Antiphase domain morphologies in Ti3Al-Si alloys

Ti₃Al based alloys with additions of Si, V, and V + Si were prepared in ingot form by argon arc co-melting and in the form of rapidly solidified ribbons by chill block melt spinning. Binary Ti₃Al and ternary Ti₃Al-Si arc-melted ingots showed equiaxed antiphase boundaries (APBs) in the ₂ (DO₁₉) grain interiors with columnar APBs adjacent to the grain boundaries. In the melt spun ribbons, which undergo high cooling rates, three types of antiphase domain (APD) morphologies (fine, columnar, and coarse equiaxed APDs) were observed in any single ₂ grain. Addition of 15 at.%V to these alloys led to retention of the ordered (B2) phase with a relatively large antiphase domain size.     

Electronic Structure of Mn Acceptor Impurity Incorporated SrTiO3 Using Embedded Cluster Method

We have performed atomic-level first principle electronic structure calculations on doped grain boundaries (GB) in SrTiO₃. This was motivated by the electron holography experiments, which were able to quantify the electrostatic potential and the associated space charge distribution across the Mn-doped GB in this material. The embedded cluster Discrete Variational (DV)-X method was used to determine the charge and the densities of states for several idealized models of a single crystal and symmetrical tilt grain boundaries in SrTiO₃. Special attention was given to the role of Mn⁺² and Mn⁺³ acceptors substituting for Ti⁺⁴ resulting in charge segregation across the grain boundaries, which was shown in the electron holography experiments. We have found that Mn replacing Ti prefers to have valence charge around +2 and this picture agrees with the experimental observation of negative grain boundary charges in the GB core.     

Giant dielectrics from modified boundary layers in n-BaTiO<Subscript>3</Subscript> ceramics involving selective melting reactions of silver/glass composites at the grain boundaries

High permittivity ceramics with _eff > 10⁵ can be realized from semiconducting BaTiO₃ by the two-step processing namely, sintering the donor doped samples in static air followed by electroding with the fired-on silver/glass composites. Doping with Sb⁵⁺ and Bi³⁺ not only enhances the grain conductivity but also increases the grain size (10–60 m), when sintered at 1370 C in static air. The ceramic samples are electroded with the paste containing nanometer particles of silver dispersed in varied amounts of low melting (600–900 C) glass compositions PbO + Bi₂O₃ + B₂O₃ ± SiO₂ ± CuO. High permittivities are obtained for these capacitors stable over a wider range of temperature and over a broad frequency range. The grain boundary layer effect superimposed with the contributions from the barrier layers formed during electroding, related to ceramic microstructure is proposed to be responsible for the unusual high permittivity in semiconducting BaTiO₃. The energy dispersive X-ray analyses indicate selective melting reactions at the grain boundary layers with higher concentrations of the low melting oxides at the grain boundaries near to the electrodes. Impedance spectroscopy on BaTiO₃ ceramics demonstrates that they are electrically heterogeneous with insulating grain boundaries together with the ceramic/electrode interface acting as barrier layers. On the basis of the symmetrical Schottky-barrier model of the grain boundary region, the barrier height and donor concentration N_d of the grains were obtained by the modified 1/C²–V plot. These modified boundary layer capacitors having high field strength withstandability can be used in a wide range of frequencies.     

Redox processes at grain boundaries in barium titanate-based polycrystalline ferroelectrics semiconductors

Barium titanate, which is characterized by a positive temperature coefficient of resistance (PTCR), is widely used in practice. At the same time, it is unknown why only a small percentage of the introduced donor dopant takes part in the formation of PTCR effect, which phases appear at grain boundaries, how the introduced acceptor dopants affect the properties of grains. Elucidation of the above questions is of considerable scientific and practical interest. It has been shown that the phases Bа₆Ti₁₇O₄₀ and Y₂Ti₂O₇ precipitate on grains of barium titanate doped with donor dopant (yttrium). We identified paramagnetic impurities (iron, manganese, chromium) in starting reagents. These impurities can occupy titanium sites. Therefore, the part of the donor dopant that is spent on the charge exchange of acceptor dopants does not participate in the charge exchange of titanium Ti⁴⁺ → Ti³⁺, which is responsible for the appearance of PTCR effect in barium titanate. It has been found that an extra acceptor dopant (manganese) is distributed mainly at grain boundaries and in the grain outer layer. It has been shown that manganese ions introduced additionally (as acceptor dopants) increase the potential barrier at grain boundaries and form a high-resistance outer layer in PTCR ceramics. The resistance of grains, outer layers, and grain boundaries as a function of the manganese content has been investigated.

