High temperature stiffening of ferroelastic LaCoO3

The cyclic ferroelastic hysteretic behavior of pure LaCoO₃ perovskite ceramic has been studied at different temperatures in four point bending. The stress-strain deformation behavior of LaCoO₃ was analyzed both in the term of the maximum stress in the cycle and in terms of the temperature used when the cyclic testing was performed. The characteristics of the stress-strain hysteresis loops, such as hysteresis loop area and irreversible strain, as well as effective Young’s modulus, were analyzed, and it was established that both the loading and the temperature history have a significant influence on the mechanical behavior of LaCoO₃. Young’s modulus values are reported to be much higher in the 700–900 °C temperature range as compared to the measurements performed in the RT-400 °C temperature range.     

Spectroscopic study of the competition between dehydration and carbonation effects in La2O3-based materials

La₂O₃ takes part in a variety of technologically interesting solids, but it has a known tendency to react with water and CO₂, which may result in nonstoichiometry of the final product and the appearance of impurities. We apply Raman spectroscopy to study hydration and carbonation effects of La₂O₃-based materials, using La(OH)₃ as starting material. The thermal evolution of La(OH)₃ reflects the competition between dehydration and carbonation kinetics, and thus is highly dependent upon environment and experimental conditions. In vacuum, La(OH)₃ evolves first to LaOOH and then to a mixture of C- and A-La₂O₃ until, upon further heating, only A-La₂O₃ phase remains. In air, La(OH)₃ yields first LaOOH, but under slow heating LaOOH converts to I, Ia and II-type oxycarbonates, in increasing order of thermal stability. II-La₂O₂CO₃, in turn, decomposes to La₂O₃ at 700 °C. Structural relations governing phase conversion within hexagonal or orthogonal-like compounds are discussed.     

Mechanical and microstructural properties of cameroonian bauxite ceramics for ballistic applications

Thermal reactions of bauxite, kaolin, and talc compound were investigated at 1550°C to obtain high rigidity ceramic for ballistic applications. The compressed strength and density of the bauxite ceramics with kaolin and talc substitutions varied from 195 to 455 MPa and density from 2.85 to 4.05 g/cm³. The Young’s modulus varied from 107 GPa to 222 GPa with water absorption varying from 1.4 wt% to 5.9 wt% for 0 wt% and 15 wt% substitution of kaolin. The substitution of bauxite-kaolin by talc up to 7.5 wt% contributes to the resorption of microporosity and increase the Young's modulus from 107 to 195 GPa. The XRD of bauxite ceramic with kaolin substitution showed the presence of corundum and mullite; whereas the XRD of bauxite ceramic with kaolin and talc substitution showed the presence of corundum, mullite, and spinel. The ballistic simulation with abaqus dassult SIMULA using the JH-2 model predict that an impact with velocity of 525 to 810 m/s on the 10 mm thick bauxite ceramic does not erode or damage for a projectile consisting of tungsten alloy with dimensions: 12 mm in diameter, 61.5 mm length, and 72 g of mass. The bauxite ceramics can be used for ballistic applications.     

PNIPAM/锂藻土纳米复合凝胶微球的制备及其弹性力学性能

利用微流控技术,以锂藻土作为交联剂,成功制备得到温度响应型聚（N-异丙基丙烯酰胺）（PNIPAM）与锂藻土的纳米复合凝胶微球,并利用一种简单的微步进单轴压缩装置,分别在25℃和37℃下对具有不同锂藻土含量的PNIPAM/锂藻土纳米复合凝胶微球的弹性力学性能进行系统研究。该微步进单轴压缩装置主要包括三个部分：一个程控进样器用以实现对凝胶微球的微步进压缩,一套配有高分辨率数码相机的侧视光学系统用以记录凝胶微球受压时发生的形变,一台精密电子天平作为力传感器用来记录凝胶微球在特定形变下所受的外力。研究结果表明,纳米复合凝胶微球在25℃和37℃下的形变量H与所受压力F的实验数据与Hertz弹性接触理论方程呈现良好的拟合关系,证明了PNIPAM/锂藻土纳米复合凝胶微球在25℃和37℃下均具有弹性形变行为。同时,随着锂藻土含量的增加,PNIPAM/锂藻土纳米复合凝胶微球的温敏性降低,但其杨氏模量增大。具有相同锂藻土含量的纳米复合凝胶微球,由于温度升高凝胶体积收缩、凝胶结构变得致密,因此在37℃下的杨氏模量大于其在25℃下的杨氏模量。研究结果可为PNIPAM/锂藻土纳米复合凝胶微球的设计与实际应用提供指导。     

