Studies on semiconductive (Ba0.8Sr0.2) (Ti0.9Zr0.1)O3 ceramics

In order to produce semiconductive (Ba_0.8Sr_0.2) (Ti_0.9Zr0.1)O₃ ceramics (BSZT), providing low resistivity for boundary-layer capacitor applications, a controlled valency method and a controlled-atmosphere method were applied and studied. In the controlled-valency method, trivalent ions (La³⁺ Sb³⁺) and pentavalent ions (Nb⁵⁺, Sb⁵⁺, Ta⁵⁺) were doped into BSZT ceramics, while in the controlled-atmosphere method, samples were sintered in air and a reducing atmosphere. The doped BSZT ceramics sintered in the reducing atmosphere showed much lower resistivities and smaller temperature coefficient of resistivity (TCR) than those sintered in air, indicating that low partial pressure of oxygen will increase the solubility of the donor dopant and enhance the grain growth. In addition, a small negative TCR at low temperature, as well as a small positive TCR at higher temperature, are also observed for specimens fired in a reducing atmosphere. The former is attributed to the semiconductive grain and the latter to the small barrier layer formed at the grain boundary.     

TiCl3-doped Ba0.92Ca0.08TiO3 PTCR ceramics with low r.t. resistivity

The influence of TiCl₃ solution on the room temperature (r.t.) resistivity and electrical properties of Ba_0.92Ca_0.08TiO₃ PTCR ceramics was studied. The results indicate that the PTC effect can be improved significantly when an appropriate amount of TiCl₃ in solution is added to the original materials. Some of the doped Ti³⁺ ions segregate at grain boundaries behaving as acceptors by substituting for Ti site or valence varying (from Ti³⁺ to Ti⁴⁺). As a result, the surface charge density N _s) increases and the barrier height at grain boundaries () is enhanced.     

Sintering condition and PTCR characteristics of porous (Ba,Sr)(Ti,Sb)O3

We fabricated porous (Ba,Sr)(Ti,Sb)O₃ ceramics by adding potato-starch (1–20 wt %) and investigated the effects of sintering temperature (1300–1450 °C) and time (0.5–10 h) on the positive temperature coefficient of resistivity characteristics of the porous ceramics. The room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O₃ ceramics decreased with increasing sintering temperature, while that of the ceramics increased with increasing sintering time. For example, the room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O₃ ceramics for the samples sintered at 1300 °C and 1450 °C for 1 h is 6.8×10³ and 5.7×10² cm, respectively, while that of the ceramics is 6.5×10² and 1.3×10⁷ cm, respectively, for the samples sintered at 1350 °C for 0.5 h and 10 h. In order to investigate the reason for the decrease and increase of room-temperature electrical resistivity of the samples with increasing sintering temperature and time, the average grain size, porosity, donor concentration of grains (N _d), and electrical barrier height of grain boundaries () of the samples are discussed.     

Structural and Electrical Properties of Bismuth Ferrite Ceramics Sintered in Different Atmospheres

Bismuth ferrite (BiFeO₃) ceramics were synthesized by the solid-state reaction method followed by rapid liquid phase sintering. The effect of sintering atmosphere (N₂, air and O₂) on the structure and electrical properties of BiFeO₃ multiferroic ceramics were investigated. XRD analysis revealed that N₂ sintering was effective in reducing impurity phases and improving the crystallization behavior. XPS analysis showed that fewer Fe²⁺ ions but more oxygen vacancies were involved in the N₂ sintered ceramics. The SEM investigations suggested that the grain size of the BiFeO₃ ceramics sintered in nitrogen are larger than those sintered in air and O₂. Electrical measurements revealed that the ceramics sintered in N₂ showed lower leakage current, superior dielectric and ferroelectric properties.     

Chemical changes induced on a TiO2 surface by electron bombardment

We study the TiO₂ (Ti⁴⁺) chemical reduction induced by electron bombardment using Auger electron spectroscopy and factor analysis. We show that the electron irradiation of a TiO₂ sample is characterized by the appearance of a lower Ti oxidation state, Ti₂O₃ (Ti³⁺), followed by a further deposition of carbon, which is present inevitably in the environment even under ultra-high vacuum conditions. The appearance of C over the surface is found to be a complex mechanism which affects the reduction process through passivation of the electron-induced oxygen desorption and formation of titanium carbide. For very high irradiation doses, we also found that the chemical changes on the surface are stopped due to the deposition of carbon in a graphitic form.     

