Densification,microstructure and mechanical properties of multicomponent (TiZrHfNbTaMo)C ceramic prepared by pressureless sintering

A multicomponent (TiZrHfNbTaMo)C ceramic has been fabricated by pressureless sintering at temperatures from 2100 ℃ to 2500 ℃,using an equimolar multicomponent carbide powder synthesized by carbothermal reduction as the starting material.Influence of sintering temperature on densification,microstructure and mechanical properties of the ceramics was investigated.The relative density increases with increasing sintering temperature,and a nearly fully dense sample is achieved by pressureless sintering at 2500 ℃.Average grain size increases from 3.7 to 15.2 μm with increasing sintering temperature from 2300 to 2500 ℃.The (TiZrHfNbTaMo)C ceramic sintered at 2400 ℃ exhibits a single phase fcc structure with homogeneous chemical composition,an average grain size of 7.0 μm and a relative density of 96.5％,while its measured hardness is 33.2 GPa at 100 mN and 23.2 GPa at 9.8 N.     

Synthesis of Ti2SnC from Ti/Sn/TiC powder mixtures by pressureless sintering technique

Ti/Sn/TiC powder mixtures were first employed to synthesize Ti₂SnC powder by pressureless sintering in the temperature range of 950–1250 °C at vacuum atmosphere. Ti₂SnC began to form at 950 °C, its content increased with increasing temperature. High purity of Ti₂SnC was obtained by sintering the mixtures with deficient Sn and TiC at 1200 °C for 15 min. A reaction mechanism was proposed to explain the formation of Ti₂SnC. The Ti₂SnC powder was characterized by scan electron microscopy (SEM) and X-ray diffraction (XRD). Using the above mixtures and process, the Ti₂SnC ceramic powder can be obtained on a larger scale.     

Densification and polymorphic transition of multiphase Y2O3 nanoparticles during spark plasma sintering

Multiphase (MP) monoclinic and cubic Y₂O₃ nanoparticles, 40 nm in diameter, were densified by spark plasma sintering for 5-15 min and100 MPa at 1000 °C, 1100 °C, and 1500 °C. Densification started with pressure increase at room temperature. Densification stagnated during heating compared to the high shrinkage rate in cubic single-phase reference nanopowder. The limited densification of the MP nanopowder originated from the vermicular structure (skeleton) formed during the heating. Interface controlled monoclinic to cubic polymorphic transformation above 980 °C led to the formation of large spherical cubic grains within the vermicular matrix. This resulted in the loss of the nanocrystalline character and low final density.     

Effects of weak boundary phases (WBP) on the microstructure and mechanical properties of pressureless sintered Al2O3/h-BN machinable composites

Al₂O₃/h-BN machinable composites were fabricated by pressureless sintering at 1750 °C for 2 h in nitrogen atmosphere. The relative density of the sintered composites decreased, while the porosity increased with increasing h-BN content. By adding 20 vol.% h-BN to the composites, the porosity increased up to 16.7%. The effects of weak boundary phases (WBP), including h-BN and pores, on the microstructure, mechanical properties and machinability of the composites were investigated. The flexural strength, fracture toughness, Young's modulus and hardness of the composites decreased with increasing WBP content. When WBP content increased up to 18.8 vol.%, the composites can be machined easily by cemented carbide drills. Furthermore, the machining mechanisms of the composites were investigated using Hertzian contact tests.     

