Compressive creep behavior of hot-pressed Si3N4-CRE2O3-Al2O3 ceramics

In this work, yttrium-rare earth oxide solid solution, CRE₂O₃, produced at FAENQUIL-DEMAR at a cost of only 20% of pure commercial Y₂O₃, was used as sintering additive of hot-pressed Si₃N₄ ceramics. The objective of this work was to characterize and to investigate the creep behavior of these ceramics. The samples were sintered by hot-pressing at 1750 °C, for 30 min using a pressure of 20 MPa. Compressive creep tests were carried out in air, between 1250 and 1300 °C, for 60 h, under stresses of 200-300 MPa. The stress exponent under all conditions was determined to be about unity. The apparent activation energy obtained was around 460 kJ mol⁻¹, corresponding to the heat of solution of the Si₃N₄ in the glassy phase. Both the stress exponent n and apparent activation energy Q are within the range of values reported in other studies of the compressive creep of Y₂O₃-Al₂O₃-doped-Si₃N₄ ceramics. X-ray diffraction (XRD) characterization shows a global reorientation of the β-Si₃N₄ grains and SEM observations detected no grain growth after the creep tests. These results indicate that grain-boundary sliding controlled by viscous flow is the dominant creep mechanism observed in the present study. The creep resistance presented of this samples indicates that this additive CRE₂O₃ can be a cheap alternative in the fabrication of Si₃N₄ ceramics, resulting in promising mechanical properties.     

Low temperature synthesis and optical properties of CaTiO3 nanoparticles from Ca(NO3)2·4H2O and TiO2 nanocrystals

Choosing low-melting-point Ca(NO₃)₂·4H₂O and high-reactive-activity TiO₂ nanocrystals as the raw materials, a simple and cost-effective route was developed for the synthesis of CaTiO₃ nanoparticles at 600 °C, which is much lower than that (about 1350 °C) used in the conventional solid state reaction methods. X-ray diffraction, energy dispersive X-ray spectroscopy and field emission scanning electron microscopy revealed the formation of orthorhombic phase CaTiO₃ nanoparticles with oxygen-deficiency at the surface. UV-vis absorption spectrum of the as-obtained CaTiO₃ nanoparticles displayed an absorption peak centered at around 325 nm (3.8 eV), together with a tail at lower energy side. Room temperature photoluminescence spectrum of the as-obtained CaTiO₃ nanoparticles upon laser excitation at 325 nm demonstrated a strong and broad visible light emission ranging from about 527 to 568 nm, which may be originated from the surface states and defect levels.     

Microstructure and mechanical properties of pulsed electric current sintered B4C-TiB2 composites

Monolithic B₄C, TiB₂ and B₄C-TiB₂ particulate composites were consolidated without sintering additives by means of pulsed electric current sintering in vacuum. Sintering studies on B₄C-TiB₂ composites were carried out to reveal the influence of the pressure loading cycle during pulsed electrical current sintering (PECS) on the removal of oxide impurities, i.e. boron oxide and titanium oxide, hereby influencing the densification behavior as well as microstructure evolvement. The critical temperature to evaporate the boron oxide impurities was determined to be 2000 °C. Fully dense B₄C-TiB₂ composites were achieved by PECS for 4 min at 2000 °C when applying the maximum external pressure of 60 MPa after volatilization of the oxide impurities, whereas a relative density of 95-97% was obtained when applying the external pressure below 2000 °C. Microstructural analysis showed that B₄C and TiB₂ grain growth was substantially suppressed due to the pinning effect of the secondary phase and the rapid sintering cycle, resulting in micrometer sized and homogeneous microstructures. Excellent properties were obtained for the 60 vol% TiB₂ composite, combining a Vickers hardness of 29 GPa, a fracture toughness of 4.5 MPa m^1/2 and a flexural strength of 867 MPa, as well as electrical conductivity of 3.39E+6 S/m.     