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Microstructural and dielectric properties of donor doped (La3+) CaCu3Ti4O12 ceramics

The effect of La³⁺ doping on Ca²⁺ sites in CaCu₃Ti₄O₁₂ (CCTO) was examined. Polycrystalline samples in the chemical formula Ca_(1-x)La_(2/3)x Cu₃Ti₄O₁₂ with x = 0, 0.5, 1 were synthesized via the conventional solid state reaction route. X-ray powder diffraction analysis confirmed the formation of the monophasic compounds and indicated the structure to be remaining cubic with a small increase in lattice parameter with increase in La³⁺ doping. The dielectric and impedance characteristics of Ca_(1-x)La_(2/3)x Cu₃Ti₄O₁₂ were studied in the 100 Hz–10 MHz frequency range at various temperatures (100–475 K). A remarkable decrease in grain size from 50 μm to 3–5 μm was observed on La³⁺ substitution. The dielectric constant of CaCu₃Ti₄O₁₂ decreased drastically on La³⁺ doping. The frequency and temperature responses of dielectric constant of La³⁺ doped samples were found to be similar to that of CaCu₃Ti₄O₁₂. The effects of La³⁺ doping on the electrical properties of CaCu₃Ti₄O₁₂ were probed using impedance spectroscopy. The conducting properties of grain decreased while that of the grain boundary increased on La³⁺ doping, resulting in a decrease of the internal barrier layer effect. A decrease in grain boundary capacitance and stable grain response in La³⁺ doped CCTO ceramics were unambiguously established by modulus spectra studies.     

Electrical conductivity and electron paramagnetic resonance investigations in manganese-doped polycrystalline Na2Ti3O7

D.c. electrical conductivity studies have been reported in Na₂Ti₃O₇ in the temperature range 370–750 K. Three distinct regions have been identified in the log(T) versus 1000/T plots, the lowest temperature region being proposed to be due to electronic hopping conduction involving loose electrons of Ti₃O ₇ ^2– groups, the intermediate region to associated extrinsic ionic conduction through dilated interlayer space and the highest temperature region to modified interlayer ionic conduction, the modification being affected by loose oxygens from Ti₃O ₇ ^2– groups. The room-temperature electron paramagnetic resonance (EPR) investigations have been interpreted in terms of manganese substitution in the lattice. For a lower percentage of doping the substitution of manganese ions occur as Mn³⁺ at Ti⁴⁺ sites, whereas for a higher percentage of doping, Mn²⁺ ions predominantly occupy the interlayer Na⁺ sites. The dilation of interlayer space has further been proposed to occur due to a Ti⁴⁺ substitution of manganese ions.     

Microstructures and mechanical properties of as-cast TiAl alloys with higher C additions

TiAl alloys with higher C additions were fabricated using a XD^TM method. The results investigated by XRD, OM and SEM indicated that in Ti-34Al-0.5C alloy, single phase TiC particles and particles having a core structure in which the TiC phase was coated by Ti₃AlC phase were formed and they uniformly distributed at the grain boundaries. However, when C content is more than 0.5%, the particles with a plate-shape were single phase Ti₂AlC and TiC phase coated by Ti₂AlC phase. These results suggested that in TiAl alloys with higher C additions, the primary is TiC, and the Ti₃AlC and Ti₂AlC result from peritectic reaction, L + TiC. The results measured by MPM show that with the increasing of C content, the microhardness both TiC, Ti₃AlC and Ti₂AlC is higher than that of the matrix Ti₃Al and TiAl. However, the elastic modulus of particle phases and matrix has little variation and no change tendency can be found with increasing C content.     

The charge state of titanium ions implanted into sapphire: An EXAFS investigation

X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) have been used to study the charge states and atomic environments of Ti⁺ ions implanted into a single crystal sapphire substrate. Fluorescence techniques have been shown capable of detecting signals from the implanted ions even though the implant density in the 100m thick specimen used is 0.02 at% (the implant profile all being within 0.2m of the surface). By comparison with TiO₂, Ti₂O₃ and TiO standards, XANES data suggest that the implant species environment is disordered. Further, EXAFS Fourier transforms show that the first shell radius of the implant species is between that of the TiO and Ti₂O₃ standards. Again, this indicates considerable structural disorder. After annealing the implanted specimen in air at 1150° C for 2 h, a shift towards the shell radius of the TiO₂ structure is observed together with a similar change in the XANES appearances. Our conclusion is that the initial titanium ion implant into sapphire exists in both the Ti²⁺ and Ti³⁺ charge states and is located in a range of sites in the radiation-damaged material. Annealing produces a shift in charge state towards Ti⁴⁺. Implications for the solid solution hardening effect of the implant in sapphire are discussed.     

