Highly conductive SiC ceramics containing Ti2CN

Highly conductive SiC ceramics were fabricated by sintering β-SiC and TiN powder mixture in N₂ atmosphere. SiC ceramics exhibited decreased electrical resistivity (ρ) with increasing TiN content. X-ray diffraction data indicated that the specimens consisted of β-SiC grains without a detectible secondary phase for low TiN content (≤2 vol%) but contained a Ti₂CN phase as the TiN content increased. The temperature-dependent resistivity ρ(T) of specimens revealed semiconductor-like behavior for TiN content up to 10 vol% and metal-like behavior above 20 vol%. For the specimen with TiN content of 15 vol%, ρ(T) remained almost constant (2.06 ± 0.01 × 10⁻³ Ω cm) in the 4–300 K range. The resistivity of metal-like specimens were as low as 3.5 × 10⁻⁴ Ω cm for TiN content of 20 vol%. For semiconductor-like specimens, ρ(T) was primarily affected by N donors in the β-SiC grains. Metal-like specimens were primarily affected by metallic Ti₂CN clusters.     

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.     

Influence of sintering aids and excess Lithium on the conductivity of perovskite-type Li3/8Sr7/16Zr1/4Nb3/4O3 solid electrolyte

Perovskite-structured Li_3/8Sr_7/16Zr_1/4Nb_3/4O₃ solid-state Lithium-conductors were prepared by conventional solid-state reaction method. Influence of sintering aids (Al₂O₃, B₂O₃) and excess Lithium on structure and electrical properties of Li_3/8Sr_7/16Zr_1/4Nb_3/4O₃ (LSNZ) has been investigated. Their crystal structure and microstructure were characterized by X-ray diffraction analysis and scanning electron microscope, respectively. The conductivity and electronic conductivity were evaluated by AC-impedance spectra and potentiostatic polarization experiment. All sintered compounds are cubic perovskite structure. Optimal amount of excess Li₂CO₃ was chosen as 20 wt% because of the total conductivity of LSNZ-20% was as high as 1.6×10⁻⁵ S cm⁻¹ at 30 °C and 1.1×10⁻⁴ S cm⁻¹ at 100 °C, respectively. Electronic conductivity of LSNZ-20% is 2.93×10⁻⁸ S cm⁻¹, nearly 3 orders of magnitude lower than ionic conductivity. The density of solid electrolytes appears to be increased by the addition of sintering aids. The addition of B₂O₃ leads to a considerable increase of the total conductivity and the enhancement of conductivity is attributed to the decrease of grain-boundary resistance. Among these compounds, LSNZ-1 wt%B₂O₃ has lower activation energy of 0.34 eV and the highest conductivity of 1.98×10⁻⁵ S cm⁻¹ at 30 °C.     

Preparation and characterization of transparent indium- doped TiO2 films deposited by MOCVD

Titanium dioxide (TiO₂) films doped with different indium (In) concentrations have been prepared on SrTiO₃ (STO) substrates by high vacuum metalorganic chemical vapor deposition (MOCVD). X-ray diffraction (XRD) analyses revealed the TiO₂ films doped with low In concentrations to be [001] oriented anatase phase and the films with high In concentrations to present polycrystalline structures. The 1.8% In-doped TiO₂ film exhibited the best electrical conductivity properties with the lowest resistivity of 8.68×10⁻² Ω cm, a Hall mobility of 10.9 cm² V⁻¹ s⁻¹ and a carrier concentration of 6.5×10¹⁸ cm⁻³. The films showed excellent transparency with average transmittances of over 85% in the visible range.     

Semiconducting properties of cold sintered V2O5 ceramics and Co-sintered V2O5-PEDOT:PSS composites

Recently we established a sintering approach, namely Cold Sintering Process (CSP), to densify ceramics and ceramic-polymer composites at extraordinarily low temperatures. In this work, the microstructures and semiconducting properties of V₂O₅ ceramic and (1-x)V₂O₅-xPEDOT:PSS composites cold sintered at 120 °C were investigated. The electrical conductivity (25 °C), activation energy (25 °C), and Seebeck coefficient (50 °C) of V₂O₅ are 4.8 × 10⁻⁴ S/cm, 0.25 eV, and −990 μV/K, respectively. The conduction mechanism was studied using a hopping model. A reversible metal-insulator transition (MIT) was observed with V₂O₅ samples exposed to a N₂ atmosphere, whereas in a vacuum atmosphere, no obvious MIT could be detected. With the addition of 1–2 Vol% PEDOT:PSS, the electrical conductivity (50 °C) dramatically increases from 10⁻⁴ to 10⁻³ ∼ 10⁻² S/cm, and the Seebeck coefficient (50 °C) shifts from −990 to −(600 ∼ 250) μV/K. All the results indicate that CSP may offer a new processing route for the semiconductor electroceramic development without a compromise to the all-important electrical properties.     