Microstructure and mechanical properties of an HfB2 + 30 vol.% SiC composite consolidated by spark plasma sintering

An ultra-high-temperature HfB₂–SiC composite was successfully consolidated by spark plasma sintering. The powder mixture of HfB₂ + 30 vol.% β-SiC was brought to full density without any deliberate addition of sintering aids, and applying the following conditions: 2100 °C peak temperature, 100 °C min⁻¹ heating rate, 2 min dwell time, and 30 MPa applied pressure. The microstructure consisted of regular diboride grains (2 μm mean size) and SiC particulates evenly distributed intergranularly. The only secondary phase was monoclinic HfO₂. The incorporated SiC particulates played a key role in enhancing the sinterability of HfB₂. Flexural strength at 25 °C and 1500 °C in ambient air was 590 ± 50 and 600 ± 15 MPa, respectively. Fracture toughness at room temperature (RT) (3.9 ± 0.3 MPa √m) did not decrease at 1500 °C (4.0 ± 0.1 MPa √m). Grain boundaries depleted of secondary phases were fundamental for the retention of strength and fracture toughness at high temperature. The thermal shock resistance, evaluated through the water-quenching method, was 500 °C.     

Effect of SiO2 coating on polyethylene separator with different stretching ratios for application in lithium ion batteries

To enhance the properties of polyethylene separators in lithium ion batteries, we tested separators with uni-axial stretching ratios of 180% and 300%. We also tested stretched separators coated with SiO₂ ceramic substance to increase ionic conductivity and thermal stability without sacrificing mechanical properties. To test the thermal and tensile properties, thermomechanical analyzer (TMA) is employed. CR 2032-type coin cells are prepared by sandwiching pristine and coated stretched separators, respectively, between the Li anode and Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode to evaluate the AC impedance and cycling performance. The coated separators are observed with superior ionic conductivity, thermal and tensile properties. The cells prepared with coated separator have slightly higher discharge capacity and a better capacity retention ratio than the cells with pristine separators. These results suggest that the coated separator is a better option for lithium ion batteries.     

Effect of CH4 and H2 on CVD of SiC and TiC for possible fabrication of SiC/TiC/C FGM

SiC and TiC coatings were deposited by CVD on graphite substrates and the effect of the variation of the methane (CH₄) and hydrogen (H₂) ratio on deposition was investigated. SiCl₄, TiCl₄ and CH₄ were used as sources of Si, Ti and C. In case of the SiC coatings, stoichiometric SiC was obtained when the ratios of CH₄/(SiCl₄ + CH₄)andH₂/(SiCl₄ + CH₄) are 0.4 and 10, respectively. Stoichiometric TiC was also obtained under similar conditions. In order to obtain non-stoichiometric materials for possible fabrication of functionally gradient materials (FGM), a change of microstructure and composition was observed with changes of the CH₄ and H₂ ratio. As a result, SiC, TiC and C contents were more easily controlled by a change of the H₂ ratio compared to the CH₄ ratio for SiC and TiC deposition. It has been verified that the change of the H₂ ratio is more desirable for possible manufacturing of SiC/TiC/C FGM.     

Microstructure and microwave dielectric properties of (1 − y)Li3NbO4+yLi2TiO3(Li2SnO3) ceramics

Phase formation, microstructure and microwave dielectric properties of (1 − y)Li₃NbO₄ + yLi₂TiO₃(Li₂SnO₃) ceramics have been studied in this paper. The structure and microstructure of the compounds were investigated using X-ray powder diffractometer (XRD), scanning electron microscope (SEM), Raman spectrometer. The microwave dielectric properties of the ceramics were studied with a network analyzer at the frequency of about 8–12 GHz. Li₃NbO₄ formed ordered solid solutions with the addition of small amount of Li₂TiO₃ (y ≤ 0.2), whereas no solid solution formed with the addition of small amount of Li₂SnO₃. Small amount of Li₂TiO₃ doping suppressed the appearance of impurity phases caused by lithium evaporation for Li₃NbO₄. The Li₂TiO₃ doped compositions with 0.02 ≤ y ≤ 0.08 demonstrated homogeneous and dense microstructure after sintering at 1150 °C/2 h, in contrast the 0.2 ≤ y ≤ 0.6 specimens exhibited porous and subgrains microstructure after sintering at 1250 °C/2 h. Short range ordering was observed in the 0.2 ≤ y ≤ 0.6 compositions. Mechanical mixture phases of Li₃NbO₄ and Li₂SnO₃ based solid solution (Li₂SnO₃ (ss)) existed in the Li₂SnO₃ added specimens_. The dielectric permittivity increased with increasing Li₂TiO₃ addition, but decreased with the increase of Li₂SnO₃ content. All specimens exhibited negative τ_f value for the Li₂TiO₃ added specimens, although its absolute τ_f value decreased with the increase of Li₂TiO₃ addition. Whereas, the τ_f value changed from negative into positive with the increase of Li₂SnO₃ addition. Optimized combined microwave dielectric properties (?_r = 19.8, Q × f = 91,200 GHz, τ_f = −24 ppm/°C and ?_r = 16, Q × f = 75,300 GHz, τ_f = 3 ppm/°C) could be obtained for the Li₂TiO₃ added (y = 0.6) and Li₂SnO₃ added specimens(y = 0.7), respectively. The microwave dielectric properties of the Li₂SnO₃ end member are ?_r = 13.5, Q × f = 61,600 GHz, τ_f = 29 ppm/°C.     

