Thermophysical property and electrical conductivity of titanium isopropoxide – polysiloxane derived ceramics

In this study, high temperature resistant Si-O-C-Ti has been successfully prepared based on the pyrolysis of polysiloxane (PSO) and titanium (IV) isopropoxide (TTIP) at 1200–1400 °C. PSO can homogeneously mix with TTIP to enhance its conversion to TiC. The carbothermal reactions between TiO₂ (product of thermal decomposition of TTIP) and carbon result in the formation of TiC. All the Si-O-C-Ti composites pyrolyzed at 1200–1300 °C are stable up to 1000 °C in an oxidizing air atmosphere. TiC leads to high electrical conductivity at elevated temperatures; the maximum conductivity is 1176.55 S/m at 950 °C, which is the first reported value of >1000 S/m conductivity for Si-O-C-Ti ceramics. However, too high a pyrolysis temperature, such as 1400 °C, can potentially ‘destabilize’ the Si-O-C-Ti system by consuming the free carbon and result in lower conductivities.     

Ceramic TiC/a:C protective nanocomposite coatings: Structure and composition versus mechanical properties and tribology

The relationship between the structure, elemental composition, mechanical and tribological properties of TiC/amorphous carbon (TiC/a:C) nanocomposite thin films was investigated. TiC/a:C thin film of different compositions were sputtered by DC magnetron sputtering at room temperature. In order to prepare the thin films with various morphology only the sputtering power of Ti source was modified besides constant power of C source. The elemental composition of the deposited films and structural investigations confirmed the inverse changes of the a:C and titanium carbide (TiC) phases. The thickness of the amorphous carbon matrix decreased from 10 nm to 1–2 nm simultaneously with the increasing Ti content from 6 at% to 47 at%. The highest hardness (H) of ~26 GPa and modulus of elasticity (E) of ~220 GPa with friction coefficient of 0.268 was observed in case of the film prepared at ~38 at% Ti content which consisted of 4–10 nm width TiC columns separated by 2–3 nm thin a:C layers. The H3/E2 ratio was ~0.4 GPa that predicts high resistance to plastic deformation of the TiC based nanocomposites beside excellent wear-resistant properties (H/E=0.12).     

Hollow porous carbon microspheres obtained by the pyrolysis of TiO2/poly(furfuryl alcohol) composite precursors

Disordered carbon materials with high porosity were prepared through the pyrolysis of TiO₂/poly(furfuryl alcohol) composites, obtained by the sol-gel method. The composites were prepared starting from titanium tetra-isopropoxide (TTIP) and furfuryl alcohol (FA) as precursors. Two different synthetic procedures for our composites were carried out, based on the addition of furfuryl alcohol (FA) before or after the TiO₂ nanoparticles formation. Also, different TTIP/FA ratio was tested. The hybrid materials obtained by both synthetic routes were pyrolyzed, under argon flow, at 900 °C producing novel TiO₂/carbon composites. All samples were characterized by XRD, FT-IR, DR-FTIR, Raman spectroscopy and TEM. Results indicated the effective FA polymerization on TiO₂ (anatase) nanoparticles, and polymer conversion to disordered carbon after the pyrolysis, simultaneously with TiO₂ anatase-rutile phase transition. The resulting TiO₂/carbon composites were treated with HF solution aiming the oxide dissolution, yielding an extremely porous carbon material as insoluble fraction. The morphology of these porous carbon materials is strongly dependent on the synthetic route adopted for the composite precursor, varying from carbon foam to highly ordered hollow microspheres.     

In-situ formation of titanium carbide in carbon-rich silicon oxycarbide ceramic for enhanced thermal stability

Silicon oxycarbide (SiOC) ceramic has attracted great attention as fascinating candidate of high-temperature material, however, its thermal stability is significantly limited by the phase separation at high temperature. Here, a TiC/SiOC ceramic was prepared by pyrolysis of a tetrabutyl titanate modified carbon-rich polysiloxane (TBT/PSO) precursor. The TiC phase is in-situ formed by the carbothermal reaction of TBT-derived amorphous TiO₂ phase with excess free-carbon phase during pyrolysis, and its size and amount increase with the pyrolysis temperature. The SiC phase appears at a higher temperature than the TiC phase and is hindered by the increased Ti content in the TBT/PSO precursor. Thus, the TiC/SiOC ceramic exhibits better thermal stability and crystallization resistance than the TiC-free SiOC ceramic under the thermal treatment (1500 °C) in argon atmosphere. The in-situ formation of metal carbide into the carbon-rich SiOC ceramic would further expand its application at high temperature environments.     