Effect of atmosphere on the PTCR characteristics of porous Ba(Ti,Sb)O3 ceramics produced by adding corn-starch

Porous Ba(Ti,Sb)O₃ ceramics were fabricated by adding corn-starch at 20 wt %. The effect of atmosphere on the PTCR characteristics of the porous Ba(Ti,Sb)O₃ ceramics and the role of oxygen on the grain boundaries in the PTCR characteristics of the Ba(Ti,Sb)O₃ ceramics were investigated. In air, O₂, N₂, and H₂ atmospheres, the electrical resistivity of Ba(Ti,Sb)O₃ ceramics below 150 °C was independent of atmosphere, while it was strongly dependent on atmosphere above 200 °C. The low electrical resistivity in reducing atmospheres was due to a decrease in potential barrier height, which originated from an increase in the number of electrons owing to the desorption of chemisorbed oxygen atoms at the grain boundaries. In a N₂ atmosphere, the electrical resistivity of Ba(Ti,Sb)O₃ ceramics during the cooling cycle was lower than that during the heating cycle, and then the electrical resistivity of the porous Ba(Ti,Sb)O₃ ceramics during subsequent heating and cooling cycles was increased again by exposure to an O₂ atmosphere.     

Effect of synthesized BaTiO3 doping on the dielectric properties of ultra temperature-stable ceramics

The high performance X9R ceramics could be sintered at as low as 1,120?°C by doping 3?mol% synthesized BaTiO₃ (SB) additives into the BaTiO₃-based ceramics, with a dielectric constant greater than 2,200 at 25?°C and dielectric loss lower than 1.7?%. The effects of SB additives on the microstructure and dielectric properties of BaTiO₃-based ceramics were investigated. The dielectric constant of BaTiO₃-based ceramics doped with 3?mol% SB was increased due to the promotion of the densification of ceramics. With SB content up to 4.5?mol%, Ti⁴⁺’s polarization was depressed, which resulted in the decrease of augmented dielectric constant at 25?°C. The partial solid solution was formed between Pb(Ti, Sn)O₃ and BaTiO₃, and the substitutions of Pb at A-sites and Sn at B-sites were existed. The strengthen of Ti–O bonds and higher Curie point of Pb(Ti_0.55Sn_0.45)O₃ was helpful to increased the Curie point of the ceramics effectively. Doped with SB additives, the volume of ferroelectric core was increased, and the sharp peak intensity at Curie point was increased accordingly. Capacitance temperature characteristics was improved attributed to the mutual effects of SB and Pb(Ti_0.55Sn_0.45)O₃. The formation of core–shell structure was sensitive to the sintering temperature, so the dielectric properties of ceramics were highly depended on the sintering temperature.     

Effect of nano-CeO2 on microstructure properties of TiC/TiN+TiCN-reinforced composite coating

TiC/TiN+TiCN-reinforced composite coatings were fabricated on Ti–6Al–4V alloy by laser cladding, which improved surface performance of the substrate. Nano-CeO₂ was able to suppress crystallization and growth of crystals in the laser-cladded coating to a certain extent. With the addition of proper content of nano-CeO₂, this coating exhibited fine microstructure. In this study, Al₃Ti+TiC/TiN+nano-CeO₂ laser-cladded coatings have been studied by means of X-ray diffraction and scanning electron microscope. X-ray diffraction results indicated that Al₃Ti+TiC/TiN+nano-CeO₂ laser-cladded coating consisted of Ti₃Al, TiC, TiN, Ti₂Al₂₀Ce, TiC_0·3N_0·7, Ce(CN)₃ and CeO₂, this phase constituent was beneficial in increasing microhardness and wear resistance of Ti–6Al–6V alloy.     