Numerical modeling of thermally induced microcracking in porous ceramics: An approach using cohesive elements

A numerical framework is developed to study the hysteresis of elastic properties of porous ceramics as a function of temperature. The developed numerical model is capable of employing experimentally measured crystallographic orientation distribution and coefficient of thermal expansion values. For realistic modeling of the microstructure, Voronoi polygons are used to generate polycrystalline grains. Some grains are considered as voids, to simulate the material porosity. To model intercrystalline cracking, cohesive elements are inserted along grain boundaries. Crack healing (recovery of the initial properties) upon closure is taken into account with special cohesive elements implemented in the commercial code ABAQUS. The numerical model can be used to estimate fracture properties governing the cohesive behavior through inverse analysis procedure. The model is applied to a porous cordierite ceramic. The obtained fracture properties are further used to successfully simulate general non-linear macroscopic stress-strain curves of cordierite, thereby validating the model.     

Al2O3/SiO2摩尔比对超薄浮法玻璃性能的影响研究

Na2O-CaO-SiO2玻璃系统中,在SiO2+Al2O3总量不变的情况下,通过调整硅铝摩尔比,分别研究了组成变化对电子用超薄浮法玻璃热膨胀系数、显微硬度、杨氏模量、高低温黏度、转变温度和软化温度的影响规律。研究表明,选择合适的Al2O3/SiO2摩尔比组成,可以有效提高玻璃切裁率和产量,同时满足超薄玻璃的使用要求。     

Microstructure and Young’s modulus evolution during re-sintering of partially sintered alumina-zirconia composites (ATZ ceramics)

Partially and fully sintered alumina-zirconia composites (ATZ ceramics), with porosities decreasing from 53.5 to 1 %, have been prepared by uniaxial pressing and firing at 1000–1500 °C and characterized by the Archimedes method and mercury intrusion porosimetry. Young’s modulus has been measured via the impulse excitation technique at room temperature, resulting in an almost exponential porosity dependence (which is unusual for partially sintered ceramics in which the microstructure is dominated by concave pore surfaces), and at elevated temperatures up to 1500 °C during heating and cooling, resulting in a temperature master curve with a low-temperature inflection point around 200 °C (accompanied by a damping maximum). Both results confirm previous findings for zirconia and are typical for zirconia-containing ceramics. When the original firing temperature is exceeded, sintering and densification continues, albeit with a temperature lag when the sintering activity (specific surface area) is reduced as a consequence of previous firing.     

Studies on nanotribological and oxidation resistance properties of yttria stabilized zirconia (YSZ), alumina (Al2O3) based thin films developed by pulsed laser deposition

The present study aims at a detailed evaluation of mechanical, tribological, and high temperature oxidation resistance (at 1000 °C under isothermal condition) properties of YSZ, and Al₂O₃ based thin films developed by pulsed laser deposition technique. The mechanical and tribological properties of YSZ and Al₂O₃ thin films showed significant improvement with increasing the deposition temperature during pulsed laser deposition process. The kinetics of oxidation was reduced due to pulsed laser deposition and Al₂O₃ coating offered a superior oxidation resistance property as compared to YSZ coating. However, the deposition temperature has no significant effect in reducing the TGO growth rate of the pulsed laser deposited thin films.     