Sputtered nanocrystalline ceramic TiC/amorphous C thin films as potential materials for medical applications

The relationship between structural behaviour of sputtered TiC/amorphous C (TiC/a:C) thin films and corrosion properties was measured in three various pH solutions (0.5 M NaCl (pH=6); 0.1 M HCl (pH=1); and 0.1 M NaOH (pH=13)). The ~400 nm thick nanocomposites were deposited by DC magnetron sputtering on different substrates (Ti6Al4V alloy and CoCrMo alloy) in argon at 25 °C and 0.25 Pa with 150 W input power of carbon target and 50 W input power of titanium target. The structure and composition of nanocomposites were investigated by Transmission and Scanning Electron Microscopy. In both samples the structural investigations confirmed columnar structure of TiC/a:C films with 25–50 nm sized cubic TiC. These columns were separated by 2–3 nm thin amorphous carbon layers. TiC/a:C /Ti6Al4V alloy implant material showed better corrosion resistance than the TiC/a:C/CoCrMo alloy in 0.5 M NaCl solution based on results of the Electrochemical Impedance Spectroscopy. For both samples, the 0.1 M NaOH solution was the most corrosive media.     

CNT-induced TiC toughened Al2O3/Ti composites: Mechanical,electrical, and room-temperature crack-healing behaviors

This study addressed novel multiphase composite of Al₂O₃/Ti/TiC that exhibited enhanced fracture toughness and room-temperature crack-healing function. Al₂O₃/Ti/TiC composites were fabricated through hot-press sintering of CNT, TiH_2, and Al₂O₃ mixed powders, where the TiC was in-situ formed by reaction of CNT and Ti. The effects of CNT (TiC) content on mechanical and electrical properties were studied. Electrochemical anodization process at room temperature was attempted to these composites to heal cracks introduced in the surface of composites. Results indicated that added CNT was invisible while metal Ti and reaction product TiC coexisted in all samples. The reaction between CNT and Ti[O] representing dissolved active oxygen into Ti was considered as the main formation route of TiC. The toughening mechanism was demonstrated as crack deflection and bridging due to the presence of TiC. In spite of the increase in electrical resistivity because of the higher resistivity of TiC than Ti, the present Al₂O₃/Ti/TiC composites still remain high enough electrical conductivity (8.0 × 10⁻³ Ωcm ~1.8 × 10⁻² Ωcm for 0-2 vol% CNT addition) which could be regarded as conductors; it allowed to heal cracks in the composites by electrochemical anodization that formed titanium dioxide phase at room temperature. It was found that crack-healing ability in 1 vol% CNT added composite exhibited higher strength recovery ratio of 95.6% to the crack-free sample than that of Al₂O₃/Ti composite (the recovery ratio of 89.6%). After crack-healing process, mechanical strength of samples increased by 52.3% compared to cracked composites. It was concluded that the formed TiC could contribute to the appropriate electrical conduction as well as interface strengthening in the Al₂O₃/Ti composites. Furthermore, it was firstly speculated that the TiC could be electrochemically anodized to form an oxide like Ti metal. These characteristics enable Al₂O₃/Ti/TiC composites as the crack-healing materials at room temperature.     

Developing high performance in titanium carbide nanocomposites containing hybrid SiC nanowire and CNT

Lightweight titanium carbide (TiC) has garnered considerable engineering applications in various advanced manufacturing industries. However, the intrinsic brittleness of TiC significantly limited its further applications. High-toughness and damage-tolerance TiC is always highly desirable for industry. Herein, we developed high-performance TiC nanocomposites reinforced by hybrid carbon nanotube (CNT) and SiC nanowire (SiC_nw) through two-step spark plasma sintering, highlighting the synergic role of CNT and SiC_nw on the microstructure and properties of the TiC matrix. Specifically, CNT was helpful in maintaining the high aspect ratio of SiC_nw during the sintering process, while SiC_nw contributed to the homogeneous distribution of CNT throughout the TiC matrix. The flexural strength and fracture toughness were simultaneously enhanced by 48.1% and 56.9% in case of CNT/SiC_nw ratio of 1:2 comparing with pure TiC, respectively. This study provided the new insight on developing high-performance TiC materials.     

