Semiconductor‐Insulator Transition in Undoped Rutile,TiO2, Ceramics

The electrical conductivity of undoped rutile ceramics is very dependent on sample processing conditions, especially the temperature and atmosphere during sintering and the subsequent cooling rate. Samples become increasingly semiconducting when quenched from temperatures above ~700°C without the need for a reducing atmosphere. Thus, samples quenched from 1400°C in air have conductivity ~1 × 10^?2 Scm^?1with activation energy ~0.01(1) eV over the temperature range 10–100 K, whereas similar samples that are slow cooled or annealed in air at 300°C–500°C are insulating with activation energy 1.67(2) eV and conductivity, e.g., 1 × 10^?7 Scm^?1 at 400°C. The very wide range of electrical properties is attributed to variations in oxygen content which are too small to be detected using thermogravimetry. Impedance analysis shows that, depending on cooling rate, partially oxidized samples may be prepared in which samples retain a semiconducting core, but have an oxidized outer layer.     

Electrical Conductivity of SiOCN Ceramics by the Powder‐Solution‐Composite Technique

SiOCN ceramics have been prepared by the polymer pyrolysis method. The preceramic polymers were synthesized from a polysiloxane cross‐linked with two different N‐containing compounds: a silazane or a ternary amine. The corresponding SiOCN ceramics were obtained by pyrolysis in nitrogen atmosphere at five different temperatures from 1000°C to 1400°C. The electrical conductivity of the powdered SiOCN ceramic samples was determined by the powder‐solution‐composite technique. The results show an increase in room temperature AC conductivity of three orders of magnitude, from ≈10^?5 (S/cm) to ≈10^?2 (S/cm), with increasing pyrolysis temperature from 1000°C to 1400°C. Furthermore, the electrical conductivity of the amine‐derived SiOCN is three to five times higher than that of the silazane‐derived ceramic at each pyrolysis temperature. The combined structural study by Raman spectroscopy and chemical analysis suggests that the increase of electrical conductivity with the pyrolysis temperature is due to the sp³‐to‐sp² transition of the amorphous carbon phase. The higher conductivity of the amine‐derived SiOCN is also discussed considering features like the volume% of the free‐carbon phase and its possible N‐doping.     

Preparation and performance study of cordierite/mullite composite ceramics for solar thermal energy storage

The employment of solar energy in recent years has reached a remarkable edge. It has become even more popular as the cost of fossil fuel continues to rise. Energy storage system improves an adjustability and marketability of solar thermal and allowing it to produce electricity in demand. This study attempted to prepare cordierite/mullite composite ceramics used as solar thermal storage material from calcined bauxite, talcum, soda feldspar, potassium feldspar, quartz, and mullite. The thermal physical performances were evaluated and characterized by XRD, SEM, EPMA, and EDS. It was found that the optimum sintering temperature was 1280°C for preparing, and the corresponding water adsorption was 11.25%, apparent porosity was 23.59%, bulk density was 2.10 mg·cm^?3, bending strength was 88.52 MPa. The residual bending strength of specimen sintered at 1280°C after thermal shock of 30 times decreased to be 57 MPa that was 36% lower than that before. The thermal conductivity of samples sintered at 1280°C was tested to be 2.20 W·(m·K)^?1 (26°C), and after wrapped a PCM (phase change materials) of K₂SO₄, the thermal storage density was 933 kJ·kg^?1 with the temperature difference (ΔT) ranged in 0‐800°C. The prepared cordierite/mullite composite ceramic was proved to be a promising material for solar thermal energy storage.     

Microstructure-property relationships in composites of 8YSZ ceramics and in situ graphitized nanocellulose

Ceramic matrix composites (CMC) of 8 mol.% yttria-stabilized zirconia (8YSZ) mixed with natural fiber nanocellulose (0.75, 1, 2 wt%) were prepared by spark plasma sintering (SPS). Nanocellulose markedly improved the densification of the 8YSZ ceramic matrix and induced significant grain size refinement. It was demonstrated that in situ graphitization of nanocellulose during the SPS processing resulted in 6 nm thin turbostratic graphite layers homogeneously covering the 8YSZ ceramic grains. The dielectric properties were analyzed by electrical impedance spectroscopy suggesting a low percolation threshold near or below ≈ 1.6 vol% graphite, above which mixed ionic-electronic conduction dominates. The CMCs are stable under reducing conditions (5%H₂/Ar atmosphere) at least until 800 °C with a high conductivity of σ_dc = 0.17 S?cm^?1 even at 900 °C (8YSZ-2%CNF). These features make the 8YSZ-nanocellulose CMCs promising candidates for application in medium- to high-temperature electrochemical devices.     

