Hot pressed and spark plasma sintered zirconia/carbon nanofiber composites

Zirconia/carbon nanofiber composites were prepared by hot pressing and spark plasma sintering with 2.0 and 3.3 vol.% of carbon nanofibers (CNFs). The effects of the sintering route and the carbon nanofiber additions on the microstructure, fracture/mechanical and electrical properties of the CNF/3Y-TZP composites were investigated. The microstructure of the ZrO₂ and ZrO₂–CNF composites consisted of a small grain sized matrix (approximately 120 nm), with relatively well dispersed carbon nanofibers in the composite. All of the composites showed significantly higher electrical conductivity (from 391 to 985 S/m) compared to the monolithic zirconia (approximately 1 × 10⁻¹⁰ S/m). The spark plasma sintered composites exhibited higher densities, hardness and indentation toughness but lower electrical conductivity compared to the hot pressed composites. The improved electrical conductivity of the composites is caused by CNFs network and by thin disordered graphite layers at the ZrO₂/ZrO₂ boundaries.     

Optimization of particle size,dispersity, and conductivity of 8 mol% Y2O3 doped tetragonal zirconia polycrystalline nanopowder prepared by modified sol-gel method via activated carbon absorption

8 mol% Y₂O₃ doped tetragonal zirconia polycrystalline (8Y-TZP) ceramic nanopowders were synthesized via a novel modified sol-gel method employing zirconium carbonate basic as zirconium resources. The activated carbon as a dispersant was added to the precursor solution during the formation of the sol. The phase behavior, thermal decomposition, microstructure morphology, and electrochemical performance of nanopowders with the addition of activated carbons were investigated by X-ray diffraction (XRD), differential thermal analysis (DTA), scanning electron microscopy (SEM), particles size distribution, and electrochemical impedance spectroscopy analysis (EIS). After adding the activated carbon, the average crystallite size of 8Y-TZP nanopowders decreased from about 53.16–33.51 nm when calcined at 900 ℃, and the 8Y-TZP nanopowders were produced loosely agglomerated. Meanwhile, compacts prepared by pressing the as-obtained 8Y-TZP nanopowders sintered to 98.8% relative density at 1600 ℃ and exhibited an average grain size of 0.89 µm, which brought a positive effect on ionic conductivity (0.079 S·cm^?1).     

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.     

Effect of in-situ formed whiskers diameter from tourmaline on microstructure and mechanical properties of 3Y-TZP ceramics

In this paper, in-situ whiskers reinforced 3 mol% Y₂O₃ stabilized tetragonal ZrO₂ (3Y-TZP) ceramics with different diameters were prepared using pressureless sintering by introducing tourmaline with different particle sizes into 3Y-TZP powders. The purpose of this research was to investigate the influence of in-situ formed whisker diameters on the densification, microstructure and mechanical properties of 3Y-TZP ceramics. The prepared ceramics were characterized by X-ray diffraction, scanning electron microscope and transmission electron microscope. Findings indicated that in-situ mullite whiskers formed by phase transformation of tourmaline particles can promote the densification of 3Y-TZP ceramics, and further improve the dispersion of mullite whiskers in the 3Y-TZP ceramics. More importantly, the average diameter of mullite whiskers can be controlled by altering the tourmaline particle size. When the average particle size of tourmaline is 500 nm, 3Y-TZP composites have a near-fully dense microstructure of 99.09%, with the ZrO₂ grain size of about 335 nm, the average diameter of mullite whiskers is 330 nm. Both the bending strength and fracture toughness reached optimal values of 836 ± 24 MPa and 10.6 ± 0.5 MPa m^0.5, respectively. This paper provides a new way to design of the microstructure and strength-toughness of zirconia composite ceramics.     

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.     

Electrical performance of orthotropic and isotropic 3YTZP composites with graphene fillers

3 mol% yttria tetragonal zirconia polycrystal (3YTZP) composites with orthotropic or isotropic microstructures were obtained incorporating few layer graphene (FLG) or exfoliated graphene nanoplatelets (e-GNP) as fillers. Electrical conductivity was studied in a wide range of contents in two configurations: perpendicular (σ_?) and parallel (σ_//) to the pressing axis during spark plasma sintering (SPS). Isotropic e-GNP composites presented excellent electrical conductivity for high e-GNP contents (σ_? ～ 3200 S/m and σ_// ～ 1900 S/m for 20 vol% e-GNP), consequence of their misoriented distribution throughout the matrix. Optimum electrical performance was achieved in the highly anisotropic FLG composites, with high electrical conductivity for low contents (σ_? ～ 680 S/m for 5 vol%), percolation threshold below 2.5 vol% FLG and outstanding electrical conductivity for high contents (σ_? ～ 4000 S/m for 20 vol%), result of the high aspect ratio and low thickness of FLG.     

