Ultralow expansion ceramics in the HfO2-TiO2 system synthesized by an hydrolysis and polycondensation technique

Ultra-refractory ceramics from the HfO₂-TiO₂ system in the range 30–40 mol% TiO₂, with a near-zero thermal expansion, have been synthesized by hydrolysis and polycondensation of titanium alkoxide and hafnium dichloride alcoholic solutions and sintered at moderate temperature. Thermal stability, crystallization, density and microstructure of these materials have been examined. The as-prepared powder, amorphous at room temperature, crystallized quickly when heated at 500 ° C. Entire crystallization occurred after treatment at 1000 °C. Sintering at 1500 °C on cold-pressed samples led to ceramics with weak porosity (7%), low expansion coefficient <1×10^–6 °C^– with a minimum for 30 mol% TiO₂ content. SEM examination on sintered materials at 1500 °C reveals a grain size from 2–6 m, increasing with TiO₂ content.     

Nanocomposites in mullite-ZrO2 and mullite-TiO2 systems synthesised through alkoxide hydrolysis gel routes: microstructure and fractography

The sol-gel process allows preparation of very homogeneous and reactive monolithic, optically clear gels. Low-temperature thermal treatments (700–1000 °C) lead to amorphous optically clear samples (glass). Amorphous mullite compositions (0.4Al₂O₃-0.6SiO₂ to 0.8Al₂O₃-0.2SiO₂) retain large amounts of Ti and Zr elements. The crystallization has been studied by differential thermal analysis, dilatometry, X-ray and electron diffraction and Raman scattering. The nucleation begins above 1000 °C with the departure of the last protonic species, the amorphous matrix being completely crystallized only above 1400 °C. The addition of Zr and Ti elements leads to a homogeneous nucleation of phases with a composition close to ZrO₂ and Al₂Ti₃O₉ (EDX analysis) above the solubility limit. TEM and SEM analyses show that the precipitate size remains submicrometric over a wide temperature range (1000–1400 °C) and consequently glass-like mechanical properties, as well as toughening effects, caused by the presence of nanoprecipitates, are observed.     

Mechanism of structure formation in samarium and holmium titanates prepared from mechanically activated oxides

We have studied the formation mechanism and phase transitions of samarium and holmium titanates prepared from mechanically activated oxide mixtures with the overall compositions Sm₂(Ho₂)Ti₂O₇ and Sm₂TiO₅. Mechanical activation of oxide mixtures leads to the formation of amorphous solid phases which crystallize in a distorted pyrochlore-like structure and contain OH groups on the oxygen site and structural vacancies up to 1000°C. In the range 800–1000°C, Sm_2?x Ti_1?y O_5?δ (OH) _n (x < 0.02; y < 0.08; δ, n < 0.19) converts to a distorted orthorhombic phase as a result of the relaxation of internal stress and removal of OH groups. Above 1000°C, the phases studied have the compositions Sm₂(Ho₂)Ti₂O₇ and Sm₂TiO₅ and ordered pyrochlore-like and orthorhombic structures, respectively. The lattice parameters of the titanates have been measured in the range 800–1350°C. The internal stress produced by mechanical activation in the phases studied here fully relaxes by ～1300°C.     

Synthesis of hexatitanate and titanium dioxide fibers by ion-exchange approach

Hexatitanate (K₂Ti₆O₁₃) and titanium dioxide (TiO₂) fibers were synthesized by ion-exchange method using fibrous potassium titanate (K₄Ti₃O₈) as initial material. Fibrous K₄Ti₃O₈ was prepared by treating TiO₂ with 80 wt.% concentrated KOH solution at 220 °C for 2 h under atmospheric pressure. Then, high qualities of K₂Ti₆O₁₃ and TiO₂ fibers were formed by a thermal treatment of the hydrolytic intermediates K_4−bH_bTi₃O₈ (1.2 ≤ b ≤ 3.0) and H₄Ti₃O₈, which were quantitatively obtained by controlling the pH value ranging from 10.9 to 12.0 and from 2.0 to 5.0 in the hydrolytic process of K₄Ti₃O₈, respectively. Based on the experimental results, the possible ion-exchange and phase transformation mechanisms in the reaction process were discussed.     

Hydrothermal synthesis of potassium titanates in Ti-KOH-H2O system

The hydrothermal oxidation of titanium metal powder was performed in concentrated potassium hydroxide solutions above 150 °C. K₂Ti₂O₅, K₄Ti₃O₈ and a new compound KTiO₂(OH) were formed as single phase and fibrous amorphous products which transformed to K₂Ti₄O₉ and K₂Ti₆O₁₃ and K₂Ti₂O₅ by calcination were also obtained depending on the reaction conditions. A formation diagram of products in the Ti-KOH-H₂O system has been constructed at the region of KOH concentration from 0 to 90 mol·kg^–1-H₂O and the temperature from 150 to 350 °C.     