Electrical conductivity of Colorado oil shale to 900 °C

Electrical conductivity of oil shale from the Anvil Points Mine, Colorado was measured to temperatures > 900 °C with conductance bridges operating at frequencies from 100 to 100 000 Hz. The conductivity of low, intermediate and high grade oil shales (15,124,233 ml kg^?1, respectively) is dependent on water content up to ≈ 100 °C. At ≈ 120 °C, values of conductivity at ≈ 10^?7 S m^?1 are observed for all grades. A strong, time-dependent, increase in conductivity, beginning at ≈400 °C, marks the loss of light hydrocarbons and the formation of a conductive char. The frequency dependence of conductivity-slightly less than a decade increase in conductivity per decade increase in frequency over the temperature range 100–400 °C-vanishes at temperatures near 500 °C. At 600–800 °C, the conductivity of these oil shales reaches a maximum value which is as much as 10⁸ times larger than the conductivity near 250 °C.     

Infrared radiation and thermal properties of Al-doped SrZrO3 perovskites for potential infrared stealth coating materials in the high-temperature environment

The Al-doped SrZrO₃ perovskite powder with low infrared emissivity at high temperatures was prepared. The infrared radiation performance and thermophysical properties of the perovskites at high temperatures were discussed. As a result, the infrared emissivity of the Al-doped SrZrO₃ perovskite powder is associated with Al³⁺-doping content, phase composition and particle morphology. The flaky particles SrZr_0.85Al_0.15O_2.925 formed by heat treatment at 1000 °C for 6 h have the lowest infrared emissivity of 0.245 in 3–5 μm wavebands at 590 °C. The perovskite powder's infrared emissivity is positively correlated with its electrical resistivity and has no apparent change after heating over 800 °C for long-term. The SrZr_0.85Al_0.15O_2.925 perovskite ceramic formed by pressureless sintering still maintains ideal heat insulation performance with the thermal conductivity from 1.17 to 2.21 W m^?1 K^?1 below 1400 °C. The Al-doped SrZrO₃ perovskite tablet exhibits significant weak radiation intensity due to its characteristics of both low infrared emissivity and thermal conductivity at high temperatures.     

Liquid‐phase sintering of highly Na+ ion conducting Na3Zr2Si2PO12 ceramics using Na3BO3 additive

Na₃Zr₂Si₂PO₁₂ (NASICON) is a promising material as a solid electrolyte for all‐solid‐state sodium batteries. Nevertheless, one challenge for the application of NASICON in batteries is their high sintering temperature above 1200°C, which can lead to volatilization of light elements and undesirable side reactions with electrode materials at such high temperatures. In this study, liquid‐phase sintering of NASICON with a Na₃BO₃ (NBO) additive was performed for the first time to lower the NASICON sintering temperature. A dense NASICON‐based ceramic was successfully obtained by sintering at 900°C with 4.8 wt% NBO. This liquid‐phase sintered NASICON ceramic exhibited high total conductivity of ~1 × 10^?3 S cm^?1 at room temperature and low conduction activation energy of 28 kJ mol^?1. Since the room‐temperature conductivity is identical to that of conventional high‐temperature‐sintered NASICON, NBO was demonstrated as a good liquid‐phase sintering additive for NASICON solid electrolyte. In the NASICON with 4.8 wt% NBO ceramic, most of the NASICON grains directly bonded with each other and some submicron sodium borates segregated in particulate form without full penetration to NASICON grain boundaries. This characteristic composite microstructure contributed to the high conductivity of the liquid‐phase sintered NASICON.     