Preparation and properties of negative thermal expansion Zr2WP2O12 ceramics

Using solid-state reaction method, Zr₂WP₂O₁₂ powder was synthesized for this study. The optimum heating condition was 1200 °C for 4 h. The obtained powder was compacted and sintered. The relative density of the Zr₂WP₂O₁₂ ceramics with no sintering additive was 60%. That of samples sintered with more than 0.5 mass% MgO was about 97%. The average grain size (D₅₀), as estimated from the polished surface of a sample sintered at 1200 °C for 4 h was about 1 μm. The obtained ceramics showed a negative thermal expansion coefficient of about −3.4 × 10⁻⁶ °C⁻¹. Young's modulus, Poisson's ratio, three-point bending strength, Vickers microhardness, and fracture toughness of the obtained ceramics were, respectively, 74 GPa, 0.25, 113 ± 13 MPa, 4.4 GPa and 2.3 MPa m^1/2.     

Effect of AlF3, CaF2 and MgF2 on hot-pressed hydroxyapatite-nanophase alpha-alumina composites

Nanophase alpha-alumina and hydroxyapatite composites (with and without 5 wt% AlF₃, CaF₂ or MgF₂, added separately) were hot pressed at 1100 °C and 1200 °C to investigate their mechanical properties and phase stability. Hydroxyapatite slightly decomposed to tri-calcium-phosphate when there was no F⁻ present. With the addition of AlF₃, CaF₂ or MgF₂ into the composite, it improved its thermal stability and mechanical properties. Substitution of OH⁻ by F⁻ ions in hydroxyapatite was verified by the change in hydroxyapatite's hexagonal lattice parameters and unit cell volume. A fracture toughness of 2.8 MPa  and μ-hardness of 8.25 GPa were calculated for the composite containing CaF₂ after the hot pressing at 1200 °C.     

Dissociation process of CH4/H2 gas mixture during EACVD

The dissociation process of CH₄/H₂ gas mixture during EACVD has been investigated using Monte Carlo simulation for the first time. The electron velocity distribution and H₂ dissociation were obtained over a wide range: 100<E/N<2000 Td. The variation of CH₄ dissociation with CH₄ concentration in the filling gas has been simulated. The electron velocity profile is asymmetric for the component parallel to the field. Most electrons possess non-zero velocity parallel to the substrate. The number of atomic H is a function of E/N. There are two peaks at E/N=177 Td and 460 Td. The appropriate E/N is suggested to be 500–800 Td for low temperature deposition. The main diamond growth precursor is proposed to be CH₃ and CH₃⁺.     

The effect of modification of KH2PO4 hardening liquid with H3PO4 and chitosan on setting reactions and compressive strength of calcium phosphate cement

The setting processes and mechanical properties of tetracalcium phosphate (TTCP) -monetite cements with H₃PO₄ and chitosan modified KH₂PO₄ hardening liquid have been studied. From XRD phase analysis, it was found that brushite was created in the first stage of the setting process by the interaction between phosphate compounds in hardening liquids and TTCP. Calcium deficient hydroxyapatite was the final product of the hardening process. The orthophosphoric acid addition to KH₂PO₄ hardening liquid caused lowering resistance to disintegration and an increase in the setting times of cements which allows the possibility for their control. Cement with pure KH₂PO₄ hardening liquid was resistant to wash-out immediately after mixing. Chitosan addition to KH₂PO₄ + H₃PO₄ hardening liquid of an amount around 1 wt.% did not affect change of setting time or improvement of disintegration behaviour of cement. Compressive strengths were around 80–100 MPa in cements soaked in SBF without an chitosan addition and chitosan caused an approximately 15% decrease in compressive strength. The compressive strength of calcium phosphate cements is more influenced by the hydroxyapatite particle morphology than their crystallinity.     