Effects of oxidation on intergranular phases in silicon nitride ceramics

The effects of oxidation on changes in the secondary phases of two Si₃N₄ ceramics were investigated by transmission electron microscopy. The Si₃N₄ materials were oxidized at 1400 °C for 168 h in laboratory air. One material, sintered with 5 vol% Yb₂O₃+0.5 vol% Al₂O₃, containing a Yb₂Si₂O₇ crystalline secondary phase, displayed no gross changes following oxidation. However, the thickness of the amorphous intergranular film was observed to have decreased by 20% from its initial thickness of 1.0 nm. The second Si₃N₄ material, sintered with 5 wt% Y₂O₃+1 wt% MgO, had a completely amorphous secondary phase. Devitrification of the secondary phase at multiple-grain junctions to -Y₂Si₂O₇ accompanied the outward diffusion of additive and impurity cations occurring in the residual amorphous intergranulàr films during oxidation. Substantial cavitation and intergranular phase depletion was observed at both multiple-grain junctions and two-grain boundaries. The equilibrium thickness of the amorphous intergranular film consequently decreased from 1.2 to 0.9 nm following oxidation. Purification of the amorphous intergranular films by diffusion of cations to the surface led to a reduction in impurity concentration, resulting in the observed thinning of grain-boundary films.     

Effect of different raw materials on the microwave dielectric properties of Ba2Ti9O20 ceramics

Dibarium nona titanate (Ba₂Ti₉O₂₀) dielectric ceramics have been prepared through solid-state route using raw materials of different origin. The dielectric resonator properties are studied in the microwave frequency region. The impurities in the raw materials drastically affect the microwave dielectric properties of Ba₂Ti₉O₂₀. Presence of TiO₂, which deteriorates the temperature coefficient of resonant frequency can be identified clearly by XRD in the range 40–45° of 2.     

Study on the formation and growth of potassium titanate whiskers

In this paper, K₂Ti₂O₅ single crystals, K₂Ti₄O₉ whiskers and K₂Ti₆O₁₃ whiskers are synthesized form the anatase-K₂CO₃ starting materials by the heating calcination and the corresponding morphologic and structural evolution of products are observed. After dissolving non-crystalline hydrosoluble products contained in sinters, the morphologic difference between the original sinter and the whiskers in it shows the sinter microstructure. The further analysis to crystal components in sinters and to the phase diagram proves that K₂Ti₄O₉ whiskers are firstly formed in K₂Ti₂O₅ crystals and there only exists the phase transformation from K₂Ti₄O₉ whiskers with layered crystal structure to~K₂Ti₆O₁₃ whiskers with tunnel crystal structure. The K₂O-rich liquid melt generated from K₂Ti₂O₅ crystals (/and K₂Ti₄O₉ whiskers) coats on the surface of K₂Ti₄O₉ whiskers (/and K₂Ti₆O₁₃ whiskers), which makes the sinters taking on the layer-by-layer structure (/and the bunch structure). The formation and growth of whiskers is dictated by the K₂O-rich non-crystalline hydrosoluble melt generated in phase transformations from solid to liquid-solid and its split effect induced by the orientation melting. A generalized liquid melt inducing mechanistic model explaining the formation and growth of potassium titanate whiskers was proposed.     

Compositional effects on densification and microstructural evolution of bismuth titanate

The effects of small compositional variations on the densification and microstructural evolution of bismuth titanate (Bi₄Ti₃O₁₂) powder compacts were investigated during sintering and during hot forging. For a nominally stoichiometric Bi₄Ti₃O₁₂ composition, sintering commenced at 870°C, leading to a relatively dense microstructure (relative density >97% of the theoretical value) with randomly aligned elongated grains after 1 h at 1100°C. Small additions (1 weight percent) of Bi₂O₃ or TiO₂ to the nominally stoichiometric Bi₄Ti₃O₁₂ composition shifted the onset of sintering to lower or higher temperatures, respectively, but did not significantly alter the final density. Hot forging produced a microstructure of aligned, elongated grains. The small compositional variations did not seriously influence the ability to develop the elongated grain alignment. However, subsequent annealing of the hot forged materials produced significant changes in the aligned grain microstructure. The elongated grain alignment in the nominally stoichiometric Bi₄Ti₃O₁₂ composition was destroyed during subsequent annealing for less than 2 h at 1100°C.     