Effects of zirconium on the structural and dielectric properties of (Ba,Sr)TiO3 solid solution

The effects of zirconium on (Ba, Sr)TiO₃ solid solution are studied by preparing (Ba_0.75Sr_0.25) (Zr_y Ti_1-y)O₃ (BSZT) ceramics with ranging from 0 to 0.15. With increased incorporation of zirconium the lattice parameters increase with decreasingc/a ratio, while a linear relationship is found between (a ² c)^1/3 values and the content of zirconium. The Curie temperature (T _c) is also lowered linearly with increasing . The Curie temperature of the (Ba_1-xSr_x (Zr_yTi_1-y)O₃ system can be estimated byT _c = 125 – 270x – 350_y (°C). Observations of microstructures also show that a small amount of zirconium promotes the grain growth, while a higher content will then inhibit it. Dielectric properties of BSZT dielectrics and BSZT-based boundary layer (BL) capacitors are also correlated to have similar temperature dependence, where the maximum dielectric constant at room temperature is found aty = 0.1.     

Dielectric responses of Na<Subscript>0.65</Subscript>Bi<Subscript>0.45</Subscript>Cu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics based on the composition design of changing the Na/Bi ratio

Na_0.65Bi_0.45Cu₃Ti₄O₁₂ ceramics were successful prepared by the conventional solid-state reaction technique. Compared to Na_0.50Bi_0.50Cu₃Ti₄O₁₂ (NBCTO), the composition of Na_0.65Bi_0.45Cu₃Ti₄O₁₂ was designed in terms of changing the Na/Bi ratio. Colossal dielectric permittivity of ~1.2 × 10⁴ at 1 kHz was obtained in Na_0.65Bi_0.45Cu₃Ti₄O₁₂ ceramics. Interestingly, three frequency dispersions were observed in the frequency dependence of dielectric constant measured at different temperatures. The investigation of electric modulus displayed that the giant low-frequency dielectric constant was attributed to Maxwell–Wagner polarization at the grain boundaries and the frequency dispersion in middle-frequency range was due to the grain polarization. Except grain response and grain boundaries response reflected by two semicircles in the impedance spectroscopy, another electrical response associated with nonzero high frequency intercept was found. The grain resistance Rg and grain boundaries resistance R _gb was ~600 Ω and 3.9 × 10⁵ Ω, respectively. The large intrinsic permittivity as high as ~700 was obtained. Furthermore, two dielectric anomalies observed in the temperature dependent of dielectric constant were discussed in detail. The results indicated change in the Na/Bi ratio had a significant effect on the electrical properties of NBCTO ceramics.     

Rate of heating and sintering temperature effect on the electrical properties of Nd ferrite

The real part of the dielectric constant () and the dielectric loss angle (tan ) as well as the ac conductivity of ferrite Mg_1+x Ti _x Nd _y Fe_2–2x–y O₄ 0.1 x 0.9 at fixed Nd concentration of 0.025 were measured at different temperatures as a function of frequencies. The variation of activation energy as a function of the applied frequency was reported. The obtained data were discussed on the basis of the valence exchange between (Fe³⁺, Fe²⁺), (Fe²⁺, Nd³⁺) and (Fe²⁺, Ti⁴⁺). Also the effect of sintering temperature and heating rate of preparation were discussed.     

A study of grain-boundary structure in rare-earth doped aluminas using an EBSD technique

Oversized rare-earth dopant ions such as Y³⁺, Nd³⁺, and La³⁺ segregate to grain boundaries and reduce the tensile creep rate of -Al₂O₃ by 2-3 orders of magnitude. It has been speculated that these dopant ions can modify the grain boundary structure in alumina by promoting the formation of special grain boundaries. If this were indeed the case, it would provide a possible explanation for the aforementioned creep rate retardation. In order to test this hypothesis, electron backscatter diffraction (EBSD) has been used to assess both the proportion of coincidence-site lattice boundaries, and the grain boundary misorientation distribution, in aluminas doped with various ions (Zr, Y, Nd, La, Nd/Zr). The results show that the grain boundary structure in alumina is not significantly altered by the addition of the above dopants, implying that the change in grain boundary chemistry is primarily responsible for the observed creep behavior.     