Effects of surface-treated cpTi and Ti6Al4V alloy on the initial attachment of human osteoblast cells

This study concerns the effect of simple surface treatments on the nature of the oxide layer, of commercially pure titanium (cpTi) and Ti6Al4V alloy substrates and their effect on human osteoblast cells (HOBS). After treatment the surfaces were analyzed by X-ray photoelectron spectroscopy (XPS) in order to identify the surface groups responsible for the cell attachment process. The assessment of cell attachment was monitored by the Alamar blue assay (AB), measuring cell activity, in three types of media: phosphate-buffered saline (PBS), serum containing and serum-free Dulbecco's modified Eagle's cell culture medium (SER+ and SERF respectively). XPS analysis of the treated surfaces revealed consistent peaks representative of TiO₂ on all surfaces and Ti⁰ and Ti₂O₃ on the non-heat-treated surfaces. The cell activity assays indicated that there were no significant differences in cellular activity caused by surface treatments, but the cellular activity compared between the three types of medium was greatest in the PBS over the initial stages of attachment.     

Identifying the Origin of Ti3+ Activity toward Enhanced Electrocatalytic N2 Reduction over TiO2 Nanoparticles Modulated by Mixed-Valent Copper

The ambient electrocatalytic N₂ reduction reaction (NRR) enabled by TiO₂ has attracted extensive recent attention. Previous studies suggest the formation of Ti³⁺ in TiO₂ can significantly improve the NRR activity, but it still remains unclear what kinds of Ti³⁺ are effective. Herein, it is demonstrated that mixed-valent Cu acts as an effective dopant to modulate the oxygen vacancy (V_O) concentration and Ti³⁺ formation, which markedly improves the electrocatalytic NRR performance. In 0.5 m LiClO₄, this electrocatalyst attains a high Faradic efficiency of 21.99% and a large NH₃ yield of 21.31 µg h⁻¹ mg_cat.⁻¹ at –0.55 V vs reversible hydrogen electrode, which even surpasses most reported Ti-based NRR electrocatalysts. Using density function theory calculations, it is evidenced that mixed-valent Cu ions modulate the TiO₂ (101) surface with multiple oxygen vacancies, which is beneficial for generating different Ti³⁺ 3d¹ defect states localized below the Fermi energy. N₂ activation and adsorption are effectively strengthened when Ti³⁺ 3d¹ defect states present the splitting of e_g and t_2g orbitals, which can be modulated by its coordination structure. The synergistic roles of the three ion pairs formed by the V_O defect, including Cu¹⁺–Ti⁴⁺, Ti³⁺–Ti⁴⁺ and Ti³⁺–Ti³⁺, are together responsible for the enhanced NRR performance.     

Fabrication of Monolithic Ti3SiC2 Ceramic Through Reactive Sintering of Ti/Si/2TiC

Fabrication of monolithic Ti₃SiC₂ has been investigated through the route of reactive sintering of Ti/Si/2TiC mixtures. Significant phase differences existed between the surface and the interior of as-synthesized products due to the evaporation of Si during the reaction process. The use of a 3Ti/SiC/C mixture as a powder bed could control the evaporation of Si and develop monolithic Ti₃SiC₂. A reaction model for the formation of Ti₃SiC₂ in the Ti/Si/2TiC system is discussed.On leave from     

Effect of nano-CeO2 on microstructure properties of TiC/TiN+nTi(CN) reinforced composite coating

TiC/TiN+TiCN reinforced composite coatings were fabricated on Ti?C6Al?C4V alloy by laser cladding, which improved surface performance of the substrate. Nano-CeO₂ was able to suppress crystallization and growth of the crystals in the laser-cladded coating to a certain extent. With the addition of proper content of nano-CeO₂, this coating exhibited fine microstructure. In this study, the Al₃Ti+TiC/TiN+nano-CeO₂ laser-cladded coatings were studied by means of X-ray diffraction and scanning electron microscope. The X-ray diffraction results indicated that the Al₃Ti+TiC/TiN+nano-CeO₂ laser-cladded coating consisted of Ti₃Al, TiC, TiN, Ti₂Al₂₀Ce, TiC_0·3N_0·7, Ce(CN)₃ and CeO₂, this phase constituent was beneficial to increase the microhardness and wear resistance of Ti?C6Al?C6V alloy.     

