Effect of the graphitization level of the free carbon on the temperature sensitivity of silicon carbonitride-based pressure sensors

Polymer-derived ceramics (PDCs) have recently attracted an increasing attention because of their applications for wireless passive pressure sensors in the harsh environment. However, due to the effect of temperature on the frequency of PDC-based wireless passive pressure sensors, it is not beneficial to accurate measurement of pressure. In this paper, a dense polymer-derived silicon carbonitride (SiCN) ceramic was prepared by precursor infiltration and pyrolysis (PIP) technique to reduce the temperature sensitivity of PDC–SiCN-based pressure sensor. The open porosity and density of SiCN ceramics varied from 13.34% and 1.89 g/cm³ without PIP process to 3.24% and 2.09 g/cm³ after three PIP cycles, respectively. Raman spectroscopy revealed that the level of graphitization of free carbon in dense SiCN ceramics is higher than that in porous SiCN ceramics, which would lead to an increase in the conductivity of dense SiCN ceramics. After three PIP cycles, the conductivity increased by almost two orders of magnitude from 3.01E − 10 to 1.28E − 08 S/cm. In addition, SiCN ceramic discs after PIP cycles and without PIP were applied to wireless passive pressure sensor based on resonator, which were tested at high temperature, respectively. Results confirmed that the temperature sensitivity of PDC–SiCN-based pressure sensor decreased from 220.5 to 50.8 kHz/°C by PIP process.     

SiCN ceramics with controllable carbon nanomaterials for electromagnetic absorption performance

Different kinds of carbon nanomaterials, free carbon (C_free), graphene, and N-containing graphene (NG), in single-source-precursors-derived SiCN ceramics, were in situ generated by modifying polysilazane with divinylbenzene, dopamine hydrochloride and melamine, respectively. Adjusting the carbon source brings phase structure and electromagnetic wave absorption (EMA) properties differences of SiCN/C ceramics. In situ C_free enhances the EMA capacity of SiCN ceramics by improving their electrical conductivity of 9.2 × 10⁻⁴ S/cm. The electrical conductivity of SiCN ceramics with 2D graphene sheets balloons to 2.5 × 10⁻³ S/cm, causing poor impedance match thus leading to a worse EMA performance. In situ NG in SiCN ceramics has a low electrical conductivity of 5.6 × 10⁻⁸ S/cm, making for excellent impedance match. The corrugated NG boosts dielectric loss, interfacial, and dipole polarization. NG-SiCN nanocomposites possess an outstanding EMA performance with RL_min of −61.08 dB and effective absorption bandwidth of 4.05 GHz, which are ∼2.4 times lower and ∼4 times higher than those of SiCN, respectively.     

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.     

Influence of the cold sintering process and post annealing on the microstructure and Li-ion conductivity of LiTa2PO8 solid electrolyte

Recently, LiTa₂PO₈ (LTPO) has attracted interest as a potential Li-ion solid electrolyte material because of its high bulk ionic conductivity and low grain boundary ionic conductivity. However, most ceramic-based solid electrolytes are fabricated via the high-temperature sintering process (typically above 1000 °C); such temperatures can cause the evaporation of Li from the compound. To replace high-temperature sintering of ceramics, the cold sintering process (CSP) was introduced; this process enables the densification of ceramics and composites at extremely low temperatures (below 300 °C). In this work, we investigate the effect of using the CSP and post annealing on the microstructure and Li-ion conductivity of LTPO pellets. It is found that the CSP pellets have an amorphous phase between particles. This intermediate amorphous phase creates a better contact between particles and is hypothesized to lead to more Li-ion migration paths. The CSP pellet is found to have a high density and high ionic conductivity of (1.19 × 10^?5 S/cm). The pellet obtained via the CSP has Li-ion conductivity similar to that of the pellet obtained via dry pressing after it has been annealed. The CSP pellet after post annealing shows good connections between particles and a high Li-ion conductivity of 1.05 × 10^?4 S/cm, which is comparable to the conductivity of a pellet obtained via high-temperature sintering. This work provides new evidence that the CSP is a promising alternative to high-temperature sintering for fabricating ceramic solid electrolytes.     

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.     

