Effects of oxidation cross-linking and sintering additives (TiN,B) on the formation and heat-resistant performance of polymer-derived SiC(Ti,B) films

Effects of oxidation cross-linking and sintering additives (TiN, B) on the microstructure formation and heat-resistant performance of freestanding SiC(Ti, B) films synthesized from Ti, B-containing polycarbosilane (TiB-PCS) precursor were investigated. TiB-PCS green films were first cross-linked for 1 h, 2 h, 3 h and 4 h, respectively, and then pre-sintered at 950 °C. Finally, they were sintered at 1800 °C to complete the conversion from organic films to inorganic SiC(Ti, B) films. The results reveal that curing time has a great impact on the uniformity and density of SiC(Ti, B) films. TiB-PCS films cured for 3 h yield the best quality SiC(Ti, B) films, which are composed of β-SiC crystals, C clusters, α-SiC nano-crystals, a small amount of TiB₂ and B₄C. TiB₂ and B₄C are both steady phases which can inhibit abnormal growth of β-SiC, effectively reduce sintering temperature and help consume excess C from decomposition of amorphous SiOxCy. After high temperature annealing at 1500 °C, 1600 °C and 1700 °C in argon, SiC(Ti, B) films still keep excellent mechanical properties, which makes them attractive candidate materials for microelectromechanical systems (MEMS) used at ultra-high temperatures (exceeding 1500 °C).     

Facile synthesis of SiOx spheres or dumbbell-shaped β-SiC whiskers on expanded graphite by silicon vapor deposition

A facile method was developed to synthesize SiO_x spheres or dumbbell-shaped β-SiC whiskers on expanded graphite (SiO_x/EG or β-SiC/EG) by silicon vapor deposition without catalyst. With the carbon black atmosphere, the above hybrids were synthesized above 1100 °C in a graphite crucible where silicon powder was placed under the expanded graphite (EG). The growth of SiO_x spheres is controlled by vapor-solid mechanism at 1100 °C and 1200 °C. Namely, the active carbon atoms absorbed SiO (g) and Si (g) to form SiC nuclei. Then, the SiO₂, residual SiO (g) and Si (g) deposited on SiC nuclei to form SiO_x spheres. At 1300 °C and 1400 °C, the same SiO_x spheres formed on EG as well as many dumbbell-shaped β-SiC whiskers. The growth of dumbbell-shaped β-SiC whiskers is controlled by vapor-vapor and vapor-solid mechanism successively. In a word, firstly, the β-SiC whiskers with defects formed via the reaction between Si (g) and CO (g). After that, the SiO₂, residual SiO (g) and residual Si (g) preferentially deposited on defects, then deposited on other parts of β-SiC whiskers to form dumbbell-shaped SiC whiskers.     

Synthesis and characterization of continuous freestanding silicon carbide films with polycarbosilane (PCS)

A technique based on melt spinning of precursor was introduced to produce continuous freestanding SiC films. An equipment including spinneret, mandril, tank and seal groove was designed and manufactured for melt spinning. The polycarbosilane (PCS) precursors were deaerated, melt spun, crosslinked (by oxidation or irradiation), and pyrolyzed at high temperature in order to convert the initial PCS into freestanding SiC films. Our results revealed that the continuous freestanding SiC films, approximately 8 μm to 190 μm in thickness depended on setting, were uniform and dense. Their microstructure consisted of amorphous SiOxCy, β-SiC nano-crystals and free carbon. The photoluminescence (PL) spectrum showed two blue emissions at 416 nm and 435 nm. The continuous freestanding SiC films with high modulus, high density, high surface hardness and optoelectronic properties may have potential applications in microelectromechanical systems (MEMS), advanced optoelectronic devices and such complex-shaped materials.     

