Refinement of Nanoscale Grain Structure in Bulk Titania via a Transformation-Assisted Consolidation (TAC) Method

Bulk nanocrystalline TiO₂ samples (100% rutile) with a relative density as high as 97% and a grain size of <20 nm have been produced via high-pressure (up to 8 GPa)/low-temperature (∼0.3 T _m, where T _m is the melting temperature) sintering, using a toroidal-type high-pressure apparatus. Nanophase TiO₂ powder with a metastable anatase structure and an initial grain size of ∼38 nm was used as the starting material. During sintering, the anatase phase transformed to either the rutile or srilankite phase, depending on the pressure–temperature ( P – T ) combination. The starting temperature of the anatase-to-rutile phase transformation decreased from ∼550°C at ambient pressure to ∼150°C at 2.5 GPa. Grain growth was limited by the low sintering temperature and the multiple nucleation events in the parent phase. The grain size of the transformed rutile decreased as the sintering pressure increased, which can be explained by the combined effect of increasing the nucleation rate and decreasing the growth rate with high pressure. We have demonstrated that it is possible to produce a dense sintered compact with a grain size even smaller than that of the starting powder. The high-pressure srilankite phase was observed at P – T conditions as low as 4.75 GPa and 250°C, respectively; however, unlike the anatase-to-rutile phase transformation, the rutile-to-srilankite phase-transformation temperature increased as the pressure increased. Also, in contrast to the irreversible anatase-to-rutile phase transformation, the srilankite will reversibly transform to rutile under the appropriate circumstances. This observation provides an opportunity to further refine the TiO₂ grain structure by switching the sintering conditions (temperature and pressure) between the regions in which the rutile or srilankite phase are stable.     

Graded evolution of anisotropic microstructure during sintering from crystal-oriented powder compact

Anisotropic sintering, including shrinkage and grain growth, was examined for c-axis-oriented (Sr,Ca)₂NaNb₅O₁₅ (SCNN) ceramics, which were prepared by colloidal processing under a magnetic field. In the c-axis-oriented SCNN powder compact, shrinkage and grain growth along the c-axis were higher than those along the a-axis. The anisotropic microstructural development was clearly associated with anisotropic sintering shrinkage. X-ray diffraction, scanning electron microscopy, and energy back scattering diffraction showed that the grain growth of oriented particles by including random grains contribute to the development of the oriented microstructure. Finally, the highly crystal-oriented SCNN ceramics with a densified microstructure were obtained through anisotropic sintering. These results clearly showed the potential to develop a well-defined anisotropic microstructure during sintering by designing and controlling the particle packing structure in a powder compact.     

两种合成方式对制备纳米二氧化钛粉体的影响

以钛酸四丁酯为原料,分别通过溶胶-凝胶法和硬脂酸凝胶法制得TiO2前驱体,在450℃煅烧后分别得到粒径在42.4,9.4 nm的混晶纳米TiO2。运用XRD和TEM等方法对TiO2进行表征,探讨了两种合成方式对纳米TiO2粒径大小及TiO2晶型转变温度的影响。结果表明,以硬脂酸凝胶法合成的纳米TiO2纯度高、粒径小,且晶型转变温度低,这是由于晶粒细化后晶型转变温度降低的缘故。     

Method to Control the Phase of TiO2 in Porous Carbon–TiO2 Composite

Activated carbon (AC)-supported composites are emerging as a novel class of materials that hold great promise for environment-protecting applications. In this paper, we present an approach for the preparation of the AC–TiO₂ composite with mold-pressing using pure anatase TiO₂ and self-sinterable corn straw derivative as the carbon mesophase. In order to explore the effects of the carbon mesophase and some dopants on the phase transition of anatase to rutile TiO₂, the samples were characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM). It was discovered that the transition could be validly influenced by the sort and content of the adulterants. As a result, the ratio between anatase and rutile could be controlled in order to improve the photocatalysis and sensing properties of TiO₂. Therefore, a method for preparing the formed AC–TiO₂ composite powders and monoliths with controllable anatase phase proportion for different applications could be established.     