Electrical properties and microstructural evolution of ZrO2–Al2O3–TiN nanocomposites prepared by spark plasma sintering

Electroconductive ZrO₂–Al₂O₃–25 vol% TiN ceramic nanocomposites were prepared by spark plasma sintering at 1200 °C for 3 min. The electrical resistivity of the composites decreased from 4.5 × 10^?4 Ω m to 3 × 10^?5 Ω m as the Al₂O₃ content in the ZrO₂–Al₂O₃ matrix increased from 0 to 100 vol%. SEM images graphically presented the microstructural evolution of the composites and a geometrical percolation model was applied to investigate the relationship between the electrical property and the microstructure. The results indicated that the addition of Al₂O₃ to ZrO₂–TiN improved the electrical conductivity of the material by tailoring the structure from “nano–nano” type for ZrO₂–TiN to “micro–nano” type for ZrO₂–Al₂O₃–TiN.     

Mechanical,electrical, and thermal properties of graphene nanosheet/aluminum nitride composites

Graphene nanosheet (GNS)/aluminum nitride (AlN) composites were prepared by hot-pressing and effects of GNSs on their microstructural, mechanical, thermal, and electrical properties were investigated. At 1.49 vol% GNSs content, the fracture toughness (5.09 MPa m^1/2) and flexural strength (441 MPa) of the composite were significantly increased by 30.17% and 17.28%, respectively, compared to monolithic AlN. The electrical conductivity of the composites was effectively enhanced with the addition of GNSs, and showed a typical percolation behavior with a low percolation threshold of 2.50±0.4 vol%. The thermal conductivity of the composites decreased with the addition of GNSs.     

Preparation and properties of B6O/TiB2-composites

B₆O/TiB₂ composites with varying compositions were produced by FAST/SPS at temperatures between 1850 and 1900 °C following a non-reactive or a reactive sintering route. The densification, phase and microstructure formation and the mechanical and thermal properties were investigated. The comparison to an also investigated pure B₆O material showed that the addition of TiB₂ in a non-reactive sintering route promotes the B₆O densification. Further improvement was obtained by sintering reactive B–TiO₂ mixtures which also results in materials with a finer grain size and thus in enhanced mechanical properties. The fracture toughness was significantly improved in all composites and is up to 4.0 MPa m^1/2 (SEVNB) and 2.6–5.0 MPa m^1/2 (IF method) while simultaneously a high hardness of up to 36 GPa (HV_0.4) and 28 GPa (HV₅) could be preserved. The high temperature properties at 1000 °C of hardness, thermal conductivity and CTE were up to 20 GPa, 18 W/mK and 6.63 × 10^?6/K, respectively.     

Improving the electrical properties of niobium-doped titania sputtering targets by sintering in oxygen-deficient atmospheres

Niobium-doped titania (TNO) film can be used as a transparent conductive oxide (TCO) film due to its excellent conductivity and visible transparency. The performances of TNO sputtering targets are thus critical issues in optimizing sputtered films. This study clarifies the influences of inert and reducing atmospheres on the microstructure, densification, crystal structure, and electrical properties of TNO sputtering targets. The results indicate that a sintering atmosphere of 90% Ar–10% H₂ can result in a lower sintered density, larger grain size, and lower resistivity than can an atmosphere of Ar, followed by one of air. Sintering in 90% Ar–10% H₂ or Ar obviously decreases the resistivity of TiO₂, from >10⁸ Ω cm to <10⁻¹ Ω cm, and the TNO target, from >10¹ Ω cm to <10⁻¹ Ω cm. The resistivity of TNO target sintered at 1200 °C in 90% Ar–10% H₂ is as low as 1.8×10⁻² Ω cm.     