无压烧结Ti3SiC2/铝基复合材料组织与性能

MAX相具有独特的层状晶体结构,不但具备常用铝基复合材料外加陶瓷颗粒的性能特征,同时具有可与石墨媲美的摩擦性能.本文以Al粉、Si粉和典型MAX相Ti_3SiC_2为原料,采用冷压成型-无压烧结方法制备了Ti_3SiC_2/Al-Si复合材料,并通过金相显微镜、X射线衍射仪(XRD)、扫描电镜(SEM)、能谱仪(EDS)等分析手段,研究了烧结温度、Si元素含量对复合材料组织与性能的影响.研究表明:随着烧结温度从500℃提高到700℃,复合材料致密度先上升后下降,摩擦系数先降低后上升,硬度逐渐增大至最大值并基本保持稳定;随着Si质量分数从0增加到20.7%,复合材料的致密度逐渐降低,硬度逐渐增大,摩擦系数先降低后增大,晶粒尺寸随之下降,12.5%Si晶粒最为细小;烧结温度为650℃,Si元素质量分数为12.5%的铝基复合材料具有最低的摩擦系数0.18,相应的硬度为62 HV,致密度为92.12%.XRD物相和扫描电镜组织分析表明,复合材料的主要相组成为Al、Ti_3SiC_2,及由界面反应产生的Al_4C_3和Al的氧化产物Al_2O_3.     

Densification and alloying of ball milled Silicon-Germanium powder mixture during spark plasma sintering

In this research, the influence of process parameters such as sintering temperature and current during alloying and densification of silicon-germanium (Si₈₀-Ge₂₀) powder mixture using spark plasma sintering (SPS) was reported. Si₈₀-Ge₂₀ powder mixture was consolidated at the temperature range 900–1200 °C with 40 MPa pressure for 5 min. soaking. X-ray diffraction (XRD) study was made on sintered compacts to confirm the Si(Ge) alloy formation. Scanning electron microscope (SEM) was used to understand the morphology, particle size and distribution of un-milled and milled Si₈₀-Ge₂₀ powder mixture. Transmission electron microscope (TEM) study was made on milled Si₈₀-Ge₂₀ powder mixture and bulk SiGe alloy to confirm the nano-crystallinity and alloy formation. Fracture toughness of sintered bulk SiGe alloy was determined from Palmqvist cracks geometry model using Vickers hardness testing. It is understood that, during spark plasma sintering nano-structured Si₈₀-Ge₂₀ powder simultaneously increases the densification and reaction kinetics. It helps to achieve homogenous nanostructured SiGe alloy of near theoretical density. The superior hardness and benchmarked fracture toughness (K_IC) values of 630 VHN and 2.19 MPa√m was achieved for SiGe alloy sintered at 1200 °C, respectively.     

Preparation of polycrystalline BaTi2O5 by pressureless sintering

Polycrystalline BaTi₂O₅ (BT₂) was prepared by pressureless sintering in air using BaCO₃ and TiO₂ as starting materials. XRD results of the calcined powder showed BaCO₃ and TiO₂ reacted completely after being calcined above 950 °C, showing a mixture of BaTiO₃ (BT), BT₂, BaTi₄O₉ and Ba₄Ti₁₃O₃₀. A small amount of ZrO₂ (less than 0.1 wt%) was effective to prepare BT₂ in a single phase and the second phase of BT and B₆T₁₇ increased with ZrO₂ content. BT₂ sintered body in a single phase was obtained at 1175-1300 °C when ZrO₂ content was 0.08 wt%. The maximum permittivity of BT₂ sintered body was 340 at the Curie temperature (T_c) of 463 °C and the frequency of 100 kHz.     

Synthesis and visible-light photocatalytic activity of Bi-doped TiO2 nanobelts

Bi-doped anatase TiO₂ nanobelts were synthesized from layer-structural titanate nanobelts using two-step hydrothermal treatment approach. X-ray diffraction (XRD) patterns and transmission electron microscopic (TEM) images show that the doping of Bi³⁺ cations does not change the crystal structure and morphology of TiO₂ nanobelts. The energy-dispersive X-ray (EDX) and inductively coupled plasma-mass spectrometry (ICP-MS) analytic results suggest that the doping cations mainly exist near the surface of the TiO₂ nanobelts. The ultraviolet-visible (UV-vis) absorption spectra show that the absorption edge for the samples with Bi³⁺ has red shift as compared with that of undoped TiO₂ nanobelts, and correspondingly, the photocatalytic degradation of methylene blue under visible-light illumination is enhanced with the increase of Bi-doping content.     