Dimensional change behavior of porous MgTi2O5 in reactive sintering

Volume-shrinkage of a sample in reactive sintering generally tends to be larger than that in conventional sintering. New techniques to suppress the volume shrinkage are eagerly needed for actual manufacturing. Recently, we have reported that reactively sintered porous MgTi₂O₅ from hydromagnesite and TiO₂ rutile showed less volume shrinkage than that from hydromagnesite and TiO₂ anatase. The result demonstrated that the compositional control of starting polymorphs can be a potential technique to optimize the volume shrinkage. In this paper, in order to evolve the reactive sintering technique, volume-changes during reactive sintering were dynamically monitored by thermomechanical analysis (TMA). The dimensional change behavior measured by TMA was linked up with the reaction behavior clarified by high-temperature X-ray diffraction (HT-XRD). In dilatometry curves, transient volume expansions were observed and they were well-explained by the formation and crystal growth of intermediate MgTiO₃ and objective MgTi₂O₅ particles.     

Effect of brookite phase on the anatase–rutile transition in titania nanoparticles

Nanosized TiO₂ powders were prepared from the precipitation in the TiCl₄ precursor under various pH values. The prepared titania existed in the form of nanocrystalline anatase with some brookite, which was evidenced by X-ray diffraction analysis and Raman spectroscopy. The average crystallite sizes of the TiO₂ particles heat treated at 450 °C for 2 h are in the range of 7–9 nm. The lattice constant c of anatase increased with increasing the synthesized pH value, whereas the volume fraction of the brookite phase increased with decreasing the synthesized pH value. The beginning and ending temperatures for the anatase–rutile transformation were found to decrease with increasing the volume fraction of the brookite phase. The brookite phase in the powder is responsible for enhancing the anatase–rutile transition.     

Role of Particle Substructure in the Sintering of Monosized Titania

Monosized titania particles (∼0.35-μ diameter) prepared by controlled hydrolysis of titanium tetraethoxide in ethanol were found to be porous agglomerates of ∼6-nm primary particles. The sintering behavior of compacts constituted of monodispersed agglomerates was evaluated, and changes in macroscopic dimensions were correlated with changes in particle microstructure and chemistry. The total volume shrinkage during sintering was ≥87%. Five contributions to the total shrinkage and the temperature ranges for the associated processes were identified: removal of chemisorbed water (from ambient to 250°C), crystallization to anatase (between 250° to 425°C), intra-agglomerate densification (425° to 800°C), conversion of anatase to rutile (600° to 800°C), and inter-agglomerate densification (>800°C). Approximately one-half the compact shrinkage was the result of agglomerate substructure changes. Studies of the agglomerate structural evolution indicated the intra-agglomerate densification and crystallite growth rates are the secondary factors, after compact packing, that influenced microstructure development.     

Phase transition between sphalerite and wurtzite in ZnS optical ceramic materials

The phase transition behavior of zinc sulfide (ZnS) ceramics consolidated via pressureless and hot press sintering has been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) analyses. Two types of ZnS powders with different particle sizes and morphologies were employed to study the influence of microstructural features of starting powders on the ZnS phase transition behaviors. The present work has revealed that during sintering of ZnS ceramics, the phase transition behavior varies based on the starting powder particle size and magnitude of the applied pressure. It has been demonstrated that smaller particle sizes lead to an increased degree of “early” phase transformation from sphalerite to wurtzite at 1000 °C. Additionally, the application of uniaxial pressure during sintering can lead to a reverse phase transition from wurtzite to sphalerite while simultaneously inducing twinning, resulting in improved optical transmittance and mechanical hardness.     

Phase analysis and microwave dielectric properties of LTCC TiO2 with glass system

Glass–ceramic composites containing TiO₂ (anatase, rutile) and modified borosilicate glasses were prepared and their sintering behaviour, phase evolution, interface reactions, and microwave dielectric properties were investigated as new candidates for low-temperature cofired ceramic (LTCC) materials. It was found that the addition of small amounts of borosilicate glasses lowered the sintering temperature of TiO₂ from 1400 to 900 °C. X-ray diffraction results showed that second phases, including Zn₂SiO₄, were formed when TiO₂+zinc-borosilicate glass was used, while no crystalline phase except rutile could be found using unmodified borosilicate glass. High-density TiO₂+zinc borosilicate glass material showed promising microwave dielectric properties: relative dielectric constant (ε_r)=74, quality factor (Q×f)=8000 GHz, and temperature coefficient of resonant frequency (τ_f)=340 ppm/°C. The effect of borosilicate glasses on the anatase–rutile phase transition was also investigated.     