Thermal,mechanical and electrical properties of hard B4C,BCN, ZrBC and ZrBCN ceramics

We study the thermal, mechanical and electrical properties of B₄C, BCN, ZrBC and ZrBCN ceramics prepared in the form of thin films by magnetron sputtering. We focus on the effect of Zr_x(B₄C)_1−x sputter target composition, the N₂+Ar discharge gas mixture composition, the deposition temperature and the annealing temperature after the deposition. The thermal properties of interest include thermal conductivity (observed in the range 1.3–7.3 W m⁻¹ K⁻¹), heat capacity (0.37–1.6×10³ J kg⁻¹ K⁻¹ or 1.9–4.1×10⁶ Jm⁻³ K⁻¹), thermal effusivity (1.6–4.5×10³ J m⁻² s^−1/2 K⁻¹) and thermal diffusivity (0.38–2.6×10⁻⁶ m² s⁻¹). We discuss the relationships between materials composition, preparation conditions, structure, thermal properties, temperature dependence of the thermal properties and other (mechanical and electrical) properties. We find that the materials structure (amorphous×crystalline hexagonal ZrB₂-like×nanocrystalline cubic ZrN-like), more than the composition, is the crucial factor determining the thermal conductivity and other properties. The results are particularly important for the design of future ceramic materials combining tailored thermal properties, mechanical properties, electrical conductivity and oxidation resistance.     

Highly resistive SiC ceramics sintered with Al2O3-AlN-Y2O3 additions

A polycrystalline SiC ceramic prepared by pressureless sintering of α-SiC powders with 3 vol% Al₂O₃-AlN-Y₂O₃ additives in an argon atmosphere exhibited a high electrical resistivity of ~10¹³ Ω cm at room temperature. X-ray diffraction revealed that the SiC ceramics consisted mainly of 6H- and 4H-SiC polytypes. Scanning electron microscopy and high resolution transmission electron microscopy investigations showed that the SiC specimen contained micron-sized grains surrounded by an amorphous Al-Y-Si-O-C-N film with a thickness of ~4.85 nm. The thick boundary film between the grains contributed to the high resistivity of the SiC ceramic.     

Preparation and mechanical performance of ductile Csf/Al2O3–BN composites—Part 2: Effects of fiber contents and ablation properties

In this paper, ductile C_sf/Al₂O₃–BN composites were prepared by hot-press sintering. Effects of fiber contents on the mechanical performance and ablation behavior of the composites were investigated systematically. The results showed that all the composites fractured in non-brittle failure mode, exhibiting elastic region and non-linear region as shown in load–displacement curves. With the increase in fiber contents from 15 to 30 vol%, mechanical properties of the obtained composites first increased, reached the peak values at fiber content of 25 vol% and then decreased. When being exposed to high-speed oxyacetylene combustion flow for 60 s, the composites with fiber contents of 15, 20 and 25 vol% showed comparable ablation property and mass loss of 5.3, 7.2 and 8.4×10⁻⁴ g/s, respectively. The ablation mechanisms include fiber and ceramic matrix oxidation, decomposition of mullite, evaporation of both B₂O₃ and SiO₂, and mechanical exfoliation.     

The preparation and properties of La3.5Ru4O13 and La2RuO5

The stability of the La_3.5Ru₄O₁₃ and La₂RuO₅ compounds in the La–Ru–O system in various atmospheres and various temperature ranges was investigated by thermal analysis, X-ray diffraction analysis and electron microscopy. The La_3.5Ru₄O₁₃ compound is stable in oxidizing and neutral atmospheres (N₂ with 10 ppm O₂), while La₂RuO₅ is partially reduced in a neutral atmosphere to form La₂RuO_4.6. In a reducing atmosphere both compounds decompose into metallic Ru and La₂O₃. The thermal expansion coefficients of La₂RuO₅ and La_3.5Ru₄O₁₃ at 800 °C are 11.2 × 10⁻⁶ K⁻¹ and 9.3 × 10⁻⁶ K⁻¹, respectively. The specific electrical resistivity for La_3.5Ru₄O₁₃ is relatively independent of temperature and is 2 × 10⁻² Ω cm at 800 °C, while for La₂RuO₅ it decreases with increasing temperature and is 1 Ω cm at 800 °C.     

Thermal conductivity of MWCNT/epoxy composites: The effects of length,alignment and functionalization

Carbon nanotubes (CNTs) show great promise to improve composite electrical and thermal conductivity due to their exceptional high intrinsic conductance performance. In this research, long multi-walled carbon nanotubes (long-MWCNTs) and its thin sheet of entangled nanotubes were used to make composites to achieve higher electrical and thermal conductivity. Compared to short-MWCNT sheet/epoxy composites, at room temperature, long-MWCNT samples showed improved thermal conductivity up to 55 W/mK. The temperature dependence of thermal conductivity was in agreement with κ ∝ Tⁿ (n = 1.9–2.3) below 150 K and saturated around room temperature due to Umklapp scattering. Samples with the improved CNT degree of alignment by mechanically stretching can enhance the room temperature thermal conductivity to over 100 W/mK. However, functionalization of CNTs to improve the interfacial bonding resulted in damaging the CNT walls and decreasing the electrical and thermal conductivity of the composites.     