Microstructure and mechanical properties of small amounts of In2O3 reinforced Pb(ZrxTi1−x)O3 ceramics

Piezoelectric Pb(Zr_xTi_1−x)O₃ (PZT) ceramics with small amount (0.5-2.0 wt.%) of In₂O₃ are prepared by conventional sintering method. Based on X-ray diffraction analysis, the tetragonality of PZT matrix decreases with In₂O₃ content, indicating that In₂O₃ diffuses into PZT matrix. The microstructure of PZT matrix is significantly refined by doping small amounts of In₂O₃. The grain size reduction and the matrix grain boundary reinforcement are the probable mechanism responsible for the high strength and hardness in the PZT/In₂O₃ materials. The enhancement in Young’s modulus is attributed to In³⁺ substitution. The decreased tetragonality with In₂O₃ addition results in less crack energy absorption by domain switching and, hence, causes the small reduction in fracture toughness.     

甲烷/氢气在超微孔金属有机骨架中的吸附与分离性能研究

使用巨正则蒙特卡罗方法(GCMC)对CH₄和H₂在3种具有不同结构的超微孔金属有机骨架(UM-MOFs)中的吸附与分离性能进行了研究。获得了这3种材料对CH₄和H₂分子的单组分吸附。通过分析它们对CH₄/H₂二元混合物的吸附数据,获得了相应的吸附选择性分布曲线,探索了UM-MOFs对CH₄/H₂分子的吸附与分离机理。     

Effect of dysprosium substitution on electrical properties of SrBi4Ti4O15

SrBi_4−xDy_xTi₄O₁₅ (with x = 0.02, 0.04, 0.06 and 0.08) powders have been synthesized using the stoichiometric amounts of nitrates and oxides of the constituent materials through sol–gel method. The compound so formed is characterized using X-ray diffraction. The density and lattice parameters are calculated. The impedance and electrical conductivity properties are investigated. The imaginary part of impedance as a function of frequency shows Debye like relaxation. Impedance data presented in the Nyquist plot which is used to identify an equivalent circuit and the fundamental circuit parameters are determined at different temperatures. The results of bulk a.c. conductivity as a function of frequency at different temperatures are presented. The dielectric behavior was investigated. Permittivity was calculated based on the relaxation frequency using an alternative approach based on the variation of the imaginary impedance component as a function of reciprocal angular frequency. The frequency dependence of real and imaginary permittivities was also investigated.     

Preparation of nitrogen doped TiO2 photocatalyst by oxidation of titanium nitride with H2O2

Nitrogen doped anatase TiO₂ (N-TiO₂) were prepared by hydrothermally treating TiN with H₂O₂. The as-synthesized samples were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), UV-vis diffuse reflectance spectrum (DRS), Fourier transform infrared spectra (FT-IR) and X-ray photoelectron spectroscopy (XPS) techniques. The results confirmed that the hydrothermal oxidation is an effective method to prepare N-doped TiO₂ anatase. The nitrogen concentration in TiO₂ could be controlled by the concentration of H₂O₂ solution. Photocatalytic degradation of methyl orange (MO) was carried out under visible light and UV-visible light irradiation, respectively. The as-prepared optimal N-TiO₂ showed higher photocatalytic activity than N-P25 and P25, and exhibited excellent reusability.     

Flower-like microparticles and novel superparamagnetic properties of new binary Co1/2Fe1/2(H2PO4)2·2H2O obtained by a rapid solid state route at ambient temperature

A new binary Co_1/2Fe_1/2(H₂PO₄)₂·2H₂O was synthesized by a simple, rapid and cost-effective method using CoCO₃-Fe(c)-H₃PO₄ system at ambient temperature. Thermal treatment of the obtained Co_1/2Fe_1/2(H₂PO₄)₂·2H₂O at 600 °C yielded as a binary cobalt iron cyclotetraphosphate CoFeP₄O₁₂. The FTIR and XRD results of the synthesized Co_1/2Fe_1/2(H₂PO₄)₂·2H₂O and its final decomposed product CoFeP₄O₁₂ indicate the monoclinic phases with space group P2₁/n and C2/c, respectively. The particle morphologies of both binary metal compounds appear the flower-like microparticle shapes. Room temperature magnetization results show novel superparamagnetic behaviors of the Co_1/2Fe_1/2(H₂PO₄)₂·2H₂O and its final decomposed product CoFeP₄O₁₂, having no hysteresis loops in the range of ±10,000 Oe with the specific magnetization values of 0.045 and 12.502 emu/g at 10 kOe, respectively. The dominant physical properties of the obtained binary metal compounds (Co_1/2Fe_1/2(H₂PO₄)₂·2H₂O and CoFeP₄O₁₂) are compared with the single compounds (M(H₂PO₄)₂·2H₂O and M₂P₄O₁₂; where M = Co, Fe), indicating the presence of Co ions in substitution position of Fe ions.     