Surface and grain boundary analysis of doped zirconia ceramics studied by AES and XPS

The surface- and grain boundary composition of Y, Ce and Ti doped zirconia were studied by X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy/Scanning Auger Microscopy. The grain boundaries and free surfaces showed the same enrichment levels. After heat treatment 1000 C all yttria doped samples showed yttrium enrichment. In the ZrO₂-Y₂O₃ system the yttrium enrichment did not depend on the bulk concentration and amounted 30–34 mol% YO_1.5 in all cases. As a consequence the segregation factor increases with decreasing solute concentration in the bulk. The thickness of the segregation layer was about 2–4 nm. In the ternary Y doped systems yttrium is the main segregant. In ceria-doped tetragonal zirconia polycrystals (Ce-TZP) systems significant segregation of cerium starts atT1300C and is mainly attributed to Ce³⁺. In Y,Ti-TZP systems also strong segregation of Ti⁴⁺ occurs. The absolute value of the increase of the surface concentration in fine grained material is smaller than in coarse grained material. This is mainly due to depletion of the bulk.     

Very high thermal stability with excellent dielectric,and non-ohmics properties of Mg-doped CaCu<Subscript>3</Subscript>Ti<Subscript>4.2</Subscript>O<Subscript>12</Subscript> ceramics

In this work, the nominal CaCu_3?xMg_xTi_4.2O₁₂ (0.00, 0.05 and 0.10) ceramics were prepared by sintering pellets of their precursor powders obtained by a polymer pyrolysis solution method at 1100 °C for different sintering time of 8 and 12 h. Very low loss tangent (tanδ)?<?0.009–0.014 and giant dielectric constant (ε′) ～?1.1?×?10⁴–1.8?×?10⁴ with excellent temperature coefficient (Δε′) less than ±?15% in a temperature range of ??60 to 210 °C were achieved. These excellent performances suggested a potent application of the ceramics for high temperature X8R and X9R capacitors. It was found that tanδ values decreased with increasing Mg²⁺ dopants due to the increase of grain boundary resistance (R_gb) caused by the very high density of grain, resulting from the substitution of small ionic radius Mg²⁺ dopants in the structure. In addition, CaCu_3?xMg_xTi_4.2O₁₂ ceramics displayed non-linear characteristics with the significant enhancements of a non-linear coefficient (α) and a breakdown field (E_b) due to Mg²⁺doping. The high values of ε′ (14012), α (13.64) and E_b (5977.02 V/cm) with very low tanδ value (0.009) were obtained in a CaCu_2.90Mg_0.10Ti_4.2O₁₂ ceramic sintered at 1100 °C for 8 h.     

Dielectric properties and electrical response of grain boundary of Na1/2La1/2Cu3Ti4O12 ceramics

The dielectric properties and electrical response of grain boundaries of Na_1/2La_1/2Cu₃Ti₄O₁₂ ceramics were investigated as a function of temperature. High dielectric permittivity (6.1–8.7 × 10³) and low loss tangent (0.032–0.038 at 10 kHz) were observed in Na_1/2La_1/2Cu₃Ti₄O₁₂ ceramics. Through analyses using complex impedance and electric modulus, it was found that the dielectric properties of Na_1/2La_1/2Cu₃Ti₄O₁₂ ceramics are closely related to the electrical response of grain boundaries. The investigation of non-Ohmic characteristics at various temperatures suggests that the potential barrier at the grain boundaries of Na_1/2La_1/2Cu₃Ti₄O₁₂ ceramics is due to the Schottky effect. The giant low frequency dielectric response in Na_1/2La_1/2Cu₃Ti₄O₁₂ ceramics is attributed to Maxwell–Wagner polarization at the grain boundaries.     