Novel electrochromic devices based on complementary nanocrystalline TiO2 and WO3 thin films

Electrochromic devices were elaborated based on two complementary electrodes made of a nanocrystalline metal oxide thin film deposited on conducting glass. The first electrode holds a 5 μm thick nanocrystalline TiO₂ film derivatized by a monolayer of a phosphonated triarylamine which can be rapidly oxidized by electron transfer to the conducting support followed by charge percolation inside the monolayer. The oxidation in accompanied by a blue coloration due to the absorption band at 730 nm of the stable triarylamminum radical cation. The second electrode bears a 0.2 μm thick nanocrystalline WO₃ film which turns from colorless to blue by reduction and lithium ion insertion. The former electrode reaches an absorbance of at least 3 between 700 and 730 nm after full oxidation (16 mC/cm²) at 1.0 V vs. NHE while for the second, complete reduction at −1.3 V (74 mC/cm²) leads to A=2.4 at 774 nm. An electrochromic device comprising both electrodes separated by an electrolytic solution of 0.1 Li⁺ in 4,7-dioxaoctanitrile reaches an absorbance of 2.2 at 700 nm, 4 s after a voltage step to 1.5 V. The system was shown to sustain at least 14400 coloration-discoloration cycles without degradation.     

Fabrication of TiO2 nanotube arrays by electrochemical anodization in an NH4F/H3PO4 electrolyte

Highly ordered TiO₂ nanotube arrays were fabricated by electrochemical anodization of titanium in an NH₄F/H₃PO₄ electrolyte. A TiO₂ crystal phase was identified by X-ray diffraction, and the morphology, length and pore diameter of the TiO₂ nanotube arrays were determined by field-emission scanning electron microscopy (FE-SEM). The anodization parameters including the rate of magnetic stirring, F⁻ concentration, calcination temperature, anodization voltage and anodization time were investigated in detail. The results show that the as-prepared TiO₂ nanotube arrays possessed good uniformity, a well-aligned morphology with a length of 750 nm and an average pore diameter of 62 nm at a 150 rpm rate of magnetic stirring for 120 min at 20 V in an electrolyte mixture of 0.2 M H₃PO₄ and 0.3 M NH₄F with a 500 °C calcination to obtain 100% anatase phase. The adsorption of N-719 dye at different tube lengths was determined by UV-vis analysis and found to increase with increasing tube length. We also discuss the formation mechanism of the TiO₂ nanotube arrays. The findings indicate that the formation of the TiO₂ nanotube arrays proceeds by the combined action of the electrochemical etching and chemical dissolution.     

Strain analysis of photocatalytic TiO2 thin films on polymer substrates

Titania (TiO₂) thin films have been deposited on polymer sheets by magnetron sputtering at room temperature. Previous X-ray diffraction experiments revealed, for a wide range of deposition parameters, that the as-deposited titania thin films are predominantly amorphous; however, Raman scattering experiments revealed small traces of crystalline phases. The photocatalytic behaviour of the titania coatings was determined by combined ultra-violet (UV) irradiation and absorption measurements of a chosen dye (pollutants) in the presence of this catalyst. In order to assess the mechanical behaviour of the as-sputtered films, the film/substrate composite system was loaded unidirectionally using a tensile testing machine. As the system was stretched, cracks transverse to the loading direction developed in the film. The number of cracks increased as the applied strain increased, thus the relation between the measured crack density and the applied strain has been used to characterize the film strength and has also been correlated with the film photocatalytic efficiency. As a result of moderate fissuring on the titania film, it was found that for strain deformations up to 5% the photocatalytic activity is enhanced due to the exposure of more catalyst surface area for the pollutant to be adsorbed and subsequently dissociated upon UV illumination.     