Nano‐structured MgH2 catalyzed by TiC nanoparticles for hydrogen storage

BACKGROUND: Magnesium hydride is considered to be a promising hydrogen storage material because of its high gravimetric and volumetric storage capacities. However, its slow kinetics and high desorption temperature of > 300 °C limit practical applications. In this work, TiC nanoparticles were selected to modify the hydrogen storage properties of MgH₂. Composite mixtures (MgH₂ + TiC) were prepared using both cryogenic milling and high‐energy ball milling. RESULTS: The resulting morphology and crystallite structure of the composites were identified by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X‐ray diffraction (XRD). The milled samples show good mixing of the hydride and carbide particles, with MgH₂ particles around 0.09–1 µm and TiC particles 10–20 nm. The (MgH₂ + TiC) composites consist of γ‐MgH₂, β‐MgH₂ and TiC. MgH₂ nano‐crystallites of 25 nm were formed after cryomilling. Thermogravimetry reveals that the composites release ～6.5 mass % hydrogen from 190–400 °C at a heating rate of 10 °C min^?1 under He flow, with the onset and peak temperatures at 190 and 280 °C, respectively, for the (MgH₂ + TiC) after 8 h cryomilling and 60 h ball milling. CONCLUSION: Results indicate that TiC is an effective catalyst for hydrogen desorption of MgH₂. Copyright © 2010 Society of Chemical Industry     

Optical transmittance and electrical conductivity of silica glass with biserial and hierarchical network structures made of carbon nanofibers

Silica glass composites, with biserial and hierarchical percolative network made of carbon nanofibers (CNFs), was fabricated using a layer-by-layer technique and spark plasma sintering to obtain high optical transmittance and electrical conductivity. Owing to the network, the critical volume fraction, V_c, for the CNF percolation in the silica glass-matrix composite (0.5–0.7 vol%), when the electrical conductivity of the composite drastically increased with change from insulator (～10^?10 S/m) to conductor (～10^?1 S/m), is smaller than theoretical V_c predicted for the three-dimensional random orientation of CNFs (2.6 vol% for the CNF aspect ratio of 30). The conductivity of the composite with above the V_c of CNFs (～10 S/m) is higher than that reported for the polymer-matrix composite (～10^?5–～10^?3 S/m). Furthermore, high optical transmittance was observed for the electrically conductive composite with V_c of CNFs.     

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Nanofiller on ion conductivity,transmittance, and glass transition temperature of PEI:LiTFSI:PC:EC polymer electrolytes

In the present study, ion conductivity, optical properties, and glass transition temperatures are characterized for polymer electrolytes composed of poly(ethyleneimine) (PEI), lithium bis(trifluoromethane)sulfonylimide (LiTFSI) salt, propylene carbonate (PC), and ethylene carbonate (EC). It was doped with nanoceramic particles in different ratio (0–15 wt.%) to see the effect of ceramic particles. The salt concentration was fixed as 1.04 mol.kg^?1. Although valuable improvement in ion conductivity could not be achieved due to nano-Al₂O₃ fillers, ion conductivity results are placed between 10^?2 and 10^?4 S/cm. Differential scanning calorimetry (DSC) measurements and optical measurements of all electrolytes were performed between ?80 and 140 °C, in the wavelength range between 400 and 700 nm for sample with 80 μm thickness, respectively. The results showed that transmittance of electrolytes decreased monotonically for increasing Al₂O₃ contents. In particular, its transmittance value at 550 nm where human sight is at its greatest sensitivity went from 100% without nanoparticles to 50% for 15 wt% of Al₂O₃.     

