vol%) titanium boride (TiB₂) composites with 10 vol% of Y₂O₃–AlN additives were fabricated at a relatively low temperature of 1800°C by spark plasma sintering in nitrogen atmosphere. Phase analysis of sintered composites reveals suppressed β→α phase transformation due to low sintering temperature, nitride additives, and nitrogen sintering atmosphere. With increase in TiB₂ content, hardness increased from 20.6 to 23.7 GPa and fracture toughness increased from 3.6 to 5.5 MPa m^1/2. The electrical conductivity increased to a remarkable 2.72 × 10³ (Ω cm)^–1 for SiC–30 vol% TiB₂ composites due to large amount of conductive reinforcement, additive composition, and sintering in nitrogen atmosphere. The successful electrical discharge machining illustrates potential of the sintered SiC–TiB₂ composites toward extending the application regime of conventional SiC-based ceramics.     

Electrical,thermal, and mechanical properties of porous silicon carbide ceramics with a boron carbide additive

The effects of the boron carbide (B₄C) content and sintering atmosphere on the electrical, thermal, and mechanical properties of porous silicon carbide (SiC) ceramics were investigated in the porosity range of 58.3%–70.3%. The electrical resistivities of the nitrogen-sintered porous SiC ceramics (∼10^–1 Ω·cm) were two orders of magnitude lower than those of argon-sintered porous SiC ceramics (∼10¹ Ω·cm). Both the thermal conductivities (3.3–19.8 W·m^–1·K^–1) and flexural strengths (8.1–32.9 MPa) of the argon- and nitrogen-sintered porous SiC ceramics increased as the B₄C content increased, owing to the decreased porosity and increased necking area between SiC grains. The electrical resistivity of the porous SiC ceramics was primarily controlled by the sintering atmosphere owing to the N-doping from the nitrogen atmosphere, and secondarily by the B₄C content, owing to the B-doping from the B₄C. In contrast, the thermal conductivity and flexural strength were dependent on both the porosity and necking area, as influenced by both the sintering atmosphere and B₄C content. These results suggest that it is possible to decouple the electrical resistivity from the thermal conductivity by judicious selection of the B₄C content and sintering atmosphere.     

Effect of graphite and Mn3O4 on clay-bonded SiC ceramics for the production of electrically conductive heatable filter

Electrically conductive porous SiC ceramics are attracting substantial attention due to their application in heatable filters, vacuum chuck, and semiconductor processing parts, etc. The main problem is their high processing cost. Ideal candidates from an engineering ceramic perspective will be mechanically durable and have the required electrical properties with sufficiently low fabrication costs. To decrease the sintering temperature, kaolin has been added, but it tended to render the material an insulator. Graphite was used to effectively decrease the electrical resistivity. Additionally, manganese oxide was used to decrease the quantity of kaolin (the component that leads to an insulator material after sintering) and decrease the electrical resistivity while maintaining the mechanical properties. In our study, we found that SiC with 35% kaolin, 20% graphite and 10% manganese oxide can produce samples with 6.5 × 10^?1 Ω cm electrical resistivity and 43.5 MPa flexural strength at a low sintering temperature of 1200 °C.     

Effect of sintering temperature on electrical properties of SiC/ZrB2 ceramics

SiC/20?wt% ZrB₂ composite ceramics were fabricated via pressureless solid phase sintering in argon atmosphere at different temperature. The effect of sintering temperature on microstructure, electrical properties and mechanical properties of SiC/ZrB₂ ceramics was investigated. Electrical resistivity exhibits twice significant decreases with increasing sintering temperature. The first decrease from 1900?°C to 2000?°C is attributed to the obvious decrease of continuous pore channels in as-sintered materials. The second decrease from 2100?°C to 2200?°C results from the improvement of carbon crystallization and the disappearance of amorphous layers enveloping ZrB₂ grains. Additionally, the increase of sintered density with increasing temperature caused greatly advance of flexural strength, elastic modulus and Vickers hardness. But excessive temperature is detrimental to flexural strength because of SiC grain growth.     

Mechanical and thermal properties of silicon carbide ceramics with yttria–scandia–magnesia

Two different SiC ceramics with a new additive composition (1.87 wt% Y₂O₃–Sc₂O₃–MgO) were developed as matrix materials for fully ceramic microencapsulated fuels. The mechanical and thermal properties of the newly developed SiC ceramics with the new additive system were investigated. Powder mixtures prepared from the additives were sintered at 1850 °C under an applied pressure of 30 MPa for 2 h in an argon or nitrogen atmosphere. We observed that both samples could be sintered to ≥99.9% of the theoretical density. The SiC ceramic sintered in argon exhibited higher toughness and thermal conductivity and lower flexural strength than the sample sintered in nitrogen. The flexural strength, fracture toughness, Vickers hardness, and thermal conductivity values of the SiC ceramics sintered in nitrogen were 1077 ± 46 MPa, 4.3 ± 0.3 MPa·m^1/2, 25.4 ± 1.2 GPa, and 99 Wm⁻¹ K⁻¹ at room temperature, respectively.     