Synthesizing and Comparing the Photocatalytic Properties of High Surface Area Rutile and Anatase Titania Nanoparticles

Rutile titania nanocrystalline particles with high specific surface areas were directly prepared by thermal hydrolysis of titanium tetrachloride aqueous solution. The as-prepared rutile titania powder was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer–Emmett–Teller surface area analysis, and Fourier transform Raman and IR spectroscopies. Neither anatase nor amorphous titania could be detected in this titania powder by XRD, Raman spectroscopy, and high-resolution TEM. In the phenol degradation reaction, the rutile titania powder with an initial crystalline size of 7 nm was found to have higher photocatalytic activity than that of anatase titania with the same specific surface area. The rutile titania powders calcined at 300° and 450°C also showed a relatively high photocatalytic property. The high activity of the as-prepared rutile titania was attributed to the abundance of hydroxy groups in the powder, as was proven by thermogravimetric analysis data, which provided more active sites for the degradation reaction.     

煅烧过程中二氧化钛微结构参数的变化和相变

采用X射线衍射研究室温到1273K煅烧过程中TiO2微结构特征和相变。结果发现：锐钛型TiO2向金红石型TiO2的转变出现在1223-1273K的高温区,此结果为高温烧结附着力强的锐钛型TiO2薄膜和将TiO2水溶胶直接加入釉料中制备光催化活性的釉面陶瓷提供了理论依据。773～973K之间的锐钛型TiO2基体中出现了质量分数小于12％的板钛型TiO2。锐钛型TiO2微晶尺寸分布在低温比在高温的均匀,其应变能量密度随煅烧温度升高明显降低。     

Photocatalytic activity of titanium dioxide coatings: Influence of the firing temperature of the chemical gel

Heterogeneous photocatalysis can be exploited for the decomposition of micro-organisms which have developed on the surfaces of building materials. In this work, the efficiency of titanium dioxide coatings on fired clay products is examined. The sol–gel method is used to synthesize a fine TiO₂ powder with a specific surface area of 180 m² g^?1. Thermal treatment of the chemical gel at 340 °C leads to crystallisation in the anatase phase and with further temperature increase, crystallite growth. For thermal treatments in the range 580–800 °C, there is a progressive transition from anatase to rutile. However, despite a decrease in specific surface area of the powder attributed to aggregation/agglomeration, the coherent domain size deduced from X-ray diffraction measurements remains almost constant at 23 nm. Once the transition is completed, increase of thermal treatment temperature above 800 °C leads to further crystallite growth in the rutile phase. The thermally treated titania powders were then sprayed onto fired clay substrates and the photocatalytic activity was assessed by the aptitude of the coating to degrade methylene blue when exposed to ultraviolet light. These tests revealed that the crystallite size is the important controlling factor for photocatalytic activity rather than the powder specific surface area or the anatase/rutile polymorph ratio.     

Effect of Chlorine Atmosphere on the Anatase-Rutile Transformation

The anatase-rutile phase transformation in Ar–Cl₂ atmosphere has been investigated by X-ray powder diffraction, thermogravimetry, and scanning electron microscopy. At 950°C the phase transformation in Ar–Cl₂ atmosphere is about 300 times faster than in air. The growth of crystals of rutile from the vapor phase has been observed. We also determined that anatase powders with adsorbed chlorine rapidly rapidly transformed to rutile and underwent a color change from white to gray when they were heated in Ar atmosphere.     

Anatase–rutile transformation in doped titania under argon and hydrogen atmospheres

Abstract

Pure titania pulp containing amorphous titania was heated at different temperatures and times. Above 650°C anatase phase was evolved and between 900 and 1000°C, anatase–rutile transformation occurred. The anatase–rutile transformation in TiO₂ in the presence of different transition metal oxides, namely Fe₂O₃, Cr₂O₃, NiO, CuO and MnO₂ under argon and hydrogen atmospheres was investigated. The different phases of TiO₂ were determined using powder X-ray diffraction (XRD). The anatase–rutile transformation temperature was found to be lowered in the presence of transition metal oxides. The transformation temperature was found to vary much in argon and hydrogen atmospheres compared to air in the presence of the metal oxides. Also the method of preparation of metal oxide doped TiO₂ influences rutilation. Other methods such as chemical analysis, surface area measurements and crystallite size calculation were used for the characterisation of the samples. The surface area of heated samples was found to be decreased while crystallite size increased due to rutilation on heating. The samples were also observed under a scanning electron microscope to characterise the microstructural changes associated with each thermal treatment and atmosphere. The morphology of doped titania changes much on heating due to phase modification. The atmosphere of heating also has important effect on deciding the morphology of rutilated titania.     