熔盐辅助碳诱导Ti3AlC2分解制备碳化钛纳米片

二维过渡金属碳化物因其独特的性质而成为极具吸引力的纳米材料。以炭黑和Ti₃AlC₂粉体为原料,以期通过在熔盐环境下实现碳还原Ti₃AlC₂合成碳化钛（TiC）纳米片。研究结果表明,炭黑可以诱导Ti₃AlC₂分解,随着加热温度的升高,Ti₃AlC₂的分解程度逐渐加剧。Ti₃AlC₂在低温（1 000~1 100 ℃）下可以严重分解为Al、Ti、Ti_2.9Al_2.1和TiC,Al和Ti可与微量氧反应生成少量的TiO₂和Al₂O₃,生成的TiC为纳米颗粒,而引入熔盐是确保TiC纳米片形成的必要条件。通过优化制备工艺得到合成高含量TiC纳米片的适宜条件：炭黑与Ti₃AlC₂物质的量比为1.05∶1,氯化钠与氯化钾的质量比为1∶1,盐的总质量与炭黑和Ti₃AlC₂混合物的质量比为2∶1;氩气保护,1 000 ℃保温2 h。通过熔盐热处理得到以TiC为主并含有少量Al₂O₃的试样。产物中TiC的形貌为细小的纳米片,厚度约为10 nm、长度约为170 nm。     

Mechanisms of TiB2 and graphite nucleation during TiC–B4C high temperature interaction

Reactive hot pressing of TiC–B₄C precursors was undertaken at 1800 °C to produce TiB₂ with carbon inclusions. Atomic mechanisms of titanium diboride nucleation, as well as sponge-like carbon inclusions and submicron platelets of graphite precipitation have been investigated. Precursor grain size, green body composition and synthesis time were varied to analyze phase transformation. The carbon left after B₄C high temperature decomposition is shown remaining as graphite sponge-like inclusions. Ab-initio calculations confirm that the boron atoms accumulation on (111) TiC plains leads to tensile stress. The developed stress cleaves TiC grains and enhances further reaction. Most of carbon expelled from TiC during its transformation into TiB₂ forms graphite submicron platelets.     

Preparation of titanium dioxide/activated carbon composites using supercritical carbon dioxide

The penetration of titanium tetraisopropoxide (TTIP) dissolved in supercritical CO₂ into the nano-spaces of an activated carbon was studied for the preparation of a TiO₂-coated activated carbon. The conversion of TTIP to TiO₂ through thermal decomposition was confirmed by evolved gas analysis during heat treatment under a N₂ flow. Acetone was detected in the evolved gas, which suggested that some isopropoxide groups in TTIP reacted with the carbonyl groups on the activated carbon surface. This chemical reaction with carbon is expected to be advantageous for favorable attachment to the carbon surface. The crystallite size of anatase in the TiO₂/carbon composites was 4.1 nm, as estimated from the X-ray diffraction pattern, which almost corresponded to the graphene crystallite size; L_a (3.3-3.4 nm), as estimated from both the Raman spectrum and X-ray diffraction pattern. As the size of the crystallite prepared by bulk condensation of TTIP was more than 15 nm, these results confirmed that the anatase crystals were present in the carbon pores. Also, it was suggested that the crystal growth of TiO₂ was influenced by the carbon nano-spaces.     

Synthesis and characterization of precursor derived TiN@Si–Al–C–N ceramic nanocomposites for oxygen reduction reaction

The development of efficient and durable catalysts is critical for the commercialization of fuel cells, as the catalysts’ durability and reactivity dictate their ultimate lifetime and activity. In this work, amorphous silicon-based ceramics (Si–C–N and Si–Al–C–N) and TiN@Si–Al–C–N nanocomposites were developed using a precursor derived ceramics approach. In TiN@Si–Al–C–N nanocomposites, TiN nanocrystals (with sizes in the range of 5–12 nm) were effectively anchored on an amorphous Si–Al–C–N support. The nanocomposites were found to be mesoporous in nature and exhibited a surface area as high as 132 m²/g. The average pore size of the nanocomposites was found to increase with an increase in the pyrolysis temperature, and a subsequent graphitization of free carbon was observed as revealed from the Raman spectra. The ceramics were investigated for electrocatalytic activity toward the oxygen reduction reaction using the rotating disk electrode method. The TiN@Si–Al–C–N nanocomposites showed an onset potential of 0.7 V versus reversible hydrogen electrode for oxygen reduction, which seems to indicate a 4-electron pathway at the pyrolysis temperature of 1000°C in contrast to a 2-electron pathway exhibited by the nanocomposites pyrolyzed at 750°C via the Koutecky–Levich plot.     