Percolative property and microwave dielectric characterization of Ba0.4Sr0.6TiO3-Mg2TiO4 diphasic ceramics

Ba_0.4Sr_0.6TiO₃-Mg₂TiO₄ composite ceramics were fabricated via conventional solid state reaction process. Dielectric properties of the composites were investigated systematically. It was shown that their dielectric constants decreased monotonously from 1197 to 60 with the contents of increasing Mg₂TiO₄ from 0 to 70 wt%. Tunability had a slight change from 24.0% to 20.5% as the volume fraction of Mg₂TiO₄ was increased from 0 to 62 vol%. However, it decreased substantially from 20.5% to 13.4% above the threshold from 62 vol% to 71 vol%. Such an obvious change in tunability was well explained by connectivity levels of the binary composites. Q values of the composite ceramics were more than 200 (tg δ < 0.005) at microwave frequencies of less than 10 GHz. Dielectric dispersion was observed at about 22 GHz using interdigital capacitor (IDC) stucture.     

Modified (K0.5Na0.5)(Nb0.9Ta0.1)O3 ceramics with high Qm

Lead-free ceramics (K_0.5Na_0.5)(Nb_0.9Ta_0.1)O₃ (KNNT) + x mol% K₄CuNb₈O₂₃ (KCN) + y mol% MnO₂ have been prepared using the conventional solid-state reaction technique. Crystalline structures and Microstructures were analyzed by X-ray diffraction and scanning electron microscope (SEM) at room temperature. The low dielectric loss tanδ and relatively high piezoelectric properties were obtained when KCN and MnO₂ were added into KNNT ceramics. The ceramics with x = 1.0, y = 0.50 exhibited excellent piezoelectric properties: high mechanical quality factor Q_m = 1563, piezoelectric coefficient d₃₃ = 96pC/N, electromechanical coupling coefficient k_p = 42.2%, k_t = 44.5%, k₃₃ = 58.4%, relative dielectric constant ε′ = 308, tanδ = 0.4%. This material is a promising candidate for the lead-free piezoelectric transformer applications.     

Ca3Co4O9 ceramics consolidated by SPS process: Optimisation of mechanical and thermoelectric properties

Ca₃Co₄O₉ (349) thermoelectric (TE) oxide ceramics were successfully prepared by Spark Plasma Sintering process. The effects of the uniaxial pressure (30-100 MPa), the dwell temperature (700-900 °C) and the cooling rate were investigated. Microstructure analyses have revealed strong enhancements of the bulk density as the pressure level and the applied temperature during the SPS process are increased. Mechanical properties were investigated by using instrumented nanoindentation and three point bending tests. Hardness, elastic modulus, strength and fracture toughness were shown to improve drastically and depend on the processing parameters. Thermal expansion measurements reveal a noticeable anisotropy induced by unidirectional hot pressing. The mechanical, thermal and thermoelectric properties were correlated to the microstructure and crystallographic texture of the resulting ceramics.     

Growth of silicon carbide thin films by hot-wire chemical vapor deposition from SiH4/CH4/H2

Silicon carbide (SiC) thin films were prepared by hot-wire chemical vapor deposition from SiH₄/CH₄/H₂ and their structural properties were investigated by X-ray diffraction, Fourier transform infrared absorption and Raman scattering spectroscopies. At 2 Torr, Si-crystallite-embedded amorphous SiC (a-Si_1 − xC_x:H) grew at filament temperatures (T_f) below 1600 °C and nanocrystalline cubic SiC (nc-3C-SiC:H) grew above T_f = 1700 °C. On the other hand, At 4 Torr, a-Si_1 − xC_x:H grew at T_f = 1400 °C and nc-3C-SiC grew above T_f = 1600 °C. When the intakes of Si and C atoms into the film per unit time are almost the same and H radicals with a high density are generated, which takes place at high T_f, nc-3C-SiC grows. On the other hand, at low T_f the intake of Si atoms is larger than that of C atoms and, consequently, Si-rich a-Si_1 − xC_x:H or Si-crystallite-embedded a-Si_1 − xC_x:H grow.