Vacuum heat treatment of LiAlO2 densified silicon nitride ceramics

The aim of the present work has been to produce high-dense Si₃N₄ ceramics by a cheaper pressureless sintering method and then to attain vacuum heat treatment to remove residual grain boundary glass in gaseous form. LiAlO₂ was used as a sintering additive rather than using Li₂O, since its grain boundary glass is not stable above 1200 °C. LiAlO₂ was synthesised from 42% Li₂CO₃ and 58% Al₂O₃ powder mix reacting together at 1450 °C for 3 h in a muffle furnace. X-ray analysis showed that 95% LiAlO₂ was obtained. LiAlO₂ was milled and added to silicon nitride powder as a sintering additive. Hot-pressing and pressureless sintering of LiAlO₂ containing Si₃N₄ compacts were carried out at temperatures between 1450–1750 °C. The sintered samples were vacuum heat-treated at elevated temperatures under high vacuum to remove intergranular glass and to increase refractoriness of Si₃N₄ ceramics. Scanning electron microscope images and weight loss results showed that Li in grain boundary glass (Li–Al–Si–O–N) was successfully volatilised, and oxidation resistance of the sintered samples was increased.     

A simple one-step fabrication of micrometer-scale hierarchical TiO2 hollow spheres

Hierarchical TiO₂ hollow spheres had been prepared based on bubble templates by a simple one-step hydrothermal method. The diameter of hollow spheres was about 700 nm and the shell thickness of them was 69 nm. They were composed of similar spindle- or needle-like building units. Furthermore, hydrothermal time had an important influence on the morphology and crystallinity of hollow spheres. Moreover, the UV-Vis diffuse reflectance spectra of TiO₂ hollow spheres heated at 150 °C for 10 h showed the strongest absorption in the UV-Vis region and the Raman spectrum demonstrated the anatase sample. Additionally, a possible formation mechanism of TiO₂ hollow spheres was proposed. So this novel and simple method would provide a development direction to fabricate all kinds of inorganic hollow spheres by one-step procedure.     

Catalyst-free growth of Sb2Te3 nanowires

In this study, a catalyst-free growth method was discovered to prepare the high-quality single crystal Sb₂Te₃ nanowires from the Al:Ge:Sb:Te thin films. The diameters of Sb₂Te₃ nanowires were found to be ~ 100 nm and their lengths were as great as tens of micrometers. The Al content and the annealing temperature play an important role in the growth of Sb₂Te₃ nanowires. When the Al content (> 12.4 at.%) was sufficiently contained in Al:Ge:Sb:Te film, Sb₂Te₃ nanowires were extruded spontaneously on the surface of thin film with increase in annealing temperatures. Compared with the vapor-liquid-solid method, our method has advantages of low temperature (~ 300 °C) and no impurities, such as a metal catalyst.     

A low-temperature reactive sintering process for the 5Li2O–1Nb2O5–5TiO2 microwave dielectric ceramics

The B₂O₃-doped 5Li₂O–1Nb₂O₅–5TiO₂ composite microwave dielectric ceramics prepared by conventional and low-temperature single-step reactive sintering processes were investigated in the study. Without any calcinations involved, the Nb₂O₅ mixture of Li₂CO₃ and TiO₂ was pressed and sintered directly in the reactive sintering process. More uniform and finer grains could be obtained in the 5Li₂O–1Nb₂O₅–5TiO₂ ceramics by reactive sintering process, which could effectively save energy and manufacturing cost. And relatively good microwave dielectric properties of _r = 41, Q × f = 9885 GHz and τ_f = 43.6 ppm/°C could be obtained for the 1 wt.% B₂O₃-doped ceramics reactively sintered at 900 °C.     