Coalescence of single-walled carbon nanotubes and formation of multi-walled carbon nanotubes under high-temperature treatments

Electric arc-discharge single-wall carbon nanotubes are annealed between 1600 and 2800 °C under argon flow. Their stability and evolution are studied by coupling TEM, X-ray diffraction and Raman spectroscopy. The first modifications appear at 1800 °C with a significant decrease of the crystalline order. It is due to SWNTs coalescence leading to smaller bundles but with an increase of the tube diameters from 2 to 4 nm. From 2200 °C, SWNTs progressively disappear to the benefit of MWNTs having at first two to three carbon layers then reaching 7 nm external diameter. The possible mechanisms responsible for the SWNTs coalescence and instability and their transformation in MWNTs are discussed.     

Effect of the calcium on the textural, structural and catalytic properties of La1–x Ca x Co1–y Fe y O3 perovskites

In La_1-x Ca _x Co_1-y Fe _y O₃ perovskites, the calcium substitution modifies the crystalline structure toward a pseudocubic one and produces an electronic unbalance, compensated by the formation of oxygen vacancies and Fe⁴⁺ ions. It also increases slightly the ethanol conversion in total combustion, compensating the detriment of catalytic activity caused by the iron substitution and it increases notably the selectivity to total oxidation.     

Microstructural,mechanical and tribological properties of nanostructured YSZ coatings produced with different APS process parameters

Plasma sprayed ceramic coatings can be used in turbine engines as thermal barrier or abradable coatings, in order to improve the durability of the components as well as the efficiency. The presence of nanostructures, deriving from partial melting of agglomerated nanostructured particles, represents an interesting technological solution in order to improve their functional characteristics. In this work nanostructured yttria stabilized zirconia (YSZ) coatings were deposited by air plasma spraying (APS). The influence of the main process parameters on their microstructural, mechanical and tribological properties was investigated by scanning electron microscopy (SEM), indentation techniques at micro- and nano-scale and wear tests, respectively. Their porous microstructure was composed of well melted overlapped splats and partially melted nanostructured areas. This bimodal microstructure led to a bimodal distribution of the mechanical properties. An increase of plasma power and spraying distance was able to produce denser coatings, with lower content of embedded nanostructures, which exhibited higher elastic modulus and hardness as well as lower wear rate.     

Effect of dysprosium doping on structural and vibrational properties of lead-free (Na0.7K0.3)0.5Bi0.5TiO3 ferroelectric ceramics

This study describes the effect of Dy³⁺ doping of lead-free ferroelectric sodium potassium bismuth titanate ((Na_0.7K_0.3)_0.5Bi_0.5TiO₃; NKBT) ceramics on their structural, optical, and vibrational properties. X-ray diffraction and Rietveld refinement analyses revealed that NKBT samples exhibited a rhombohedral perovskite structure belonging to the R3c space group, with the incorporation of Dy³⁺ resulting in structural parameter change and local atomic structure shift to tetragonal P4mm, as supported by the appearance of new Raman modes. Increased Dy³⁺ content decreased the NKBT band gap from 3.4 to 3.3 eV, which was ascribed to the introduction of Dy 4f levels between valence and conduction bands. Ferroelectric behavior studies combined with transport measurements explained the unexpected ferroelectric response weakening of Dy³⁺-modified NKBT by increased leakage current densities.     

Effect of Ni addition on the formation mechanism of Ti5Si3 during self-propagation high-temperature synthesis and mechanical property

The addition of Ni to Ti–Si system significantly decreased the ignition difficulty of self-propagation high-temperature synthesis (SHS), increased the product purity and improved the mechanical properties of bulk Ti₅Si₃ material. The formation of Ti₅Si₃ was controlled by two mechanisms: (i) solid-state reaction: evolution of unstable Ti–Si intermediate phases to Ti₅Si₃ through the mutual diffusion of Ti and Si atoms; and (ii) Ni-induced dissolution-precipitation: Ni reacted with Si and Ti to form Ni–Si and Ni–Ti intermetallics and the heat released from these reactions created massive Ni–Si and Ni–Ti liquids; subsequently, these liquid diffused mutually to form Ni–Ti–Si ternary liquid and finally massive Ti₅Si₃ precipitated out of the liquid. The latter mechanism played a dominant role in forming Ti₅Si₃ and led to the direct formation of Ti₅Si₃, which decreased the product impurity. The good binder effect of Ni for Ti₅Si₃ contributed to the improved fracture toughness and enhanced density.     