Electrical characterization of La9.33Si2Ge4O26 oxyapatite for prospective intermediate-temperature solid oxide fuel cells

La_9.33Si₂Ge₄O₂₆ materials have been fabricated from La₂O₃, SiO₂ and GeO₂ powders by high speed mechanical alloying followed by conventional and microwave hybrid sintering at different temperatures and holding times. XRD data showed that the apatite phase is formed after 1 h of mechanical alloying at 850 rpm. This phase remained stable after conventional sintering in an electric furnace with density increasing as sintering temperatures and holding times were increased. However, the highest density was achieved for samples sintered in the microwave furnace (5.44 g cm⁻³), corresponding to a relative density of 98%. The electrical conductivity of the samples microwave sintered at 700 and 800 ºC are 4.72×10⁻³ and 1.93×10⁻² S.cm⁻¹, respectively, with a correspondent activation energy of 0.952 eV.     

Fabrication of highly efficient TiO2/Ag/TiO2 multilayer transparent conducting electrode with N ion implantation for optoelectronic applications

Fabrication of highly conductive and transparent TiO₂/Ag/TiO₂ (referred hereafter as TAT) multilayer films with nitrogen implantation is reported. In the present work, TAT films were fabricated with a total thickness of 100 nm by sputtering on glass substrates at room temperature. The as-deposited films were implanted with 40 keV N ions for different fluences (1×10¹⁴, 5×10¹⁴, 1×10¹⁵, 5×10¹⁵ and 1×10¹⁶ ions/cm²). The objective of this study was to investigate the effect of N⁺ implantation on the optical and electrical properties of TAT multilayer films. X-ray diffraction of TAT films shows an amorphous TiO₂ film with a crystalline peak assigned to Ag (111) diffraction plane. The surface morphology studied by atomic force microscopy (AFM) and field emission scanning electron microscope (FESEM) revealed smooth and uniform top layer of the sandwich structure. The surface roughness of pristine film was 1.7 nm which increases to 2.34 nm on implantation for 1×10¹⁴ ions/cm² fluence. Beyond this fluence, the roughness decreases. The oxide/metal/oxide structure exhibits an average transmittance ~80% for pristine and ~70% for the implanted film at fluence of 1×10¹⁶ ions/cm² in the visible region. The electrical resistivity of the pristine sample was obtained as 2.04×10⁻⁴ Ω cm which is minimized to 9.62×10⁻⁵ Ω cm at highest fluence. Sheet resistance of TAT films decreased from 20.4 to 9.62 Ω/□ with an increase in fluence. Electrical and optical parameters such as carrier concentration, carrier mobility, absorption coefficient, band gap, refractive index and extinction coefficient have been calculated for the pristine and implanted films to assess the performance of films. The TAT multilayer film with fluence of 1×10¹⁶ ions/cm² showed maximum Haacke figure of merit (FOM) of 5.7×10⁻³ Ω⁻¹. X-ray photoelectron spectroscopy (XPS) analysis of N 1s and Ti 2p spectra revealed that substitutional implantation of nitrogen into the TiO₂ lattice added new electronic states just above the valence band which is responsible for the narrowing of band gap resulting in the enhancement in electrical conductivity. This study reports that fabrication of multilayer transparent conducting electrode with nitrogen implantation that exhibits superior electrical and optical properties and hence can be an alternative to indium tin oxide (ITO) for futuristic TCE applications in optoelectronic devices.     

Physical,thermal and ablative performance of CVI densified urethane-mimetic SiC preforms containing in situ grown Si3N4 whiskers

A two-step process has been developed for silicon carbide (SiC) coated polyurethane mimetic SiC preform containing silicon nitride (Si₃N₄) whiskers. SiC/Si₃N₄ preforms were prepared by pyrolysis/siliconization treatment at 1600 °C, of powder compacts containing rigid polyurethane, novolac and Si, forming a porous body with in situ grown Si₃N₄ whiskers. The properties were controlled by varying Si/C mole ratios such as 1–2.5. After densification using a chemical vapour infiltration, the resulting SiC/Si₃N₄/SiC composites showed excellent oxidation resistance, thermal conductivity of 4.32–6.62 Wm⁻¹ K⁻¹, ablation rate of 2.38 × 10⁻³ − 3.24 × 10⁻³ g cm⁻² s and a flexural strength 43.12–55.33 MPa for a final density of 1.39–1.62 gcm⁻³. The presence of a Si₃N₄ phase reduced the thermal expansion mismatch resulting in relatively small cracks and well-bonded layers even after ablation testing. This innovative two-step processing can provide opportunities for expanded design for using SiC/Si₃N₄/SiC composites being lightweight, inexpensive, homogeneous and isotropic for various high temperature applications.     