Improved oxidation resistance of expanded graphite through nano SiC coating

Expanded graphite with nano SiC and amorphous SiC_xO_y coating was successfully prepared through pyrolysing silane coupling agent (SCA), where the grafting of SCA dominated the final products. The results show that mainly amorphous SiC_xO_y coating covers expanded graphite at 1000 °C, regardless of the SCA concentration. In comparison, nano SiC coating can be synthesized at 1200 °C depending on the good dispersion of SCA (with a SCA concentration of 50 vol%). The formed SiC coating contributes to much higher peak oxidation temperature (812.1 °C) than 678.0 °C of the pure expanded graphite. Meanwhile, the oxidation activation energies of expanded graphite are remarkably improved from 149.15 kJ/mol to 176.16 kJ/mol (based on Kissinger method), attributing to the derived nano SiC and SiC_xO_y coating.     

Synthesis and properties of AlN/MAS/Si3N4 ternary glass-ceramic composites with in-situ grown rod-like β-Si3N4 crystals

The AlN/MAS/Si₃N₄ ternary composites with in-situ grown rod-like β-Si₃N₄ were obtained by a two-step sintering process. The microstructure analysis, compositional investigation as well as properties characterization have been systematically performed. The AlN/MAS/Si₃N₄ ternary composites can be densified at 1650 °C in nitrogen atmosphere. The in-situ grown rod-like β-Si₃N₄ grains are beneficial to the improvement of thermal, mechanical, and dielectric properties. The thermal conductivity of the composites was increased from 14.85 to 28.45 W/(m K) by incorporating 25 wt% α-Si₃N₄. The microstructural characterization shows that the in-situ growth of rod-like β-Si₃N₄ crystals leads to high thermal conductivity. The AlN/MAS/Si₃N₄ ternary composite with the highest thermal conductivity shows a low relative dielectric constant of 6.2, a low dielectric loss of 0.0017, a high bending strength of 325 MPa, a high fracture toughness of 4.1 MPa m^1/2, and a low thermal expansion coefficient (α_{25–300 °C}) of 5.11 × 10^?6/K. This ternary composite with excellent comprehensive performance is expected to be used in high-performance electronic packaging materials.     

Process-tolerant pressureless-sintered silicon carbide ceramics with alumina-yttria-calcia-strontia

Process-tolerant SiC ceramics were prepared by pressureless sintering at 1850–1950 °C for 2 h in an argon atmosphere with a new quaternary additive (Al₂O₃-Y₂O₃-CaO-SrO). The SiC ceramics can be sintered to a > 94% theoretical density at 1800–1950 °C by pressureless sintering. Toughened microstructures consisting of relatively large platelet grains and small equiaxed grains were obtained when SiC ceramics were sintered at 1850–1950 °C. The presently fabricated SiC ceramics showed little variability of the microstructure and mechanical properties with sintering within the temperature range of 1850–1950 °C, demonstrating process-tolerant behavior. The thermal conductivity of the SiC ceramics increased with increasing sintering temperature from 1800 °C to 1900 °C due to decreases of the lattice oxygen content of the SiC grains and residual porosity. The flexural strength, fracture toughness, and thermal conductivity of the SiC ceramics sintered at 1850–1950 °C were in the ranges of 444–457 MPa, 4.9–5.0 MPa m^1/2, and 76–82 Wm^?1 K^?1, respectively.     

Phase composition,crystal structure and microwave dielectric properties of Mg2−xCuxSiO4 ceramics

In this work, the Mg_2-xCu_xSiO₄(x = 0–0.40) microwave dielectric ceramics were prepared using solid-state reaction method. Compared with the Mg₂SiO₄ sample, the Cu-substituted Mg samples could be sintered at a lower temperature. The Mg_2?xCu_xSiO₄ ceramics exhibit the composite phases of Mg₂SiO₄ and a small quantity of MgSiO₃. The Cu²⁺ ion presented a solid solution with the Mg₂SiO₄ phase and preferentially occupy Mg(1) site. The distortion of MgO₆ octahedron was modified by Cu²⁺ ions, resulting in a positive change in the temperature coefficient of resonance frequency (τ_f) values. Excellent microwave dielectric properties of ε_r = 6.35, high Qf of ～ 188,500 GHz and near zero τ_f = ?2.0 ppm/°C were achieved at x = 0.08 under sintering at 1250 °C for 4 h. Thus, the fabricated ceramics were considered as possible candidates for millimeter-wave device applications.     