Effect of Microstructure on the Thermal Expansion Coefficient of Sintered Cordierite Prepared from Sol Mixtures

Dense cordierite ceramics were prepared from a sol mixture of alumina, silica, and magnesia, and the relationship between microstructure and thermal expansion was clarified for sinters with relative density greater than 97%. In the dense cordierite ceramics, submicrometer‐sized primary cordierite crystals aligned in the same crystal orientation and constituted the domain structure. We discovered that these domain structures could be easily observed by optical polarizing microscopy and quantified by digital image analysis of the photographs. The occurrence of microcracks between domains larger than 40 μm was induced by the thermal expansion anisotropy of the cordierite crystal axes. As a result, the mean thermal expansion coefficient of the cordierite ceramics decreased to 0.4 × 10^?6 K^?1 from the average value of the crystal axes of 1.7 × 10^?6 K^?1. This lower thermal expansion coefficient could be theoretically explained by partial microcracking.     

Electrical characterization of multidoped ceria ceramics

Ceria ceramics was obtained from multi-doped nanosized ceria powders prepared by both modified glycine nitrate procedure (MGNP) and self-propagating reaction at room temperature (SPRT). Rare earth elements such as Nd, Sm, Gd, Dy, Y, Yb were used as dopants. The overall mole fraction of dopants was 0.2. One-hour long sintering of powder compacts was performed at 1500 °C in oxygen atmosphere. Phase composition, microstructure and ionic conductivity of sintered samples were analysed. Single-phase ceria was detected in all samples. In general, the increase in the number of dopants improved the ionic conductivity. The samples doped simultaneously with five dopants had the highest ionic conductivity, as evidenced by the impedance measurements. At 450 °C, the conductivity of sample obtained by MGNP was 3.94×10^?3 Ω^?1 cm^?1 whereas the conductivity of sample obtained by SPRT was 2.61×10^?3 Ω^?1 cm^?1. The conductivity activation energy for MGNP and SPRT samples was measured to be 0.348 and 0.385 eV, respectively. Finally, the conductivity decreased as the number of dopants increased to six.     

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.     

Dielectric and electromagnetic properties of amorphous PDCs-Si(Al)CNO with different Al contents in X-band

Amorphous polymer-derived Si(Al)CNO ceramics with different Al contents were successfully prepared using polyvinylsilazane (PVSZ) doped with different mass fractions (0 to 15 wt%) of aluminum tri-sec-butoxide (ASB) as the precursors. The thermogravimetric, crystal morphology, microstructure, electrical conductivity and dielectric properties of Si(Al)CNO ceramics were characterized and tested, respectively. The results show that the porosity of Si(Al)CNO ceramics first decreased and then increased with the increase of ASB doping content. The electrical conductivity of SiAlCNO-5, SiAlCNO-10 and SiAlCNO-15 ceramics gradually decreased from 2.80 × 10^-9 to 0.89 × 10^-9 (Ω cm)^-1. Their conductivity was related to free carbon in the ceramics based on Raman spectroscopy. The average permittivity of SiCNO, SiAlCNO-5, SiAlCNO-10 and SiAlCNO-15 ceramics gradually increased from 3.63 to 3.93 with the increase of ASB doping content in X-band. The dielectric loss of SiAlCNO ceramics doped with ASB was smaller than that of SiCNO ceramics. But with the continuous increase of ASB doping content, the dielectric loss of SiAlCNO ceramics also increased. The permittivity and dielectric loss of Si(Al)CNO ceramics were related to their porosity and electrical conductivity. The average absorption coefficient of Si(Al)CNO ceramics were less than 0.020, while their transmission coefficient were greater than 0.500. It was found that the amorphous Si(Al)CNO ceramics exhibit good electromagnetic wave transmission properties.     

Hierarchically Fe-doped porous carbon derived from phenolic resin for high performance supercapacitor

Supercapacitors are promising for high power application in the recent years. In particular, the conversion of simple and available carbon materials into economic and high performance electrical devices receives excellent scientific and technological interest. This paper reports a one-step strategy for synthesizing hierarchical porous carbon derived from phenolic resin (PR), which is then used to configure electric double-layer capacitors (EDLCs). Here, a carbon material with a flexible porous structure, large specific surface area, and high graphitization degree is prepared using potassium ferrate (K₂FeO₄) to catalytically activate PR and to realize synchronous carbonization and graphitization. This method overcomes the disadvantage of time-consuming, high-cost, and environmentally unfriendly. In addition, the as-prepared carbon material has a high specific surface area (1086 m² g^?1) and a large pore size (3.07 nm), which can increase the transfer rate of electrolyte ions. The specific capacitance of the obtained electrode material is 315 F g^?1 at 1.0 A g^?1, and the optimized electrode material has an ultra-long cycle lifetime (capacitance retention rate is 96.3% after 10,000 cycles). Thus, the hierarchically Fe-doped porous carbon material derived from PR material is expected to realize high rate capacitance for supercapacitor applications.     