Fabrication of TiO<Subscript>x</Subscript>–Si photoanode and its energetic photoelectrochemical performance

Pristine Si is oxidized to insulative SiO₂ when it comes in contact with air and water. Covering it with a protection layer inhibits passivation of Si and significantly improves its photoelectrochemical performance. In this study, TiO_x with gradient change of oxygen stoichiometry ratio (TiO_x) was designed as a protection layer and fabricated via a chemical vapour deposition process in Ar flow under 400 °C for 1 min. The anaerobic atmosphere and short heating duration synergistically produced the ratio of O and Ti lower than two in the prepared film. XPS analysis suggested the existance of TiO₂ only at the surface of TiO_x film and Ti³⁺ and Ti²⁺ appeared successively with the increase of distance to the surface. The first advantage of lower-valence-state Ti and oxygen deficiency was to inhibit the oxidation of Si and to reduce electric resistance of the interface and the protection layer. The second advantage was to create a defect energy level under the conduction band of TiO₂ which provided the possibility for holes in the valence band of Si to be transferred to this defect level. This tunnel like transfer enhanced the photogenerated charge separation and redox ability of TiO_x–Si which brought a 3.25 folds enhancements in photocurrent density compared to that of stoichiometric TiO₂–Si at 0 V (SCE) under simulated sunlight. This study highly motivates further research on transparent and conductive protection layer of Si photoelectrode.     

Structure of a HAp/TiO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Nanocomposite Studied by Vibrational Spectroscopy Techniques

A hydroxyapatite/TiO_y nanocomposite material has been studied by X-ray diffraction, infrared spectroscopy, and Raman spectroscopy. The results demonstrate that annealing at 400°C changes the valence state of the titanium and leads to the formation of [Ti(OH)₂]²⁺ and [TiHPO₄]²⁺ surface groups and terminal carbonyls and partial heterovalent cation substitutions of Ti³⁺ and Ti⁴⁺ for Ca²⁺. As the annealing temperature is raised to 600°C, surface groups disappear. The amount of inflowing oxygen is insufficient for the complete oxidation of TiO_0.92 to TiO₂ in hydroxyapatite/TiO_0.92. The result is the formation of an intermediate nanophase with the composition Ti_4.5O₅.     

Microstructural characterization of a rapidly solidified Al-Sr-Ti alloy

The microstructure of the melt-spun Al-7Sr-3Ti alloy has been characterized using X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The results show the microstructure of the melt-spun Al-7Sr-3Ti alloy is composed of the equilibrium α-Al, Al₄Sr, Al₃Ti and metastable Al₂₃Ti₉, different from that of the ingot-like alloy comprising α-Al, Al₄Sr and Al₃Ti. Moreover, the amount of Al₃Ti is much less than that of Al₄Sr and metastable Al₂₃Ti₉ in the melt-spun alloy. The melting temperature of primary phases and enthalpies of fusion for the melt-spun Al-7Sr-3Ti alloy are lower than those for the ingot-like alloy. The formation mechanism of the microstructure of the melt-spun alloy has also been discussed.     

Proton transport and structural relations in hydroxyl-bearing BaTiO3 and its doped compositions synthesised by wet-chemical methods

Hydrothermally synthesised powders of BaTiO₃ and its Fe- or Nd-doped analogues contain hydroxyl groups in the lattice substitutional to oxide ion, as confirmed from TGA/DTA, IR spectral analysis of D₂O-treated powders, EGA-MS, the contraction in lattice constant with heat treatment by XRD and surface examination by XPS. Electrical resistivity measurements were carried out on the pellets from 298 to 1000 K by ac impedance spectroscopy and dc methods in dry or moist air and 8% H₂+Ar environments. The electrical conductivity observed for unsintered pellets between 298 and 500 K, is in the range of 10⁻³ to 10⁻⁷ S/cm and can be attributed to the extrinsic hydroxyls in BaTiO₃. The acceptor-doped composition, BaTi_0.9Fe_0.1O_3−δ: 2δ(OH) exhibits higher electrical conductivity than BaTiO₃ or the donor-doped Ba_0.9Nd_0.1TiO_3−δ: 2δ(OH) in moist air. The hydrothermally prepared powders heat treated below 1000 K having cubic symmetry at room temperature, possess higher proton conductivity and reabsorption capability for hydroxyls on exposure to moisture than the powders sintered at 1673 K (tetragonal symmetry). The conductivity at 298-500 K is due to the mobility of proton along OHO octahedra in the perovskite lattice. The conduction at 550-1000 K is a combined effect of proton as well as oxygen vacancy mobility in BaTiO₃ and Ba_0.9Nd_0.1TiO₃; electron hole (Ti⁴⁺, Ti³⁺, Fe³⁺, Fe²⁺) participation is the additional contribution in acceptor-doped composition in this temperature range.     