Preparation and characterization of porous,biomorphic SiC ceramic with hybrid pore structure

Bio-carbon template (charcoal) was prepared by carbonizing pine wood at 1200 °C under vacuum, and was impregnated with phenolic resin/SiO₂ sol mixture by vacuum/pressure processing. Porous SiC ceramics with hybrid pore structure, a combination of tubular pores and network SiC struts in the tubular pores, were fabricated via sol–gel conversion, carbonization and carbothermal reduction reaction at elevated temperatures in Ar atmosphere. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM) were employed to characterize the phase identification and microstructural changes during the C/SiO₂ composites-to-porous SiC ceramic conversion. Experimental results show that the density of C/SiO₂ composite increases with the number of impregnation procedure, and increases from 0.32 g cm⁻³ of pine-derived charcoal to 1.5 g cm⁻³ of C/SiO₂ composite after the sixth impregnation. The conversion degree of charcoal to porous SiC ceramic increases as reaction time is lengthened. The resulting SiC ceramic consists of β-SiC with a small amount of α-SiC. The conversion from pine charcoal to porous SiC ceramic with hybrid pore structure improves bending strength from 16.4 to 42.2 MPa, and decreases porosity from 76.1% to 48.3%.     

Improved Curie temperature,electromechanical properties and thermal stability in ZnO-modified 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 ceramics with coexisting monoclinic and tetragonal phases

Further improving electromechanical properties and overcoming relatively low Curie temperature (T_c) of (1-x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (PMN-100xPT) are still two scientific issues. Here, we demonstrate a stable coexistence of monoclinic-tetragonal (M_C-T) phases in ZnO-modified PMN-32PT (PMN-32PT:xZnO) due to the diffusion-induced substitution of Zn²⁺ for Mg²⁺. The Curie temperature, saturated polarization, remnant polarization, piezoelectric coefficient (T_c, P_s, P_r, d₃₃) are increased from (160 °C, 22.0 μC/cm², 13.3 μC/cm², 350 pC/N) for x = 0 to (180 °C, 30.3 μC/cm², 22.4 μC/cm², 470 pC/N) for x = 0.06. Moreover, the thermal stability is improved. After annealing at 150 °C, the x = 0.06 sample shows retrained d₃₃ value of 209 pC/N, about 4 times larger than that of x = 0 counterpart. The improved properties are attributed to the substituting increased polar nanoregions and easy domain switching in M_C phase.     

Research on microstructure and oxidation resistant property of ZrSi2-SiC/SiC coating on HTR graphite spheres

ZrSi₂-SiC/SiC coating was prepared on the surface of high temperature gas-cooled reactor (HTR) matrix graphite spheres by two-step pack cementation and sintering process. The microstructure, oxidation resistance and thermal shock resistance properties of the as-prepared coatings with different original powder mixtures were investigated. Results show that dense microstructure of the ZrSi₂-SiC/SiC coating and continuous ZrSiO₄-SiO₂-ZrO₂ glass phase generated during the oxidation process were the key factors for the outstanding thermal properties. When the mole ratio of Zr:Si:C reaches 1:7:3 in the second pack cementation powders, the coated graphite spheres have optimum oxidation resistant ability. The weight gain is only 0.6 wt% after 15 times thermal shock tests and 0.12 wt% after isothermal oxidation test at 1500 °C for 20 h in air. The oxidation resistant mechanism of the coating was also discussed. The dense inner SiC layer and the outer glass layer generated during the oxidation process could protect the ZrSi₂-SiC/SiC coating from further oxidation.     