Effect of molar ratio of TiO2/SiO2 on the properties of particles synthesized by flame spray pyrolysis

Titania (TiO₂)–silica (SiO₂) nanoparticles were synthesized from sprayed droplets of a mixture of TEOS and TTIP by flame spray pyrolysis (FSP). The effect of molar ratio between TEOS and TTIP in the mixture on the particle properties such as particle morphology, average particle diameter, specific surface area, crystal structure, etc., were determined using TEM, XRD, BET, and FT-IR. A UV-spectrometer was also used to measure the absorption spectrum and the band gap energy of the product particles. As the molar ratio of TEOS/TTIP increased by increasing TEOS concentration at the fixed TTIP concentration, the average particle diameter of the mixed oxide nanoparticles increased with maintaining uniform dispersion between TiO₂ and SiO₂, and crystal structure was transformed from anatase to amorphous. The band gap energy of the TiO₂–SiO₂ nanoparticles increased with respect to the increase of the molar ratio due to the decrease of width of UV-absorption spectrum. Photocatalytic activity of TiO₂–SiO₂ composite particles decreased with the concentration of TEOS.     

Formation of TiO2 hollow microparticles by spraying water droplets into an organic solution of titanium tetraisopropoxide (TTIP) — Effects of TTIP concentration and TTIP-protecting additives

Hollow, spherical TiO₂ microparticles of several tens of micrometers in diameter can be prepared by spraying water into an organic phase containing titanium tetraisopropoxide (TTIP) as a titanium source. The concentration of TTIP did not affect the shell thickness. On the contrary, the shell thickness was increased with the concentration of the additives such as acetic acid and acetylacetone, having effects to protect TTIP from hydrolysis and condensation. The formation of a hollow particle was described by a simple model involving the hydrolysis of TTIP at the water–oil interface, the inward diffusion of hydrolyzed titanium hydroxide through the passage in the shell and its incorporation into the TiO₂ shell by condensation. The reduction of porosity of shell inhibits the diffusion, resulting in the formation of hollow structure. The simulation based on this model predicted that the shell thickness increased as the diffusion rate increased or the reaction rate decreased, and was independent of the outer TTIP concentration. These predictions were in qualitative agreement with the experimental results.     

Models Proposed to Explain the Electrical Conductivity of Polymer‐Derived Silicon Carbide Fibers

Two types of silicon carbide fibers (SiC_f) were prepared employing different pyrolysis techniques. The relationship between the microstructure and the electrical resistivity of the fibers was investigated. The results indicated that the carbon layer present on the fiber surface acted as the main conductive phase in the SiC_f obtained by direct pyrolysis, whereas a free carbon phase determined the conductivity of the SiC_f prepared by the preheated pyrolysis method. A core‐shell model and a general effective media (GEM) theory were proposed to explain the conductivity of different types of SiC_f. Quantitative analysis based on these models indicated an electrical resistivity of ~10^?2 Ω·cm for the carbon layer on the surface of SiC_f obtained by direct pyrolysis. The electrical resistivity and the percolation threshold of the free carbon in SiC_f prepared by the preheated pyrolysis method were 10^?1 Ω·cm and 11.3% respectively.     

Synthesis of TiC–TiB2 composite powders via carbothermal reduction and its reaction mechanism

Titanium carbide–titanium diboride (TiC–TiB₂) composite powders were synthesised through a carbothermal reduction method by using titanium dioxide, boric acid, and different carbon sources (namely, carbon black, sucrose, and glucose) as starting materials. The thermal decomposition behaviour of the precursors was studied by thermogravimetry–differential thermal analyser. Phase compositions and morphologies of the synthesised products were characterised by X-ray diffractometer and scanning electron microscope. When n(Ti):n(B):n(C)?=?1.0:2.5:5.0, the blended stock mainly formed TiB₂ with sucrose or glucose as a carbon source, whereas the stock produced TiC when carbon black was the source. At an optimum reaction temperature, the particles of the powders synthesised from carbon black as a carbon source were the smallest at approximately 100?nm. With increasing amount of boric acid in the precursor, the morphologies of the samples changed into less spherical particles, and more flaky grains and small particles with irregular structures were observed.     

Fabrication of core-shell structured MWCNT-Ti(TiC) using a one-pot reaction from a mixture of TiCl3, TiH2, and MWCNTs

Multi-walled carbon nanotubes (MWCNTs) coated with metal titanium or carbide layers were prepared by heating a mixture of TiCl₃, TiH₂, and MWCNTs under vacuum. The resulting MWCNT-Ti(TiC) materials had a uniform and conformal core-shell structure. The thickness of the Ti(TiC) layer was determined by the deposition temperature and time. The amount of TiC in the shell depended primarily on the deposition temperature. Whether a metal or carbide coating was obtained was determined by controlling the deposition temperature. Ti(TiC)-coated MWCNTs showed better field emission properties than did pristine MWCNTs. This deposition method should prove to be a versatile route for fabricating other one-dimensional core-shell materials.     