Fabrication and thermal expansion properties of ZrW2O8/Zr2WP2O12 composites

ZrW₂O₈/Zr₂WP₂O₁₂ composites were fabricated by sintering ZrW₂O₈–Zr₂WP₂O₁₂ powder mixtures at 1473 K for 1 h, and their negative thermal expansion properties were investigated. The relative density of sintered pure-phase ZrW₂O₈ was 72.3%, while that of the sintered composites was 88.4–92.3%. In the composites, the observed hysteresis in the thermal expansion data was small because of the small difference between the CTEs of ZrW₂O₈ and Zr₂WP₂O₁₂. The CTE of the composites was negative and increased with the Zr₂WP₂O₁₂ content. When the Zr₂WP₂O₁₂ volume fraction in the composites was increased from 0 to 75 vol%, the CTEs of the composites increased from ?9.1 × 10^?6 to ?3.1 × 10^?6 K^?1 and from ?5.0 × 10^?6 to ?1.9 × 10^?6 K^?1 over the temperature ranges of 323–373 and 473–673 K, respectively. In composites with Zr₂WP₂O₁₂ volume fractions of 0–25 vol%, the experimentally obtained CTE values were in good agreement with the calculated values obtained by assuming mixed law behavior.     

Microwave sintered Bi0.90La0.10Fe0.95Mn0.05O3 nanocrystalline ceramics: Impedance and modulus spectroscopy

Multiferroic Bi_0.90La_0.10Fe_0.95Mn_0.05O₃ (BLFMO) nanoceramics were synthesized by PVA sol-gel method, followed by microwave sintering. The structural, microstructural and electrical properties of BLFMO were investigated. The crystal symmetry and unit cell dimensions were determined from the experimental data using Rietveld analysis. BLFMO revealed only one electroactive region as verified from impedance and modulus spectroscopy. Overlapping large polaron tunneling transport mechanism was observed from AC conductivity analysis. Conduction below 250 °C (−30 °C≤T≤250 °C) was attributed to translational hopping while above 250 °C (250 °C≤T≤350 °C) corresponds to electron hopping between charge defects. The relative permittivity varies from 66 to 203 at 1 kHz over the measured temperature range (−150 °C≤T≤350 °C). The electrical conductivity of the microwave sintered BLFMO has been discussed based on defect reaction with Mn doping. The measured DC conductivity in the range of 10⁻¹³ S/cm at −130 °C to 10⁻⁴ S/cm at 350 °C revealed the insulating behavior of the sample. At room temperature, the DC resistivity of the sample was over ~50 MΩ cm. The stretching constant (β) obtained from KWW (Kohlrausch-Williams-Watts) equation indicates that the sample inclined towards ideal Debye behavior as the temperature increases.     

Electrical resistivity of α-SiC ceramics sintered with Al2O3 or AlN additives

Highly resistive SiC ceramics were prepared by hot pressing α-SiC powders with Al₂O₃-Y₂O₃ additives with a 4:1 molar ratio. X-ray diffraction patterns, Raman spectra, electron probe microanalysis (EMPA), and scanning electron microscopy (SEM) images revealed that the bulk SiC ceramics consisted mostly of micron-sized 6H-SiC grains along with Y₂O₃ and Si clusters. As the additive content increased from 1 to 10 vol%, the electrical resistivity of the ceramics increased from 3.0 × 10⁶ to 1.3 × 10⁸ Ω cm at room temperature. Such high resistivity is ascribed to Al₂O₃ in which Al impurities substituting Si site act as deep acceptors for trapping carriers. More resistive α-SiC ceramics were produced by adding AlN instead of Al₂O₃. The highest resistivity (1.3 × 10¹⁰ Ω cm) was achieved by employing 3 vol% AlN-Y₃Al₅O₁₂ (yttrium aluminum garnet, YAG) as an additive.