Effects of oxidation curing and Al atoms on the formation of near-stoichiometric freestanding SiC(Al) films derived from polyaluminocarbosilane (PACS)

The effects of oxygen pick-up and Al atoms on the formation and microstructure of freestanding SiC(Al) films by melt spinning of polyaluminocarbosilane (PACS) precursor were studied. PACS green films were cross-linked for 1 h, 2 h, 3 h and 4 h, pre-pyrolyzed at 900 °C, respectively. They were continuously pyrolyzed at 1800 °C to convert initial PACS into SiC(Al) ceramic films. Results reveal that the strict control of oxygen content during the oxidation curing is essential to produce near-stoichiometric SiC(Al) films. The microstructure of the dense films is a mixture of β-SiC crystals, α-SiC nano-crystals, C clusters and a small amount of Al₄O₄C and Al₄SiC₄. Al atoms which play important roles as both sintering aids and grain growth inhibitor are well distributed in the films due to the presence of stable composition and structure. SiC(Al) films with excellent mechanical properties would be attractive candidate materials for MEMS in harsh environments.     

Effect of Nd-doping on structural,thermal and electrochemical properties of LaFe0.5Cr0.5O3 perovskites

The structural, thermal and electrochemical properties of the perovskite-type compound La_1−xNd_xFe_0.5Cr_0.5O₃ (x=0.10, 0.15, 0.20) are investigated by X-ray diffraction, thermal expansion, thermal diffusion, thermal conductivity and impedance spectroscopy measurements. Rietveld refinement shows that the compounds crystallize with orthorhombic symmetry in the space group Pbnm. The average thermal expansion coefficient decreases as the content of Nd increases. The average coefficient of thermal expansion in the temperature range of 30–850 °C is 10.12×10⁻⁶, 9.48×10⁻⁶ and 7.51×10⁻⁶ °C⁻¹ for samples with x=0.1, 0.15 and 0.2, respectively. Thermogravimetric analyses show small weight gain at high temperatures which correspond to filling up of oxygen vacancies as well as the valence change of the transition metals. The electrical conductivity measured by four-probe method shows that the conductivity increases with the content of Nd; the electrical conductivity at 520 °C is about 4.71×10⁻³, 6.59×10⁻³ and 9.62×10⁻³ S cm⁻¹ for samples with x=0.10, 0.15 and 0.20, respectively. The thermal diffusivity of the samples decreases monotonically as temperature increases. At 600 °C, the thermal diffusivity is 0.00425, 0.00455 and 0.00485 cm² s⁻¹ for samples with x=0.10, 0.15 and 0.20, respectively. Impedance measurements in symmetrical cell arrangement in air reveal that the polarization resistance decreases from 55 Ω cm⁻² to 22.5 Ω cm⁻² for increasing temperature from 800 °C to 900 °C, respectively.     

Effect of Ce and Yb co-doping on conductivity of LaNbO4

La_1−x−yCe_xYb_yNbO₄ specimens with various Ce and Yb contents were prepared by solid reactions, and their crystal structure, element valence, sinterability and conductivity were investigated. LaNbO₄-type single phase was formed at 1200 °C in air, and the lattice of La_1−x−yCe_xYb_yNbO₄ was distorted from that of LaNbO₄ to various extents, depending on the added amount of Ce and Yb. Both La and Nb remained the same valence as they are in LaNbO₄; Ce⁴⁺, Ce³⁺ and Yb³⁺ were detected at room temperature. Highly densified La_1−x−yCe_xYb_yNbO₄ specimens were achieved by sintering at above 1215 °C in air with conductivity 1–2 orders higher than that of pure LaNbO₄ in dry air, wet air and wet 5% H₂–N₂ atmospheres. The conductivity changed with testing atmosphere owing to the competition of electron and proton conduction, and maximal value 4.7 × 10⁻⁴ S cm⁻¹ was obtained in wet air at 900 °C.     