Preparation and electrical properties of monophase cubic spinel,Mn1.5Co0.95Ni0.55O4, derived from rock salt type oxide

The purpose of this study was to prepare a sintered body consisting of monophase cubic spinel type oxide, Mn_1.5Co_0.95Ni_0.55O₄, and to evaluate its electrical properties. It was found that cooling from 1400 to 1000 °C in nitrogen did not affect the preservation of the sintered rock salt type oxide formed at 1400 °C. A crack free sintered body of monophase cubic spinel may be obtained by heat treatment at 1000 °C in air, using a specimen cooled from 1400 °C at a rate of 500 °C min ^–1. A heat treatment time in air at 1000 °C of more than 48 h was required to convert the rock salt type structure into a perfect cubic spinel structure. The electrical conductivity, , of the sintered cubic spinel oxide synthesized in this work was found to be stable at 100 and 200 °C in air and at 100, 200 and 300 °C in nitrogen. The sintered spinel oxide was a p-type semiconductor, based on small polaron hopping conduction. The intrinsic hole concentration, n, was estimated to be constant, with a value of 1.6–1.8×10²⁸m^–3. The mobility, , increased exponentially with increasing annealing temperature in both atmospheres, suggesting that the change in is dependent on .     

Effect of TiO2 addition on oxidation behavior of sintered bodies composed of Si3N4-Y2O3-Al2O3-AlN

The effect of TiO₂ content on the oxidation of sintered bodies from the conventional Si₃N₄-Y₂O₃-Al₂O₃-AlN system was investigated. Sintered specimens composed of Si₃N₄, Y₂O₃, Al₂O₃, and AlN, with a ratio of 100 : 5 : 3 : 3 wt% and containing TiO₂ in the range of 0 to 5 wt% to Si₃N₄, were fabricated at 1775 °C for 4 h at 0.5 MPa of N₂. Oxidation at 1200 to 1400 °C for a maximum of 100 h was performed in atmospheres of dry and wet air flows. The relation between weight gain and oxidation time was confirmed to obey the parabolic law. The activation energies decreased with TiO₂ content. In the phases present in the specimens oxidized at 1300 °C for 100 h in dry air, Y₃Al₅O₁₂ and TiN, which had existed before oxidation, disappeared. Alpha-cristobalite and Y₂O₃·2TiO₂ (Y2T) appeared in their place and increased with increasing TiO₂ content. In those oxidized at 1400 °C, -cristobalite was dominant and very small amounts of Y₂O₃·2SiO₂ and Y2T were contained. There was a tendency for more -cristobalite to form in oxidation in wet air than in dry air. Therefore, moisture was confirmed to affect the crystallization of SiO₂ formed during oxidation. Judging from the lower activation energy, the crystallization, and the pores formation, we concluded that the addition of TiO₂ decreases oxidation resistance.     

Fabrication of fine-grain piezoelectric ceramics using reactive calcination

Fine-grain piezoelectric ceramics with the formula Pb(Zr_0.53Ti_0.47)O₃(PZT) were prepared by a reactive calcination process. Using conventional materials and processing techniques, highly reactive powders of PZT were achieved by calcining to or near the point of maximum volume expansion, whereby associated morphological changes resulted in highly reactive powder. Upon milling, powders <0.3 m were readily obtained allowing densification at temperatures <1000°C. The B-site precursor method, whereby the ZrO₂ and TiO₂ oxides were pre-reacted prior to reaction with PbO, further enhanced reactivity by eliminating intermediate reactions and subsequent phase(s) which can hinder densification and overall homogeneity. Highly dense piezoceramics with grain sizes 1 to 2 m exhibiting dielectric and piezoelectric characteristics comparable to conventionally prepared large grain size materials were obtained.     

Ageing effect on microstructural and mechanical properties of mullite-ZrO2-TiO2 composites

The microstructural and mechanical properties of mullite-zirconia composites with TiO₂ (0.25 and 1.0 mol) additions have been studied, after ageing the samples over a wide temperature range (1000 to 1500° C) for long periods of time (100 to 200 h). In the sample with 0.25 mol TiO₂ addition, changes in mullite composition and in the solid state compatibility at temperatures below 1450° C were detected. In the sample containing 1 mol TiO₂, decomposition of Al₂TiO₅ occurs atT1200° C. Both compositions exhibit no increment in zirconia average grain size during ageing and, concomitantly, there is no strength degradation until higher temperatures (>1400° C) are reached, which become more drastic when Al₂Ti₅ is present.     

Effects of binary additives B2O3–Y2O3 on the microstructure and thermal conductivity of aluminum nitride ceramics

Aluminum nitride (AIN) ceramics, with binary additives B₂O₃-Y₂O₃, were sintered at temperatures from 1700 to 1850 °C. The microstructure and sintering characteristics were studied by XRD, HREM, SEM and TEM/EDS, which showed that Y₂O₃ gave different yttrium aluminates through the reaction with Al₂O₃ under different conditions. With the increase of sintering temperature, the yttrium-to-aluminum atomic ratio Y/Al decreased in the secondary phases of the sintered bodies. It was discovered that B₂O₃ could dissolve in the yttrium aluminates, forming some ordered structure with a superlattice. After sintering at 1850 °C for 4 h, a specimen with a fine microstructure and a thermal conductivity of 190 Wm^–1K^–1 was obtained.     