Novel triethanolamine assisted sol-gel synthesis of N-doped TiO2 hollow spheres

A facile preparation of N-doped anatase TiO₂ hollow spheres in sub-micron size with good morphology was developed in this work. Polystyrene latexes in size of 470 nm were used as the templates to fabricate polystyrene/TiO₂ core-shell spheres in the sol-gel process. Here the ammonia/triethanolamine positive/negative catalyst pair was first employed to control the coating of TiO₂ onto the surface of the PS templates. And synchronously triethanolamine served also as the N source. The N-doped TiO₂ hollow spheres with good morphology were first obtained here after calcinations of the core-shell spheres. It was proved that the hollow spheres have distinct visible light response from 390 to 600 nm. The content of the N doping could be easily adjusted by changing the amount of triethanolamine added and the optical response of TiO₂ hollow spheres shifted more to the visible light as the content of the N doping increasing. The N doping could increase the separation efficiency of the photoinduced electron and hole, so the intensity of photoluminescence decreased obviously with increased content of the N doping.     

Room temperature synthesis of nearly monodisperse rodlike rutile TiO2 nanocrystals

A new method was developed to synthesize uniform rodlike rutile TiO₂ nanocrystals by the hydrolysis of tetrabutyl titanate [Ti(OC₄H₉)₄] in hydrochloric acid-alcohol aqueous solutions at room temperature. The hydrolytic sol-gel reaction generated 44 nm (diameter) × 200 nm (length) sized rutile TiO₂ nanocrystals. Transmission electron microscopic images showed that the particles have a uniform shape and narrow size distribution. X-ray diffraction and electron diffraction patterns combined with high-resolution transmission electron microscopic image showed that the rodlike TiO₂ nanoparticles prepared at room temperature were crystalline rutile structure grown along the [001] direction. The morphology and photocatalytic activity of the TiO₂ nanocrystals formed at different urea concentrations were showed. The rutile TiO₂ nanocrystals formed in the absence of urea exhibited higher photocatalytic activity than the commercial photocatalyst P25 on the photocatalytic degradation of Rhodamine B.     

Nitridation of nanocrystalline TiO2 thin films by treatment with ammonia

Nanocrystalline anatase (TiO₂) thin films prepared by a physical vapour deposition method were nitrided by annealing in flowing NH₃ at temperatures ranging between 650 °C and 700 °C. It was established that there was a narrow window of temperatures which allowed both incorporation of interstitial nitrogen into the films with retention of the anatase phase without chemical reduction and preservation of the characteristic nanocrystalline morphology. These optimally modified films responded to visible light in photowetting tests and showed the ability to degrade an organic dye under visible light irradiation.     

Investigation on solar photocatalytic activity of TiO2 loaded composite: TiO2/Eggshell, TiO2/Clamshell and TiO2/CaCO3

The TiO₂/Eggshell, TiO₂/Clamshell and TiO₂/CaCO₃ loaded composites were prepared by sol-gel method and characterized by XRD and SEM. Their photocatalytic activities were measured through the degradation of Acid Red B under solar light irradiation. The influences of TiO₂ loaded content, heat-treated temperature and time on the photocatalytic activities were reviewed. The effects of irradiation time and dye initial concentration on the photocatalytic degradation were also investigated. The results showed that the photocatalytic activity can be greatly enhanced by appropriate TiO₂ loaded content.     

Crystal structure of dense nanocrystalline BaTiO3 ceramics

The dense nanocrystalline BaTiO₃ ceramics with the average grain size of 30 nm was sintered by the pressure assisted method. The Raman spectroscopy and the Rietveld refinement were employed to characterize the structural information of the nanocrystalline BaTiO₃ ceramics. The structural parameters and the reliability factors for the nanocrystalline BaTiO₃ ceramics were successfully determined by the Rietveld refinement based on the analysis of Raman spectra. A multiphase coexistence of tetragonal and orthorhombic phases was observed in 30 nm BaTiO₃ ceramics at room temperature. The phenomenon can be explained by the internal stress.