Improving specific stiffness of silicon carbide ceramics by adding boron carbide

A strategy for improving the specific stiffness of silicon carbide (SiC) ceramics by adding B₄C was developed. The addition of B₄C is effective because (1) the mass density of B₄C is lower than that of SiC, (2) its Young’s modulus is higher than that of SiC, and (3) B₄C is an effective additive for sintering SiC ceramics. Specifically, the specific stiffness of SiC ceramics increased from ~142 × 10⁶ m²?s^?2 to ~153 × 10⁶ m²?s^?2 when the B₄C content was increased from 0.7 wt% to 25 wt%. The strength of the SiC ceramics was maximal with the incorporation of 10 wt% B₄C (755 MPa), and the thermal conductivity decreased linearly from ~183 to ~81 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 25 wt% B₄C were ~690 MPa and ~95 W?m^?1?K^?1, respectively.     

Synthesis of Ga-doped Li7La3Zr2O12 solid electrolyte with high Li+ ion conductivity

Garnet-type Li₇La₃Zr₂O₁₂ (LLZO) Li⁺ ion solid electrolyte is a promising candidate for next generation high-safety solid-state batteries. Ga-doped LLZO exhibits excellent Li⁺ ion conductivity, higher than 1 × 10^?3 S cm^?1. In this research, the doping amount of Ga, the calcination temperature of Ga-LLZO primary powders, the sintering conditions and the evolution of grains are explored to demonstrate the optimum parameters to obtain a highly conductive ceramics reproducibly via conventional solid-state reaction methods under ambient air sintering atmosphere. Cubic LLZO phase is obtained for Li_6.4Ga_0.2La₃Zr₂O₁₂ powder calcined at low temperature 850 °C. In addition, ceramic pellets sintered at 1100 °C for 320 min using this powder have relative densities higher than 94% and conductivities higher than 1.2 × 10^?3 S cm^?1 at 25 °C.     

Effect of MgO and PVA on the Synthesis and Properties of Negative Thermal Expansion Ceramics of Zr2(WO4)(PO4)2

A green synthesis of Zr₂(WO₄)(PO₄)₂ ceramics from ZrO₂, WO₃ and P₂O₅ is presented. It is shown that the ceramics can be synthesized by one‐step sintering within 60 min. The relative density of the ceramics can be enhanced from about 75% without sintering additives to 99.8% of the theoretical value with 1.0 wt% MgO and 2.0 wt% polyvinyl alcohol. The grain sizes of the ceramics are smaller and more uniform with MgO added in the raw materials than with MgO added in the Zr₂(WO₄)(PO₄)₂ powder. The coefficients of thermal expansion are about ?2.325 × 10^?6, ?1.406 × 10^?6, ?1.509 × 10^?6 and ?1.384 × 10^?6°C^?1 for the samples without MgO, with MgO added in the raw materials, with MgO added in the Zr₂(WO₄)(PO₄)₂ powder and with MgO and PVA added in the raw materials, respectively.     

Evaluation of Air Permeation Behavior of Porous SiC Ceramics Synthesized by Oxidation‐Bonding Technique

Porous SiC ceramics were synthesized by oxidation bonding of compacts of commercial α‐SiC powder at 1300°C. Different volume fractions of petroleum coke powder were used for variation of porosity of ceramics from 36% to 56%. The material exhibited variations of pore size from 3 to 15 μm, flexural strength from 5.5 to 29.5 MPa, and elastic modulus from 3.3 to 27.6 GPa. Air permeation behavior was studied at 26–650°C. At room temperature Darcian (k₁) and non‐Darcian (k₂) permeability parameters vary from 1.64 to 18.42 × 10^?13 m² and 0.58 to 2.95 × 10^?7 m, respectively. Temperature dependence of permeability was explained from structural changes occurring during test conditions.     

Enhanced “overall” ionic conductivity in Li1+xAlxTi2−x(PO4)3 (0.3 ≤ x ≤ 0.7) ceramics prepared from sol-gel powders by spark plasma sintering

The Li_1+xAl_xTi_2?x(PO₄)₃ (LATPx) series displays the highest “bulk” reported conductivity, but a much lower “overall” contribution, that changes with the powder preparation and sintering conditions. In this work, the preparation of LATPx ceramics is discussed, by using the sol-gel technique for powders synthesis and mild spark plasma (SPS) for ceramics sintering at 800 °C. An “overall” conductivity ~ 2.10^?3 Ω^?1 cm^?1 was obtained for the x = 0.4 composition, that was the result of a high “bulk” conductivity, an optimized microstructure and almost full density, in absence of micro-cracks, with a small content of secondary phases and clean grain boundaries. Fast-ion ceramics prepared by SPS are good candidates for solid electrolytes in all solid state batteries (ASSB).     