Synthesis and performance of TiB2–B4C composites by high pressure of TiC–B mixture

TiB₂–B₄C composites were in situ synthesized and consolidated by high pressure synthesis method from a mixture of TiC and B powders at the pressure and temperature of 5.0 GPa and 1500℃-1900℃. The phase composition, microstructure, density, hardness, thermal conductivity, and electrical resistivity of TiB₂–B₄C composites were analyzed. As the increase in the synthesis temperature, the products were TiB₂ and B₄C phases and that crystallinity improved. TiB₂–B₄C composites were dense without obvious pores. TiB₂–B₄C composites synthesized at 1800℃ obtained the optimized performance, including the relative density of 98.2%, the Vickers hardness of 31.7 ± 1.2 GPa with the load of 9.8 N, the thermal conductivity of 30.3 ± 0.7 W/(m K), and the electrical resistivity of 3.3 × 10⁻³ Ω cm, respectively. The grain size of the TiB₂–B₄C composites changed with the increase in synthesis temperature, leading to the changes in hardness, thermal conductivity, and electrical resistivity.     

Thermoelectric properties of A-site deficient La-doped SrTiO3 at 100–900 °C under reducing conditions

Lanthanum doped strontium titanate is a potential n-type thermoelectric material at moderate and high temperatures. (La_0.12Sr_0.88)_0.95TiO₃ ceramics were prepared by two different routes, conventional sintering at 1500 °C and spark plasma sintering at temperatures between 925 and 1200 °C. Samples with grain size between 40 nm and 1.4 μm were prepared and characterized with respect to their thermoelectric transport properties at temperatures between 100 and 900 °C under reducing conditions (H₂/H₂O-buffer mixtures). The thermal conductivity was significantly reduced with decreasing grain size reaching a value of 1.3 W m^−1. K⁻¹ at 600 °C for grain size of 40 nm and porosity of 19%. Electrical conductivity increased with increasing grain size showing a maximum of 500 S cm⁻¹ at 200 °C for a grain size of 1.4 μm. The highest figure-of-merit (zT) was measured for samples with 1.4 μm average grain size reaching 0.2 at 500 °C.     

Gasphasensynthese von Kern/Mantel‐Partikeln am Beispiel von TiO2‐Nanopartikeln mit elektrisch leitendem Mantel

An advanced process enables synthesis and coating of individual TiO₂‐core particles with a shell of transparent conducting oxide (TCO) from the gas phase in one reactor. TiO₂ particles were coated with fluorine‐doped tin oxide (SnO₂:F) or antimony‐doped tin oxide (SnO₂:Sb). Specific electrical conductivity of the core/shell particles was up to 8 · 10^–3 S cm^–1. Variation of process parameters allows modifying dopant level and conductivity in an easy way.     

Sintering,microstructure and hardness of Y-TZP- 64S bioglass ceramics

3 mol% yttria-partially stabilized zirconia (Y-TZP) powder and a sol-gel derived CaO- P₂O₅- SiO₂ (64S) bioglass, were used to produce Y-TZP- 64S slip cast compacts. The compacts with 10.5 and 19.9 vol% 64S were sintered at different temperatures up to 1500 °C using 5 and 10 °C/min heating/cooling rates. The densification behaviour, crystalline phase formation and zirconia grain growth were investigated as a function of sintering temperature and 64S glass content. Ca₃(PO₄)₂ along with SiO₂ as a major phase were obtained from thermal decomposition of the 64S glass at 950–1500 °C. Both 64S additions, 10.5 and 19.9 vol%, promoted the sintering process at a lower temperature with respect to Y-TZP (1500 °C); the SiO₂ phase markedly increased the Y-TZP solid state sintering rate at the intermediate stage. The rapidly cooling at 10 °C/min inhibited the t-m transformation of Y-TZP and markedly reduced that of Y-TZP- 64S at 1300–1500 °C. Sintered Y-TZP with 10.5 vol% 64S, nearly fully densified at 1300–1400 °C, was constituted by polygonal ZrSiO₄ particles and elongated Ca₂P₂O₇ particles uniformly distributed in the tetragonal zirconia fine grain matrix. This ceramic exhibited similar hardness to that of Y-TZP sintered at 1500 °C; the in situ formation of calcium phosphate will have the potential to improve the Y-TZP biological properties without significantly affecting its hardness.     