Effect of grain growth on the thermal conductivity of liquid-phase sintered silicon carbide ceramics

The effect of grain growth on the thermal conductivity of SiC ceramics sintered with 3 vol% equimolar Gd₂O₃-Y₂O₃ was investigated. During prolonged sintering at 2000 °C in an argon or nitrogen atmosphere, the β → α phase transformation, grain growth, and reduction in lattice oxygen content occurs in the ceramics. The effects of these parameters on the thermal conductivity of liquid-phase sintered SiC ceramics were investigated. The results suggest that (1) grain growth achieved by prolonged sintering at 2000 °C accompanies the decrease of lattice oxygen content and the occurrence of the β → α phase transformation; (2) the reduction of lattice oxygen content plays the most important role in enhancing the thermal conductivity; and (3) the thermal conductivity of the SiC ceramic was insensitive to the occurrence of the β → α phase transformation. The highest thermal conductivity obtained was 225 W(m K)⁻¹ after 12 h sintering at 2000 °C under an applied pressure of 40 MPa in argon.     

Silicon oxycarbide-based composites produced from pyrolysis of polysiloxanes with active Ti filler

Phenyl (PPS) and methyl (PMS) containing polysiloxanes were pyrolyzed at elevated temperatures (900–1500 °C) under argon atmosphere to investigate the phase developments within the polymers. It was found that pyrolysis of the polymers under inert atmosphere up to 1300 °C leads to amorphous silicon oxycarbide (SiO_xC_y) ceramics. Conversions at higher temperatures results in the transformations into the crystalline β-SiC phases. Ceramic matrix composites (CMCs) were developed based on the active filler controlled pyrolysis (AFCOP) of polysiloxanes with active Ti filler additions. CMC monoliths were prepared with 60–80 wt.% of active Ti particulates blended into polymer precursors. Green bodies of the composites were made by warm pressing under 15 MPa pressure and ceramics were obtained by pyrolysis at elevated temperatures between 900 and 1500 °C under argon atmosphere. The results showed that due to the incorporation of active Ti fillers, formation of crystalline phases such as TiC, TiSi, and TiO occured within the amorphous matrix due to the reactions between the Ti and the polymer decomposition products. The microstructural and mechanical characterization results of the composites are presented within the paper.     

Synthesis and electrical properties of lead-free piezoelectric Bi0.5Na0.5TiO3 thin films prepared by Sol-Gel method

Lead-free Bi_0.5Na_0.5TiO₃ (BNT) piezoelectric thin films were deposited on Pt/TiO_x/SiO₂/Si substrates by Sol-Gel method. A dense and well crystallized thin film with a perovskite phase was obtained by annealing the film at 700 °C in a rapid thermal processing system. The relative dielectric constant and loss tangent at 12 kHz, of BNT thin film with 350 nm thickness, were 425 and 0.07, respectively. Ferroelectric hysteresis measurements indicated a remnant polarization value of 9 μC/cm² and a coercive field of 90 kV/cm. Piezoelectric measurements at the macroscopic level were also performed: a piezoelectric coefficient (d_33effmax) of 47 pm/V at E = 190 kV/cm was obtained. The piezoresponse force microscopy data confirmed that BNT thin films present ferroelectric and piezoelectric behavior at the nanoscale level.     