Effect of Sn doping on the structural,optical, electrical and anticancer properties of WO3 nanoplates

This report is on the synthesis of Sn doped WO₃ nanoplates by a facile co-precipitation method. Various characterization tools have been employed to study the effect of Sn doping on the structural, optical and dielectric properties of WO₃ nanoplates. The successful incorporation of dopant ions in the monoclinic structure of WO₃ has been verified by XRD, EDX, FTIR and Raman spectroscopy. It has been observed that there is a broadening in optical band gap for doped samples due to the band filling effects caused by the crystal defects. Furthermore, the frequency dependent electrical properties of WO₃ nanoplates are found to be significantly tuned by Sn doping. Most importantly, this is one of the initial reports that Sn doped WO₃ nanoplates are an excellent candidate for anticancer applications. The anticancer activity of WO₃ nanoplates against MCF-7 cancer cells is increased with Sn doping which is attributed to several factors such as particle size, defects density and reactive oxygen species (ROS) production.     

Effects of B/C ratio on the structural and mechanical properties of TiBCN coating deposited by PACVD

TiBCN coating is known as a hard, self-lubricant and wear resistant coating which can be applied on industrial tools to increase their working life time under severe wear conditions. In this paper, TiBCN coatings with different B/C ratios were applied on H13 steel using plasma-assisted chemical vapor deposition from BBr₃, TiCl₄, CH₄, N₂ and H₂ reactants at 500 °C. The results signified that the introduction of B and C elements to TiN changed its preferred crystalline orientation from (200) to (111) and decreased crystal size from 12 to 9 nm as a result of the formation of amorphous phases which constrain grain growth. The addition of B and C altered the coating's nucleation and growth mechanisms and generated a strong surface etching agent of HBr which significantly changed surface morphology and roughness. Increasing flow ratio of CH₄ to BBr₃ from 0.125 to 0.25 influenced the coating's mechanical properties and increased coating's hardness from 18.1 to 23.2 GPa and Young's modulus from 296 to 334.7 GPa. Rising coating's C content remarkably improved its nano-wear resistance and the coating with the highest C content exhibited a wear volume of 1*10⁻¹⁹ m³ which was about 63% lower than that of TiN coating.     

Effect of Nd-doping on structural,thermal and electrochemical properties of LaFe0.5Cr0.5O3 perovskites

The structural, thermal and electrochemical properties of the perovskite-type compound La_1−xNd_xFe_0.5Cr_0.5O₃ (x=0.10, 0.15, 0.20) are investigated by X-ray diffraction, thermal expansion, thermal diffusion, thermal conductivity and impedance spectroscopy measurements. Rietveld refinement shows that the compounds crystallize with orthorhombic symmetry in the space group Pbnm. The average thermal expansion coefficient decreases as the content of Nd increases. The average coefficient of thermal expansion in the temperature range of 30–850 °C is 10.12×10⁻⁶, 9.48×10⁻⁶ and 7.51×10⁻⁶ °C⁻¹ for samples with x=0.1, 0.15 and 0.2, respectively. Thermogravimetric analyses show small weight gain at high temperatures which correspond to filling up of oxygen vacancies as well as the valence change of the transition metals. The electrical conductivity measured by four-probe method shows that the conductivity increases with the content of Nd; the electrical conductivity at 520 °C is about 4.71×10⁻³, 6.59×10⁻³ and 9.62×10⁻³ S cm⁻¹ for samples with x=0.10, 0.15 and 0.20, respectively. The thermal diffusivity of the samples decreases monotonically as temperature increases. At 600 °C, the thermal diffusivity is 0.00425, 0.00455 and 0.00485 cm² s⁻¹ for samples with x=0.10, 0.15 and 0.20, respectively. Impedance measurements in symmetrical cell arrangement in air reveal that the polarization resistance decreases from 55 Ω cm⁻² to 22.5 Ω cm⁻² for increasing temperature from 800 °C to 900 °C, respectively.     