Ultrasound assisted synthesis of Gd and Nd doped ceria electrolyte for solid oxide fuel cells

In this study, the ceramic powders of Ce_1?xGd_xO_2?x/2 and Ce_1?xNd_xO_2?x/2 (x=0.05, 0.10, 0.15, 0.20 and 0.25) were synthesized by ultrasound assisted co-precipitation method. The ionic conductivity was studied as a function of dopant concentration over the temperature range of 300–800 °C in air, using the impedance spectroscopy. The maximum ionic conductivity, σ_800 °C=4.01×10^?2 Scm^?1 with the activation energy, E_a=0.828 kJmol^?1 and σ_800 °C=3.80×10^?2 Scm^?1 with the activation energy, E_a=0.838 kJmol^?1 were obtained for Ce_0.90Gd_0.10O_1.95 and Ce_0.85Nd_0.15O_1.925 electrolytes, respectively. The average grain size was found to be in the range of 0.3–0.6 μm for gadolinium doped ceria and 0.2–0.4 μm for neodymium doped ceria. The uniformly fine crystallite sizes (average 12–13 nm) of the ultrasound assisted prepared powders enabled sintering of the samples into highly dense (over 95%) ceramic pellets at 1200 °C (5 °C min^?1) for 6 h.     

Electrical conductivity characteristics of Sr substituted layered perovskite cathode (SmBa0.5Sr0.5Co2O5+d) for intermediate temperature-operating solid oxide fuel cell

The high electrical conductivity of the cathode is one of the important factors for reducing the polarization resistance. For this reason, we here report the electrical conductivity characteristics of SmBa_0.5Sr_0.5Co₂O_5+δ (SBSCO) as a function of sintering temperature and current ranges. Calcined SBSCO samples were sintered at 1000, 1050, 1100, and 1150 °C. The current ranges applied in the process of measuring electrical conductivity were subdivided as 1.0A [0.05step], 0.5A [0.025step], and 0.1A [0.005step]. It was found that the sintering temperature affected the electrical conductivity in the following way: when the sintering temperature increases, an increase in the observed electrical conductivity is the result. However, as the current range decreases, it was found that the electrical conductivity would increase. The maximum and minimum conductivities of SBSCO sintered at 1150 °C were 2263S?cm^?1 at 50 °C and 382 S?cm^?1 at 900 °C with metallic behavior in air condition. When a current of 0.1A was applied to SBSCO sintered at 1150 °C, the electrical conductivity at the 800 °C was 1377.15 S/cm. It can be determined that the increase in the internal charge carrier flux of the SBSCO is associated with the decrease in the overall electrical conductivity of the Co-based metallic electrical conductivity. These results show that the high sintering temperature and low current range enable higher electrical conductivity at high operating temperature.     

Temperature-dependent electrical transport behavior and structural evolution in hollandite-type titanium-based oxide

Electrical transport behavior and structural characteristics directly determine the use of functional ceramic materials in electronic information storage, catalytic conversion, and energy field applications. However, these properties are poorly understood because most of the relevant experiments were performed in a rather narrow temperature range. Herein, we used hollandite-type K_xTi₈O₁₆ as an example to systematically study the temperature-dependent structure and electrical transport properties in a wide temperature range from 25 to 900°C. The electrical transport involves both potassium ionic conduction and electronic conduction. With increasing temperature, the ionic conductivity increases below 800°C and decreases above 800°C. The electronic conductivity displays two maxima at 0.15 S/cm at 400°C and 5.2 × 10⁻⁴ S/cm at 800°C. These interesting variations in the conductivities are related to the presence of Ti³⁺ and the structural transformation from hollandite to a mixture of rutile and jeppeite. The findings reported herein support the potential application of titanium-based hollandites and provide an understanding of the electrical transport properties of functional ceramic materials.     

Stability and electrical properties of MoSi2‐ and WSi2‐oxide electroconductive composites

MoSi₂‐ and WSi₂‐based electroconductive ceramic composites were fabricated using 40‐80 vol% fine‐ and coarse‐Al₂O₃, and ZrO₂ particles (refractory oxides) after sintering in argon. Their chemical and thermal stability was tested between 1400°C‐1600°C for up to 48 hours. X‐ray diffraction analysis showed the formation of secondary 5‐3 metal silicide (Mo₅Si₃, W₅Si₃) and silica phases on the grain boundaries and surface. The fraction of the W₅Si₃ (11.4‐38.8 vol%) was significantly higher than that of the Mo₅Si₃ (3.3‐7.3 vol%) in the composites after annealing at 1400°C for 48 hours. The rates of grain growth in the composites (0.013‐0.023 μm/h) were highly decreased by a grain‐boundary pinning effect. This effect was relatively better with the addition of the coarse‐grained oxides due to their more homogeneous distribution throughout the microstructure. The 20–80 vol% MoSi₂‐Al₂O₃ (fine‐grained) composite exhibited an electrical conductivity of 8.8 S/cm at 900°C. At the 60 vol% silicide content, MoSi₂–Al₂O₃ (coarse‐grained) and WSi₂–Al₂O₃ (fine‐grained) showed higher electrical conductivity (126‐128 S/cm) at 900°C. The density, porosity level, particle distribution, intrinsic conductivity of silicide phase, particle size, and fraction of the secondary 5‐3 silicide phase highly influenced their electrical properties.     