Pressureless sintering of SiC ceramics with improved specific stiffness

The effects of B₄C content on the specific stiffness and mechanical and thermal properties of pressureless-sintered SiC ceramics were investigated. SiC ceramics containing 2.5 wt% C and 0.7–20 wt% B₄C as sintering aids could be sintered to ≥ 99.4% of the theoretical density at 2150 °C for 1 h in Ar. The specific stiffness of SiC ceramics increased from 136.1 × 10⁶ to 144.4 × 10⁶ m²‧s⁻² when the B₄C content was increased from 0.7 to 20 wt%. The flexural strength and fracture toughness of the SiC ceramics were maximal with the incorporation of 10 wt% B₄C (558 MPa and 3.69 MPa‧m^1/2, respectively), while the thermal conductivity decreased from ∼154 to ∼83 W‧m⁻¹‧K⁻¹ when the B₄C content was increased from 0.7 to 30 wt%. The flexural strength and thermal conductivity of the developed SiC ceramic containing 20 wt% B₄C were ∼346 MPa and ∼105 W‧m⁻¹‧K⁻¹, respectively.     

Controllable preparation of porous ZrB2–SiC ceramics via using KCl space holders

In this work, a novel and facile technique based on using KCl as space holders, along with partial sintering (at 1900 °C for 30 min), was explored to prepare porous ZrB₂–SiC ceramics with controllable pore structure, tunable compressive strength and thermal conductivity. The as-prepared porous ZrB₂–SiC samples possess high porosity of 45–67%, low average pore size of 3–7 μm, high compressive strength of 32–106 MPa, and low room temperature thermal conductivity of 13–34 W m⁻¹ K⁻¹. The porosity, pore structure, compressive strength and thermal conductivity of porous ZrB₂–SiC ceramics can be tuned simply by changing KCl content and its particle size. The effect of porosity and pore structure on the thermal conductivity of as-prepared porous ZrB₂–SiC ceramics was examined and found to be consistent with the classical model for porous materials. The poring mechanism of porous ZrB₂–SiC samples via adding pore-forming agent combined with partial sintering was also preliminary illustrated.     

Novel thermal-sensitive properties of NBT-BZT composite ceramics for high-temperature NTC thermistors

We found for the first time that (1 − x) Na_0.5Bi_0.5TiO₃-xBiZn_0.5Ti_0.5O₃ (NBT–BZT) composite ceramics showed negative temperature coefficient (NTC) at a high temperature. The NBT–BZT nanopowders were successfully prepared by Pechini method. Their ceramics were sintered at 1100°C. The NBT–BZT ceramic exhibited a good linear relationship between logarithm of electrical resistivity (Inρ) and reciprocal of absolute temperature (1000/T) at 250°C–1050°C. The obtained ρ₆₀₀, ρ₉₀₀, and B_600/900 constants of the NBT–ZBT NTC thermistors are approximately 5.92 × 10⁶ to 3.01 × 10⁴ Ω cm, 7.03 × 10³ to 7.60 × 10² Ω cm and 2.3 × 10⁴-1.3 × 10⁴ K, respectively. The electrical characteristics can be tuned to the desired value by changing the Na_0.5Bi_0.5TiO₃ content in the compound. The electrical conductivity in these compounds is due to the electron jumps between Ti³⁺ and Ti⁴⁺ and oxygen-ion conductivity. Results demonstrate a tremendous potential of the studied system for perovskite materials with NTC performance.     

Effects of Y2O3–RE2O3 (RE = Sm,Gd, Lu) Additives on Electrical and Thermal Properties of Silicon Carbide Ceramics

In this study, we investigated the electrical and thermal properties of SiC ceramics with 2 vol% equimolar Y₂O₃–RE₂O₃ (RE = Sm, Gd, Lu) additives. The three SiC ceramics with 2 vol% equimolar Y₂O₃–RE₂O₃ additives showed electrical conductivities on the order of ~10³ (Ω·m)^?1, which is one order of magnitude higher than that of the SiC ceramics sintered with 2 vol% Y₂O₃ only. The increase in electrical conductivity is attributed to the growth of heavily nitrogen‐doped SiC grains during sintering and the confinement of oxide additives in the junction area. The thermal conductivities of the SiC ceramics were in the 176–198 W·(m·K)^?1 range at room temperature. The new additive systems, equimolar Y₂O₃–RE₂O₃, are beneficial for achieving both high electrical conductivity and high thermal conductivity in SiC ceramics.     

Final-stage densification kinetics of direct current–sintered ZrB2

Final-stage sintering was analyzed for nominally phase pure zirconium diboride synthesized by borothermal reduction of high-purity ZrO₂. Analysis was conducted on ZrB₂ ceramics with relative densities greater than 90% using the Nabarro–Herring stress–directed vacancy diffusion model. Temperatures of 1900°C or above and an applied uniaxial pressure of 50 MPa were required to fully densify ZrB₂ ceramics by direct current sintering. Ram travel data were collected and used to determine the relative density of the specimens during sintering. Specimens sintered between 1900 and 2100°C achieved relative densities greater than 97%, whereas specimens sintered below 1900°C failed to reach the final stage of sintering. The average grain size ranged from 1.0 to 14.7 μm. The activation energy was calculated from the slope of an Arrhenius plot that used the Kalish equation. The activation energy was 162 ± 34 kJ/mol, which is consistent with the activation energy for dislocation movement in ZrB₂. The diffusion coefficients for dislocation motion that controls densification were 5.1 × 10⁻⁶ cm²/s at 1900°C and 5.1 × 10⁻⁵ cm²/s at 2100°C, as calculated from activation energy and average grain sizes. This study provides evidence that the dominant mechanism for final-stage sintering of ZrB₂ ceramics is dislocation motion.     

Fabrication and properties of macro-porous SiC using Al2O3–Y2O3–SiO2 as bonding additives

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.