Structure–Property Relationship in an Electroconductive Hydroxyapatite–Titanium Disilicide Composite

We elucidate here the structure–property relationship in a novel composite system consisting of hydroxyapatite (HA) and titanium disilicide that addresses the challenge of low electrical conductivity and fracture toughness of hydroxyapatite. Theoretical considerations indicated that the addition of 20 wt.% titanium disilicide (σ ~ 10⁶ S/m) favorably contributed to the increase in electrical conductivity (σ ~ 10⁵ S/m) of the HA–titanium disilicide system. As compared to theoretical value, a lower value of electrical conductivity of HA‐20 wt.%TiSi₂ composite (σ ~ 67.117 ± 3.57 S/m) was observed in experimental measurement using four probe method. However, this electrical conductivity of HA‐20 wt.%TiSi₂ composite was significantly higher than the HA, because no current was recorded in case of HA in the sensitivity range of the instrument. During spark plasma sintering, to raise per unit temperature, higher magnitude of current was utilized in case of HA‐20TiSi₂ than HA. This resulted in higher densification of HA‐20TiSi₂ during initial stage of sintering. Also, a significant improvement in fracture toughness was observed on the addition of TiSi₂ to HA from 0.6 MPa.m^1/2 in HA to 1.2 MPa.m^1/2 in HA‐20TiSi₂. The mechanism of increase in fracture toughness involved crack deflection, friction bridging, wake debonding, and elastic bridging.     

Electrical conductivity of spark plasma sintered Al2O3–SiC and Al2O3-carbon nanotube nanocomposites

The electrical conductivity of alumina-silicon carbide (Al₂O₃–SiC) and alumina-multiwalled carbon nanotube (Al₂O₃-MWCNT) nanocomposites prepared by sonication and ball milling and then consolidated by spark plasma sintering (SPS) is reported. The effects of the nanophase (SiC and MWCNTs) and SPS processing temperature on the densification, microstructure, and functional properties were studied. The microstructure of the fabricated nanocomposites was investigated using field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The phase evolution was determined using X-ray diffraction (XRD). The room-temperature direct current (DC) electrical conductivity of the monolithic alumina and nanocomposites was determined using the four-point probe technique. The EDS mapping results showed a homogenous distribution of the nanophases (SiC and MWCNTs) in the corresponding alumina matrix. The room-temperature DC electrical conductivity of monolithic alumina was measured to be 6.78 × 10⁻¹⁰ S/m, while the maximum electrical conductivities of the alumina-10 wt%SiC and alumina-2wt%MWCNT samples were 2.65 × 10⁻⁵ S/m and 101.118 S/m, respectively. The electrical conductivity increased with increasing nanophase concentration and SPS temperature. The mechanism of electrical conduction and the disparity in the electrical performance of the two investigated nanocomposite systems (alumina-SiC and alumina-MWCNT) are clearly described.     

Formation of novel mesoporous TiC microspheres through a sol–gel and carbothermal reduction process

Novel mesoporous TiC microspheres with uniform size are synthesized via a sol–gel combined carbothermal reduction process. A microfluidic aerosol nozzle was used to produce droplets which were subsequently dried into gel microspheres under different conditions. The influence of drying temperatures and sol aging time on the diameters of obtained gel microspheres was investigated. The spherical morphology of TiC spheres can be maintained after a two-step heat treatment. Moreover, the TiC microspheres exhibit a high surface area of 267 m²/g and consist of 30–50 nm nano TiC grains and 4.5 nm pores. This unique nanostructure is directly formed from the carbothermal reduction of non-porous and template-free titania/carbon spheres.     

Y-TZP,Ce-TZP and as-synthesized Ce-TZP/Al2O3 materials in the development of high loading rate digital light processing formulations

Y-TZP, Ce-TZP and Ce-TZP/Al₂O₃ materials are widely investigated in dentistry. Digital Light Processing (DLP) is considered as a breakthrough technology for the dental field to fine print Y-TZP green parts. High loading photocurable formulations (>45 vol%) with Y-TZP, Ce-TZP commercial powders and Ce-TZP/30 vol% Al₂O₃ as-synthesized powder suitable to DLP printing were achieved in this study. A low specific surface area (5–13 m²/g) of particles without any pores and 1 wt% to 2 wt% of steric dispersant are required to obtain high loading formulations. The as-synthesized composites provide these properties by increasing the calcination temperature from 800 °C to 1200 °C. The as-prepared ceramic formulations based on the same photocurable resin exhibit a curing behavior suitable to DLP process for Y-TZP formulations (thickness > 50 μm in few seconds with a high conversion rate) in comparison with ceria ceramic. The ceria is a strongly UV absorbing material and a specific formulation is developed to obtain 80% of conversion and a cured thickness of 75 μm in 0.5 s.     