Novel polyimide-based electrospun carbon nanofibers prepared using ion-beam irradiation

This paper describes a novel study focused on preparing carbon nanofibers, with a narrow fiber diameter distribution, from a fluorinated polyimide using both electrospinning and ion-beam irradiation. We specifically focused on the effects of ion species and ion fluences on the electrical conductivity of the nanofiber. The nanofibers were successfully prepared in the diameter range from 340 nm to 1500 nm by varying the concentration of polyimide solution using electrospinning. The Raman spectrum of the ion-irradiated nanofiber included the two well-known D (1360 cm^?1) and G (1580 cm^?1) peaks, indicating that the nanofiber surface changed to a carbon-enriched material. The carbon nanofibers underwent a more ordered graphitic carbon structure with an increase in the ion fluence and the electrical conductivity of the nanofiber irradiated at 1 × 10¹⁶ ions/cm² of Ar⁺ was 0.18 S/cm. In addition, the electrical conductivities of the ion-irradiated nanofibers increased in the order, He⁺ < Ne⁺ < Ar⁺, which indicated that the amount of nuclear energy in the ion species had the most influence on the electrical conductivity. However, the higher electrical conductivity of the carbon nanofibers is required to realize their industrial applications. This paper is the first to address the properties of the electrical conductivity of the carbon nanfibers prepared by electrospinning and ion irradiation as a new approach.     

Piezoresistive effect of individual electrospun carbon nanofibers for strain sensing

Piezoresistive behavior of individual electrospun carbon nanofibers (CNF) was studied for the first time via a microelectromechanical systems platform. The gage factor of CNFs was found to vary from 1.96 to 2.55, not correlating with nanofiber diameter. The measured strain sensitivity of electrical resistance of individual CNFs could not be solely explained based on strain induced dimensional changes of CNFs, pointing to piezoresistivity in nanofibers. The microstructure of CNFs was studied via TEM imaging and Raman spectroscopy, suggesting the presence of sp² and sp³ hybridized carbon atoms in CNFs. The piezoresistivity of CNFs was explained in light of their hybrid structure. A one-dimensional model was adopted to relate CNFs piezoresistivity to their microstructure and electron tunneling between sp² hybridized regions through sp³ hybridized regions. The calibrated model revealed tunneling distances of 0.15–0.3 nm between sp² hybridized atoms. Moreover, our study pointed to the degree of graphitization and elastic mismatch between differently hybridized carbon atom regions in CNFs as critical parameters controlling CNFs’ piezoresistivity. This study sets the stage for the utilization of CNFs, not just as load bearing elements, but also as multifunctional nanoscale components with strain sensing capabilities, for instance in Nanoelectro-mechanical systems applications.     

Studies on structural,morphological, electrical and electrochemical properties of activated carbon prepared from sugarcane bagasse

Activated carbon composite was prepared from sugarcane bagasse. The X-ray diffraction revealed the evolution of crystallites of carbon and silica during activation at higher temperature. FTIR spectrum shows the presence of functional groups and silica in the carbon composite. The morphology of the carbon sample was determined by SEM. The surface area, pore volume and pore size distribution of carbon composites were measured. The dc conductivity was determined and conductivity at room temperature was found to increase from 10.22 × 10^?3 to 25.131 × 10^?3 S cm^?1. The samples show good electrochemical property and the specific capacitance in the range of 92–340 F g^?1.     

High-entropy titanate pyrochlore as newly low-thermal conductivity ceramics

Low-thermal conductivity ceramics play an indispensable role in maximizing the efficiency and durability of hot end components. Pyrochlore, particularly zirconate pyrochlore, is currently a highly promising and widely studied candidate for its extremely low thermal conductivity. However, there are still few pyrochlores that offer both stiffness, insulation, and good thermal expansion properties. In this work, the solidification method was innovatively introduced into the preparation of titanate pyrochlore, and combined it with the compositional design of high-entropy. Through careful composition design and solidification control, the high-density and uniform elements distributed high-entropy titanate pyrochlore ceramics were successfully prepared. These samples possess high hardness (15.88 GPa) and Young’s modulus (295.5 GPa), low thermal conductivity (0.947 W·m^?1·K^?1), excellent thermal expansion coefficient (11.6 ×10^?6/K) and an exquisite balance between stiffness and insulation (E/κ, 312.1 GPa·W^?1·m·K), in which the E/κ exhibits the highest value among the current reported works.     

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.