Influence of Li doping on the morphological evolution and optical & electrical properties of SnO2 nanomaterials and SnO2/Li2SnO3 composite nanomaterials

SnO₂ nanomaterials and SnO₂/Li₂SnO₃ composite nanomaterials doped with different Li contents were synthesized via a simple one-step thermal evaporation method. X-ray diffraction patterns showed that with the increase of Li doping, the intensity of Li₂SnO₃ diffraction peaks gradually increased, while that of SnO₂ diffraction peaks gradually decreased. With the increase of Li doping, the width of nanobelts gradually increased, with the morphology changing from banded structure to standard hexagonal sheet structure. The Raman scattering spectra indicated that with the increase of Li doping, the peak of Li₂SnO₃ at 588.8 cm^?1 kept increasing, and the strongest vibration mode A_1g in SnO₂ gradually weakened. X-ray photoelectron spectroscopy revealed that with the increase of Li doping, the surface electrophilic oxygen species in SnO₂/Li₂SnO₃ composite nanomaterials greatly increased. Under the condition of light irradiation with a wavelength of 505 nm, the bright current of the Li-doped SnO₂ samples was higher than the dark current, while that of the SnO₂/Li₂SnO₃ composite nanomaterials was higher than the dark current, which was mainly due to more oxygen vacancies in SnO₂/Li₂SnO₃ composite nanomaterials than electrons excited by light. Consequently, positive photoconductivity gradually weakened, and even the negative photoconductivity emerged.     

Resistive,capacitive and conducting properties of Bi0.5Na0.5TiO3-BaTiO3 solid solution

In the present paper, the effect of addition of a small amount (8 wt%) of barium titanate (BT) on electrical properties of bismuth sodium titanate (BNT) forming a solid solution of a composition (0.92)(Bi_0.5Na_0.5TiO₃)+(0.08)(BaTiO₃) (BNT-BT-8) has mainly been reported. The solid solution of BNT-BT-8 was prepared by a cost effective and standard mixed-oxide method. Preliminary structural analysis using X-rays diffraction pattern and data showed the existence of two phases; orthorhombic (major) and tetragonal (minor impurity/secondary) phase. Analysis of scanning electron micrograph and energy dispersive spectrum of the pellet sample reveals the formation of high density with homogeneously distributed grains of varying dimension. The locations, phonon modes statistics, width and intensity of peaks of Raman spectra of BNT-BT-8 was analyzed by Raman spectroscopy and provided some data on molecular structure of the material. The effect of temperature and frequency on some ferroelectric characteristics of the material were studied. The frequency-temperature dependence of electrical characteristical such as impedance of the material was studied by impedance spectroscopy. The electric conductivity follows the Arrhenius equation and provided activation energy at different frequency. The dielectric and impedance spectroscopy suggest the existence of a non-Debye relaxation mechanism in the material.     

Correlation of La-mediated structural transition and dielectric relaxation in Bi2Mg2/3Nb4/3O7 pyrochlores

Bismuth based pyrochlores have attracted a massive interest in the modern research areas owing to their structural tenability and variety of physical properties. The structural variation and its consequent effects on dielectric relaxation have been presented in this work by synthesizing a series of La-mediated Bi_2-xLa_xMg_2/3Nb_4/3O₇ (x = 0.00, 0.05, 0.10, 0.15, 0.20 and 0.25) ceramics using sol–gel auto-combustion technique. Coexistence of fluorite and cubic pyrochlore phases in the parent composition and gradual enrichment of pyrochlore phase with increase of substitution contents was observed from the structural analysis. Distortion of spherical symmetry of grains with reduced grain size from 950 to 500 nm was revealed by field emission scanning electron microscope. The presence of required constituent elements with their correct proportion was analysed using energy dispersive X-ray spectroscopy. Complex impedance study and frequency dependent dielectric dispersion analysis was performed to check ac-conductivity and to explore potential of material for energy storage applications. With the help of complex modulus analysis, the range of carrier transportation inside the material was distinguished in different frequency regimes.