微晶纤维素模板法制备纳米二氧化钛及表征

以TiCl4为原料,微晶纤维素为模板,制备了具有微晶纤维素特征形貌的纳米二氧化钛。借助透射电子显微镜、环境扫描电子显微镜、傅里叶变换红外光谱、X射线衍射、热重分析等分析方法对样品进行表征,探讨纳米二氧化钛的形成机理。结果表明:在500℃焙烧得到准球形锐钛矿纳米二氧化钛,粒径为15 nm;在700℃和900℃焙烧分别得到夹有金红石型的锐钛矿和红晶石型纳米二氧化钛,并且随着焙烧温度的升高,纳米二氧化钛晶型由锐钛矿逐渐转变成金红石型,其粒径增大,烧结加剧致使形貌变得更不规则。700℃焙烧产物在紫外光下对罗丹明B具有较佳的光催化降解性能。     

Transformation of Crystal Phase of Micron-sized Rutile TiO2 and Investigation on its Sonocatalytic Activity

The partial transformation of crystal phase of micron-sized TiO₂ powder from rutile to anatase was realized utilizing microwave irradiation in hydrogen peroxide solution. Afterwards, the ultrasound of low power was used as an irradiation source to induce the transition crystal TiO₂ powder to perform the sonocatalytic activity through the degradation of azo fuchsine in aqueous solution. The results show that the sonocatalytic activity of the transition crystal TiO₂ powder is obviously higher than that of pure micron-sized rutile and anatase TiO₂ powders. The degradation ratio of azo fuchsine in the presence of the transition crystal TiO₂ powder attains nearly 80% within 80 min ultrasonic irradiation.     

Kinetics of anatase transition to rutile TiO2 from titanium dioxide precursor powders synthesized by a sol-gel process

Kinetics of anatase transition to rutile TiO₂ from titanium dioxide precursor powders synthesized by a sol-gel process have been studied using differential thermal analysis (DTA), X-ray diffraction, transmission electron microscopy (TEM), selected area electron diffraction (SAED), nano beam electron diffraction (NBED) and high resolution TEM (HRTEM). The DTA result shows residual organic matter decomposed at 436 K. The transition temperature for amorphous precursor powders converted to anatase TiO₂ occurred at 739 K. Moreover, the full anatase transition to rutile TiO₂ occurred at 1001 K. The activation energy of anatase TiO₂ formation was 128.9 kJ/mol. On the other hand, the activation energy of anatase transition to rutile TiO₂ was 328.4 kJ/mol. Mesoporous structures can be observed in the TEM image.     

Phase Transformation of Nanocrystalline Anatase Powders Induced by Mechanical Activation

The microstructural evolution of nanocrystalline anatase TiO₂ during high-energy planetary ball milling was studied. The results show that mechanical activation induces the transformations of nanocrystalline TiO₂ from anatase to srilankite and rutile at room temperature and under ambient pressure. As the milling time increases, more anatase powder transforms to the srilankite and rutile phase, and the particle size of the powder decreases. There is no indication of the formation of amorphous phase during ball milling.     

磷酸盐调控纳米TiO2微结构及其表面羟基密度

以冰乙酸作诱导剂,研究磷酸盐在溶胶-凝胶法合成纳米二氧化钛过程中对其微结构及其表面羟基密度的影响,并利用XRD、BET、EDS、SEM、FT-IR和TGA等方法对二氧化钛相变过程和表面结构变化进行表征.结果表明:合成过程中添加磷酸根可增加纳米二氧化钛的表面羟基密度,500℃、700℃煅烧6h,其表面羟基密度分别为10.74nm^-2、4.03nm^-2,而对照样在500℃煅烧条件下其表面羟基密度为4.95nm^-2;同时磷酸根的添加,有利于锐钛型晶相结构的热稳定性,锐钛型向金红石转变的相变温度达800℃.这可能是由于PO₄^3-与TiO₂晶相生长基元[TiO₆]八面体易发生多齿螯合,不利于金红石相形成.     

Effects of Thermal Annealing on Formation of Micro Porous Titanium Oxide by the Sol–Gel Method

We investigated the structural and optical properties of microporous titanium oxide (TiO₂) fabricated by the sol–gel method using templates of colloidal crystals with polystyrene spheres when the annealing temperature was changed between 600° and 1000°C. From X-ray diffraction patterns and SEM images, the rutile TiO₂ annealed at a high temperature did not form periodic porous bodies, while the anatase TiO₂ annealed at lower than 800°C formed periodic porous bodies. The porous TiO₂ obtained acts as an air-sphere/TiO₂ photonic crystal with an FCC structure. It is suggested that TiO₂ sol annealed at a lower temperature do not lead to phase transition from the anatase phase to the rutile phase to obtain the air-sphere/TiO₂ photonic crystal by the sol–gel method using templates of colloidal crystals.