Ablation resistant ZrC coating modified by polymer-derived SiC/TiC nanocomposites for ultra-high temperature application

To improve the ablation resistance of ZrC coating on SiC-coated carbon/carbon composites above 2000 °C, SiC/TiC nanocomposites (SiC/TiC-NCs) powders derived from single-source precursor were incorporated into ZrC coating, denoted as ZrC-SiC/TiC-NCs, via supersonic atmospheric plasma spraying (SAPS). After SAPS, the incorporated SiC/TiC-NCs evolved into TiC/(SiC and Zr_xTi_yC) embedded in amorphous SiC. The ablation resistance of the ZrC-SiC/TiC-NCs coating was evaluated by oxyacetylene flames with a heat flux of 4.18 MW/m². For comparison, the ZrC-SiC-NCs coating without Ti modification was seriously damaged due to rapid gas denudation. The good ablation resistance of ZrC-SiC/TiC-NCs coating is mainly attributed to the distinctive “capsule-like” multi-crystalline microstructure of SiC/TiC-NCs. During ablation, TiO₂ and Zr_xTi_yO₂, due to the oxidation of TiC and Zr_xTi_yC, contributed to the formation of Zr-Ti-Si-O glass with high viscosity and low evaporation pressure, improving the ablation resistance.     

Electrical conductivity and electron spin resonance studies of pyrolysed polyimide

Recent studies of polymer-based pyrolytic amorphous carbon for use in organic electrolyte-lithium batteries and as metal-doped carbon electrodes show considerable promise. Polyimide can be thermally converted to amorphous carbon films. The irreversible evolution of polyimide under progressive heat treatment is characterized by three successive structural changes: pyrolysis, carbonization and graphitization. Four-point contact conductivity and electron spin resonance (e.s.r.) measurements were used to study and correlate electrical conductivity changes with unpaired electrons. From these studies it is concluded that at low pyrolysis temperature (<650°C) the pyrolysed polyimide is a nonmetallic amorphous carbon incorporating residual oxygen and nitrogen. At higher pyrolysis temperatures (>700°C) a microcrystalline graphite-like metallic domain starts to appear in the amorphous matrix. The development of this ‘metallic domain’ causes the conductivity of the pyrolysed polyimide to increase sharply.     

高性能生物质炭还原剂的制备与表征

以生物质核桃壳为原料制备生物质炭还原剂,对其进行了表征,分析了焦炭化学成分、官能团分布等与电阻率的关系. 结果表明,热解温度从400℃增加到1450℃时,生物质炭的电阻率从6288.7 mW×m减小至1515.9 mW×m,固定碳含量增加,挥发分含量减小;随热解温度升高,其羰基和脂肪族官能团逐渐被破坏,炭晶面间距d002逐渐减小,结晶度、芳构化程度和有序化程度增强. 在500~700℃的热解温度下所得生物质炭产率≥24%,电阻率≥5800 mW×m,固定碳≥80%,灰分≤4%,且具有发达的孔隙,可用作工业硅生产炭质还原剂.     

Temperature influence on the interlayer and surface morphology of diamond coating on 3D porous titanium substrates

Nanocrystalline (NCD) and/or microcrystalline (MCD) diamond films grown on three-dimensional porous titanium (Ti) substrate were obtained by hot filament chemical vapor deposition (HFCVD) technique. The morphology variation of diamond films grown on porous three-dimensional titanium substrate was studied at four different deposition temperatures to investigate their influence on nucleation density. Scanning electron microscopy images depicted the continuous change from microcrystalline diamond grains with a random crystallographic orientation, at 500 °C and 600 °C, to a cauliflower-like structure for deposits at 700 °C and 800 °C. Visible Raman spectroscopy confirmed the good quality of diamond films and revealed that the amount of amorphous carbon increased associated to the film morphology changes from MCD to NCD. X-ray diffraction analyses, performed both through θ–2θ scans and at grazing incidence angle, allowed the investigation of the crystallographic properties and structural evolution of the different film/substrate interface phases, such as TiC(111), TiC(200) and TiH₂. The results revealed that the temperature enhanced the nucleation sites for diamond growth.