Improved electrical and thermal conductivities of polysiloxane-derived silicon oxycarbide ceramics by barium addition

The electrical and thermal conductivities of bulk barium-added silicon oxycarbide (SiOC-Ba) ceramics are investigated. The SiOC-Ba ceramics exhibited improved electrical and thermal conductivities upon increasing the sintering temperature from 1450 °C to 1650 °C. Precipitation of graphitic carbon clusters observed by Raman spectroscopy and high-resolution transmission electron microscopy is attributed to the phase separation during the fabrication process. The increase in the electrical conductivity can be rationalized in terms of an increase in the density of the sp² CC bonds within the carbon clusters. The increase in the thermal conductivity is mainly attributed to the formation of interconnected graphitic clusters in the SiOC matrix and SiC embedded in the clusters. The electrical and thermal conductivities of the SiOC-Ba ceramics sintered at 1650 °C are 14.0 Ω^?1 cm^?1 and 5.6 W/m K, respectively, at room temperature. The electrical conductivity of SiOC-Ba sintered at 1550 °C is 5.3 Ω^?1 cm^?1 and 7.0 Ω^?1 cm^?1 at 2 and 300 K, respectively.     

Improving electrical conductivity and wear resistance of hafnium nitride films via tantalum incorporation

Transition metal nitrides are being widely applied, as durable sensors, semiconductor and superconductor devices, their electrical conductivity and wear resistance having a significant influence on these applications. However, there are few reports about how to improve above properties. In this paper, tantalum was incorporated into hafnium nitride films through Hf_1-xTa_xN_y [x=Ta/(Hf+Ta), y=N/(Hf+Ta)] solid solution. The electrical conductivity and wear resistance of the films were significantly improved, due to the increase of the electron concentration (tantalum has one more valence electron than hafnium) and the increase in H/E and H³/E² ratios caused by the effect of solid solution hardening, respectively. The highest electrical conductivity of Hf_1-xTa_xN_y films is 8.3×10⁵ S m⁻¹, which is 1.7 times and 5.2 times of that of hafnium nitride and tantalum nitride films, respectively. In addition, the lowest wear rate of films is 1.2×10⁻⁶ mm³/N m, which is only 10% and 48% of that of hafnium nitride and tantalum nitride films, respectively. These results indicate that alloying with another transition metal is an effective method to improve electrical conductivity and wear resistance of transition metal nitrides.     

Nitrate–citrate combustion synthesis of Ce1−xGdxO2−x/2 powder and its characterization

The structure, thermal expansion coefficients, and electric conductivity of Ce_1−xGd_xO_2−x/2 (x = 0–0.6) solid solution, prepared by the gel-combustion method, were investigated. The uniform small particle size of the gel-combustion prepared materials allows sintering into highly dense ceramic pellets at 1300 °C, a significantly lower temperature compared to that of 1600–1650 °C required for ceria solid electrolytes prepared by traditional solid state techniques. XRD showed that single-phase solid solutions formed in all the investigated range. The maximum conductivity, σ_600 °C = 5.26 × 10⁻³ S/cm, was found at x = 0.2. The thermal expansion coefficient, determined from high-temperature X-ray data, was 8.125 × 10⁻⁶ K⁻¹ at x = 0.2.     

Influence of SiC particle addition on the nucleation density and adhesion strength of MPCVD diamond coatings on Si3N4 substrates

It is generally accepted that SiC layers are often involved in the adhesion efficiency of chemical vapour deposition (CVD) diamond films on Si-containing substrates. Si₃N₄–SiC composite substrates with different amounts of SiC particles (0–50 wt%) were then used for diamond deposition. Samples were produced by pressureless sintering (1750°C, N₂ atmosphere, 2–4 h). The diamond films were grown on a commercial MPCVD reactor using H₂/CH₄ mixtures. Despite there being no special substrate pre-treatment, the films were densely nucleated when SiC was added (N_d≈1×10¹⁰ cm⁻²) with primary nanosized (∼100 nm) particles, followed by a less dense (N_d≈1×10⁶ cm⁻²) secondary nucleation. Indentation experiments with a Brale tip of up to 588 N applied load corroborated the benefit of SiC inclusion for a strong adhesion. The low thermal expansion coefficient mismatch between Si₃N₄ and diamond resulted in very low compressive stresses in the film, as proved by micro-Raman spectroscopy.