Effect of the crystalline layer on the electrical behaviour of 17.7Li2O·5.2ZrO2·68.1SiO2·9.0Al2O3 glass ceramic monoliths

Li₂O–ZrO₂–SiO₂–Al₂O₃ (LZSA) glass ceramic systems are usually obtained from powder technology to obtain materials with a low thermal expansion coefficient (CTE). However, in these cases, there is a high residual porosity. An alternative to reduce the porosity involves the production of monoliths. Nevertheless, there is still a lack of crystallisation kinetics and the final properties of glass ceramic monoliths are affected such as electrical properties. This study aims to evaluate the electrical behaviour as function of the crystalline layer thickness formed on the monolith surface of a 17.7Li₂O·5.2ZrO₂·68.1SiO₂·9.0Al₂O₃ (molar basis) glass ceramic LZSA composition. Monoliths thermally treated at 750, 800, and 850 °C were chosen to evaluate based on the range of the crystalline layer growth. Electrochemical impedance spectroscopy was used for the electrical characterisation of LZSA glass and the glass ceramics. The resistivity increased with increasing thermal treatment temperature due to the formation of lithium-based crystalline phases. The electrical conductivity at 25 °C of the glass ceramic thermally treated at 850 °C decreased to 1.4 × 10^?13 S cm^?1 from 8.7 × 10^?11 S cm^?1 for LZSA glass. Based on the electrical behaviour, monoliths thermally treated at 850 °C can be considered potential for dielectric industrial applications.     

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.     

High‐Temperature Shear Strength and Bonding Mechanism of the Mullite Ceramic/Fiber Brick Component Joined by Phosphate Adhesive

The mullite ceramic/fiber brick system was bonded by two kinds of phosphate adhesives. The specimens were treated from 200 to 1400°C. The mechanical properties were tested at room temperature and at high temperature, and the relevant bonding mechanism was also discussed. The results show that the addition of silicon can greatly improve the adhesive's mechanical properties. The room‐temperature shear strength of the component bonded by adhesive with the silicon calcined at 800°C can reach 6.58 MPa. The shear strength of the adhesive with silicon tested at 800°C can reach 0.42 MPa.     

Low‐Temperature Sintering of β‐Spodumene Ceramics Using Li2O–GeO2 as a Sintering Additive

Low‐temperature sintering of β‐spodumene ceramics with low coefficient of thermal expansion (CTE) was attained using Li₂O–GeO₂ sintering additive. Single‐phase β‐spodumene ceramics could be synthesized by heat treatment at 1000°C using highly pure and fine amorphous silica, α‐alumina, and lithium carbonate powders mixture via the solid‐state reaction route. The mixture was calcined at 950°C, finely pulverized, compacted, and finally sintered with or without the sintering additive at 800°C–1400°C for 2 h. The relative density reached 98% for the sample sintered with 3 mass% Li₂O–GeO₂ additive at 1000°C. Its Young's modulus was 167 GPa and flexural strength was 115 MPa. Its CTE (from R.T. to 800°C) was 0.7 × 10^?6 K^?1 and dielectric constant was 6.8 with loss tangent of 0.9% at 5 MHz. These properties were excellent or comparative compared with those previously reported for the samples sintered at around 1300°C–1400°C via melt‐quenching routes. As a result, β‐spodumene ceramics with single phase and sufficient properties were obtained at about 300°C lower sintering temperature by adding Li₂O–GeO₂ sintering additive via the conventional solid‐state reaction route. These results suggest that β‐spodumene ceramics sintered with Li₂O–GeO₂ sintering additive has a potential use as LTCC for multichip modules.     