Effect of nanostructure on the thermal conductivity of La-doped SrTiO3 ceramics

A series of La-doped (10 at.%) SrTiO₃ ceramics with grain size ranging from 6 μm to 24 nm was prepared from nanocrystalline powders using high-pressure field assisted sintering (HP-FAST). A progressive reduction of thermal conductivity κ with decreasing grain size was observed. At room temperature, κ of the ceramic with grain size of 24 nm (1.2 W m⁻¹ K⁻¹) is one order of magnitude lower than that of undoped single crystals. The strong suppression of κ can be ascribed to (i) the high concentration of lattice defects, (ii) the increasing contribution of grain boundaries to phonon scattering when the grain size is decreased to the nanoscale and (iii) a moderate amount (10–15 vol.%) of nanopores. These results demonstrate that nanostructuration can be a successful strategy to attain a considerable reduction of κ in heavily doped bulk oxide ceramics. The low electrical conductivity of the La:SrTiO₃ nanoceramics represents a major obstacle for thermoelectric applications.     

Microstructure and mechanical properties of α/β-Si3N4 composite ceramics with novel ternary additives prepared via spark plasma sintering

In this study, α/β-Si₃N₄ composite ceramics with high hardness and toughness were fabricated by adopting two different novel ternary additives, ZrN–AlN–Al₂O₃/Y₂O₃, and spark plasma sintering at 1550 °C under 40 MPa. The phase composition, microstructure, grain distribution, crack propagation process and mechanical properties of sintered bulk were investigated. Results demonstrated that the sintered α/β-Si₃N₄ composite ceramics with ZrN–AlN–Al₂O₃ contained the most α phase, which resulted in a maximum Vickers hardness of 18.41 ± 0.31 GPa. In the α/β-Si₃N₄ composite ceramics with ZrN–AlN–Y₂O₃ additives, Zr₃AlN MAX-phase and ZrO phase were found and their formation mechanisms were explained. The fracture appearance presented coarser elongated β-Si₃N₄ grains and denser microstructure when 20 wt% TiC particles were mixed into Si₃N₄ matrix, meanwhile, exhibited maximum mean grain diameter of 0.98 ± 0.24 μm. As a result, the compact α/β-Si₃N₄ composite ceramics containing ZrN–AlN–Y₂O₃ additives and TiC particles displayed the optimal bending strength and fracture toughness of 822.63 ± 28.75 MPa and 8.53 ± 0.21 MPa?m^1/2, respectively. Moreover, the synergistic toughening of rod-like β-Si₃N₄ grains and TiC reinforced particles revealed the beneficial effect on the enhanced fracture toughness of Si₃N₄ ceramic matrix.     

Plastic deformation-induced improved mechanical and thermal properties in hot-forged SiC-TiC composite

A novel SiC-20 vol% TiC composite prepared via a two-step sintering technique using 6.5 vol% Y₂O₃-Sc₂O₃-MgO exhibited high deformation (60 %) on hot forging attributed to the high-temperature plasticity of TiC (ductile to brittle transition temperature ～800 °C) and fine-grained microstructure (～276 nm). The newly developed SiC-TiC composite exhibited a ～2-fold increase in nominal strain as compared to that of monolithic SiC. The plastic deformation caused by grain-boundary sliding in monolithic SiC was supplemented by the plastic deformation of TiC in the SiC-TiC composite. The hot-forged composite exhibited anisotropy in its microstructure and mechanical and thermal properties due to the preferred alignment of α-SiC platelets formed in situ. The relative density, flexural strength, fracture toughness, and thermal conductivity of the composite increased from 98.4 %, 608 MPa, 5.1 MPa?m^1/2, and 34.6 Wm^?1 K^?1 in the as-sintered specimen to 99.9 %, 718–777 MPa, 6.9–7.8 MPa?m^1/2, and 54.8–74.7 Wm^?1 K^?1, respectively, on hot forging.