Preparation of highly ordered Fe-SBA-1 and Ti-SBA-1 cubic mesoporous silica via sol-gel processing of silatrane

Silatrane prepared from fumed silica and triethanolamine (TEA) was used as a precursor for the sol-gel synthesis of M-SBA-1 (M = Fe and Ti) at room temperature using cetyltrimethylammonium bromide as a template, and dilute solutions of ferric chloride and titanium glycolate as metal sources. Powder X-ray diffraction (XRD) showed the mesoporous materials to be well-ordered cubic structures, while N₂ adsorption/desorption measurements yielded high surface areas. Diffuse reflectance UV-visible spectroscopy demonstrated that iron (Fe³⁺) and titanium (Ti⁴⁺) were incorporated in the framework of the calcined materials to loadings of 6 wt.% Fe and 10 wt.% Ti without perturbing the ordered mesoporous structure.     

Effect of Y3+ addition on the emitting level in a SrTiO3:Pr3+ phosphor

The effect of Y₃₊ addition to a SrTiO₃:Pr₃₊ phosphor on photoluminescence was investigated. The light emitting levels of Pr₃₊ were found to vary with the amount of Y₃₊ and ratios of Sr to Ti. Red emission prevails in the case of Sr(Ti_1-1Y_xO₃:Pr₃₊ whereas green and red emissions are observed from (Sr_1-xY_xTiO₃:Pr₃₊. This phenomenon can be attributed to the ³P₀¹D₂ nonradiative transition via the 4f5d band. The data herein suggests that the 4f5d band of (Sr_1-xY_x)TiO₃:Pr₃₊ is located at a higher level that of Sr(Ti_1-xY_x)O₃:Pr₃₊. Using this model the nonradiative transition, which is partially suppressed, and green emission can be explained.     

Microstructure and ferroelectric properties of soft-doped sol–gel derived PZT/SKN fibers

Sol–gel derived PZT/SKN fibers with a final composition of 0.98(PbO)_1+z (Zr_0.53Ti_0.47)O₂-0.02Sr(K_0.25Nb_0.75)O₃ and a PbO content of z = +0.04 and +0.14 in the spinning sol were sintered at different temperatures. Fiber stoichiometry, phase content and microstructure as well as the physical properties of the fibers were investigated. A fully densified microstructure independent from the initial PbO content was obtained for fibers sintered at 950 °C or higher. Enhanced porosity was found only for fibers sintered at 900 °C. The densification of the fiber batches at the lower temperatures is dominated by solid state sintering, while liquid phase sintering is promoted by sintering at temperatures above 900 °C. TEM investigations confirmed the homogeneous nature of the PZT/SKN fibers devoid of compositional gradients. Typical dielectric permittivity is in the range of 650–1000. The ferroelectric hysteresis loops are well pronounced showing typical soft-doped behavior with remanent polarization values in the range of 15 and 34 μC/cm² and coercivities of about 1.9 V/μm.     

Microstructure and thermal expansion of Ti3SiC2

Abstract

A dense, bulk ceramic of Ti₃SiC₂ containing impurities of TiC and Ti₅Si₃ was fabricated by hot pressing elemental powders of Ti, Si, and C. X-ray diffractometry, scanning electron microscopy, and transmission electron microscopy were used to determine the crystalline phases and observe the microstructure of sintered compacts, respectively. Ti₃SiC₂ exhibits anisotropic grain growth and the size of exaggerated grain is ~25 μm in length. The average coefficient of thermal expansion of Ti₃SiC₂ was measured to be 9.29 × 10^-6 K^-1 in the temperature range 25-1400°C.