Electrically conductive SiC ceramics processed by pressureless sintering

Polycrystalline SiC ceramics with 10 vol% Y₂O₃-AlN additives were sintered without any applied pressure at temperatures of 1900-2050°C in nitrogen. The electrical resistivity of the resulting SiC ceramics decreased from 6.5 × 10¹ to 1.9 × 10⁻² Ω·cm as the sintering temperature increased from 1900 to 2050°C. The average grain size increased from 0.68 to 2.34 μm with increase in sintering temperature. A decrease in the electrical resistivity with increasing sintering temperature was attributed to the grain-growth-induced N-doping in the SiC grains, which is supported by the enhanced carrier density. The electrical conductivity of the SiC ceramic sintered at 2050°C was ~53 Ω⁻¹·cm⁻¹ at room temperature. This ceramic achieved the highest electrical conductivity among pressureless liquid-phase sintered SiC ceramics.     

Morphological and Electrochemical Properties of Crystalline Praseodymium Oxide Nanorods

Highly crystalline Pr₆O₁₁ nanorods were prepared by a simple precipitation method of triethylamine complex at 500°C. Synthesized Pr₆O₁₁ nanorods were uniformly grown with the diameter of 12–15 nm and the length of 100–150 nm without any impurities of unstable PrO₂ phase. The Pr₆O₁₁ nanorod electrodes attained a high electrical conductivity of 0.954 Scm⁻¹ with low activation energy of 0.594 eV at 850°C. The electrochemical impedance study showed that the resistance of electrode was significantly decreased at high temperature, which resulted from its high conductivity and low activation energy. The reduced impedance and high electrical conductivity of Pr₆O₁₁ nanorod electrodes are attributed to the reduction of grain boundaries and high space charge width.     

Dielectric Tunability,Dielectric Relaxation,and Impedance Spectroscopic Studies on (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 Lead‐Free Ceramics

Dielectric and impedance spectroscopies were employed to study the electrical behavior of (Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃ (abbr. BCTZ) lead‐free ceramic. The dielectric properties versus dc bias electric field experiment revealed high dielectric tunability (> 65%) as well as figure of merit (> 27) at 10 kHz and room temperature. At elevated temperature range, a dielectric loss peak was observed and verified to be correlate of oxygen vacancy relaxation. The impedance spectra studies indicate that the ceramic is a mixed ionic conductor of p‐type nature at the paraelectric phase and, the grain and total conductivity at 600°C reaches 6.0 × 10^?5 and 2.0 × 10^?5 S/cm, respectively.     

Development of microstructure during creep of polycrystalline mullite and a nanocomposite mullite/5 vol.% SiC

The microstructures of as-sintered and creep tested polycrystalline mullite and mullite reinforced with 5 vol.% nano-sized SiC particles have been characterized by scanning and transmission electron microscopy. The dislocation densities after tensile creep testing at 1300 and 1400 °C were virtually unchanged as compared to the as-sintered materials which indicates diffusion-controlled deformation. Mullite matrix grain boundaries bending around intergranular SiC particles suggest that grain boundary pinning, in addition to a reduced mullite grain size, contributed to the increased creep resistance of the mullite/5 vol.% SiC nanocomposite. Both materials showed pronounced cavitation at multi-grain junctions after creep testing at 1400 °C which suggests that unaccommodated grain boundary sliding, facilitated by softening of the intergranular glass, occurred at this temperature. This is consistent with the higher stress exponents at 1400 °C.     

Synthesis and properties of conductive B4C ceramic composites with TiB2 grain network

High electrical resistance and low fracture toughness of B₄C ceramics are 2 of the primary challenges for further machining of B₄C ceramics. This report illustrates that these 2 challenges can be overcome simultaneously using core‐shell B₄C‐TiB₂&TiC powder composites, which were prepared by molten‐salt method using B₄C (10 ± 0.6 μm) and Ti powders as raw materials without co‐ball milling. Finally, the near completely dense (98%) B₄C‐TiB₂ interlayer ceramic composites were successfully fabricated by subsequent pulsed electric current sintering (PECS). The uniform conductive coating on the surface of B₄C particles improved the mass transport by electro‐migration in PECS and thus enhanced the sinterability of the composites at a comparatively low temperature of 1700°C. The mechanical, electrical and thermal properties of the ceramic composites were investigated. The interconnected conductive TiB₂ phase at the grain boundary of B₄C significantly improved the properties of B₄C‐TiB₂ ceramic composites: in the case of B₄C‐29.8 vol% TiB₂ composite, the fracture toughness of 4.38 MPa·m^1/2, the electrical conductivity of 4.06 × 10⁵ S/m, and a high thermal conductivity of 33 W/mK were achieved.