Preparation and photoconduction of rapidly quenched films in the Bi2O3-TiO2 system

The rapidly quenched films in the Bi₂O₃-TiO₂ system (0 to 60% TiO₂) were prepared using a twin-roller type apparatus. The films precipitated Bi₂O₃ solid solutions of different types in the composition ranges, with TiO₂ contents of 0 to 5, 7.5 to 10 and 12.5 to 40%, respectively. The first solid solution had a tetragonal structure of -form. The second, though also crystallized in the tetragonal structure, adopted a disordered modification of the -form. The third solid solution was -form (defect fluorite structure). The formation of amorphous phase commenced in the composition with 30% TiO₂, and the films became completely amorphous beyond 50% TiO₂. The quenched films showed a certain instability to decompose or transform into the different phase assemblage by annealing at higher temperatures (about 400 to 500° C, except 260° C for the pure Bi₂O₃ film). The quenched films were also characterized by a high photoconductivity. The photoconduction mechanism was suggested to be associated with a structural imperfection of Bi₂O₃ accompanied by a certain amount of pentavalent bismuth ion.     

Characterisation of amorphous silica in air-oxidised Ti3SiC2 at 500–1000 °C using secondary-ion mass spectrometry,nuclear magnetic resonance and transmission electron microscopy

In this paper we have described the use of secondary-ion mass spectrometry (SIMS), solid state ²⁹Si magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM) to detect the existence of amorphous silica in Ti₃SiC₂ oxidised at 500–1000 °C. The formation of amorphous SiO₂ and growth of crystalline TiO₂ with temperature was monitored using dynamic SIMS and synchrotron radiation diffraction. A duplex structure with an outer TiO₂-rich layer and an inner mixed layer of SiO₂ and TiO₂ was observed. Results of NMR and TEM verified for the first time the direct evidence of amorphous silica formation during the oxidation of Ti₃SiC₂ at the temperature range 500–1000 °C.     

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.     

Microscale meshes of Ti3O5 nano- and microfibers prepared via annealing of C-doped TiO2 thin films

A new technique to produce microscale Ti₃O₅ nano- and microfiber meshes is proposed. When a 3 wt% carbon-doped TiO₂ film on Si(1 0 0) was annealed at 1000 °C in wet nitrogen (0.8%H₂O), the amorphous TiO₂ phase gave rise to crystalline phases of λ-Ti₃O₅ (75%) and rutile + trace of TiO_2−xC_x (25%). From Raman and FTIR Spectroscopy results, it was concluded that rutile is formed at the inner layer located at the interface between the mesh and the Si that was located away from the surface such that the meshes of nano- and microfibers are predominantly composed of Ti₃O₅ grown from the reaction of rutile with Si to form Ti₃O₅ and SiO₂. On the other hand, it was noteworthy that the microscale mesh of nano- and microfibers showed increased photoluminescence compared with amorphous TiO₂. The PL spectrum which had a broad band in the visible spectrum, fitted as three broad Gaussian distributions centered at 571.6 nm (∼2.2 eV), 623.0 nm (∼2.0 eV) and 661.9 nm (∼1.9 eV).     

TiO2(B) a new form of titanium dioxide and the potassium octatitanate K2Ti8O17

TiO₂(B) a new form of titanium dioxide, the octatitanate K₂Ti₈O₁₇ and intergrowths between K₂ Ti₄ O₉ and K₂ Ti₈ O₁₇ have been prepared by hydrolysis of K₂ Ti₄ O₉ followed by heating at 500°C. TiO₂ (B) and K₂Ti₈ O₁₇ have the host covalent framework of the bronzes Na_x TiO₂ and K₃Ti₈O₁₇ respectively.     

Microstructural evolution of potassium titanate whiskers during the synthesis by the calcination and slow-cooling method

K₂Ti₆O₁₃ whisker was synthesized by the calcination and slow-cooling method, and the microstructural evolution of whiskers, and the relations between the phase and the layer structure, were investigated. The amount of K₂O in the starting powder mixture played a key role in forming a layer structure, which can be obtained by addition of excess K₂O. During calcination, rod-like K₂Ti₆O₁₃ particles with a layer structure were formed by reactions between K₂O and TiO₂. During slow-cooling, the K₂O-rich liquid phase reacted with K₂Ti₆O₁₃ layers near the boundary and k₂Ti₄O₉ whiskers were produced by the splitting of layers within rod-like k₂T₆O₁₃ particles. k₂Ti₄O₉ whiskers were reformed into k₂T₆O₁₃ whiskers by treatment in boiling water and reheating. The synthesized k₂T₆O₁₃ whiskers had a clean surface and a length exceeding 100 m.