The effect of doping process on microstructure and dielectric properties of BaTiO3-based X7R materials

The effect of doping process on the dielectric properties, sintering behavior and microstructure were investigated on the BaTiO₃–Nb₂O₅–Co₃O₄ ternary system ceramic. Temperature stable dielectric ceramics were obtained by different doping processes if only appropriate Nb⁵⁺+Co³⁺ amount and Nb⁵⁺/Co³⁺ ratio were adopted. The dielectric constant was enhanced to the largest extent by nanometer oxide doping and the temperature characteristic satisfied the X7R specification. Two kinds of grains were observed in all the samples: matrix grains (BaTiO₃) and the secondary phase grains (Ba₆Ti₁₇O₄₀) formed by the incorporation of Nb⁵⁺ and Co³⁺ into BaTiO₃ lattice and Ti⁴⁺ segregation. The matrix grains were about 1 m in diameter and showed little grain growth with increasing temperature in all the doped samples, whereas the sizes of the secondary phase grains were strongly dependent on the doping process. The secondary phase formed liquid phase during firing, but the liquid phase contributed little to the densification of ceramics.     

Oxidation of reduced Y-doped semiconducting barium titanate ceramics

Oxidation-induced microstructural changes in reduced yttrium-doped barium titanate (Ba_{1 – x}Y_xTi_{1 – x}⁴⁺Ti_x³⁺ O₃) are studied using samples sintered in a reducing atmosphere ( = 10^–4Pa) and then oxidized in air at 1150 and 1350°C. The results indicate that oxidation leads to precipitation of Ba₆Ti₁₇O₄₀ and, at relatively high doping levels, Y₂Ti₂O₇. These phases increase the electrical resistance of the outer layer of the grains in the ceramics.Translated from Neorganicheskie Materialy, Vol. 41, No. 1, 2005, pp. 93–100.Original Russian Text Copyright © 2005 by Vyunov, Kovalenko, Belous, Belyakov.     

Modified giant dielectric properties of samarium doped CaCu3Ti4O12 ceramics

Effects of Sm³⁺ substitution on the microstructure and dielectric properties of CaCu₃Ti₄O₁₂ ceramics were investigated. The grain size of CaCu₃Ti₄O₁₂ ceramics was greatly decreased by doping with Sm³⁺, resulting from the ability of Sm³⁺ to inhibit the grain growth rate. This result can cause a decrease in the dielectric constant (?′) and loss tangent (tan δ) of CaCu₃Ti₄O₁₂ ceramics. Interestingly, high dielectric permittivity (?′ ～ 10,863) and low loss tangent (tan δ ～ 0.043 at 20 °C and 1 kHz) were observed in the Ca_0.925Sm_0.05Cu₃Ti₄O₁₂ ceramic. Nonlinear electrical properties of CaCu₃Ti₄O₁₂ ceramics were modified by doping with Sm³⁺. The dielectric relaxation behavior of Sm-doped CaCu₃Ti₄O₁₂ ceramics can be well ascribed based on the internal barrier layer capacitor model of Schottky barriers at the grain boundaries.     

A small-specimen investigation of the fracture toughness of Ti5Si3

The fracture toughness of the refractory hardmetal Ti₅Si₃, with a grain size between 5 and 6 m, was measured using the controlled-flaw method in conjunction with the miniaturized disc-bend test. The specimens used in these experiments were 3 mm diameter and varied in thickness from 150–450 m. They were indented using a Vickers pyramid indentor to indention loads varying from 2.9–79.2 N. Indentation cracking was experienced at all indentation loads, and R-curve behaviour was exhibited. The fracture toughness was determined to be 2.69 ± 0.21 MPam^1/2 using a straightforward graphical procedure involving an empirical R-curve equation. This value is almost 30% higher than that of similar material (2.1 MPam^1/2) with a larger grain size, suggesting that the fracture toughness of this material, which fractures intergranularly, might be grain-size dependent.     

Thermal behaviours of mechanically alloyed Ti50Al50 in a nitrogen atmosphere

Thermal behaviours of mechanical alloyed Ti₅₀Al₅₀ powders in a nitrogen atmosphere are investigated in this paper. X-ray diffraction and differential thermal analysis were used to determine their characteristics. At the initial milling stage, large amounts of defects were introduced and the grain sizes were gradually refined. The enthalpy changes of formation of -TiAl and ₂-Ti₃Al were decreased with increasing milling times. No obvious dissolution of nitrogen into the powder particles occurred at this stage. With increasing milling time, an amorphous phase containing nitrogen gradually occurred. The amorphous phase and small amounts of Ti solid solution were obtained after milling for 30 h in a N₂ atmosphere. The thermal process included two stages. Firstly, the amorphous phase crystallized at low temperature and resulted in the formation of a nanophase; secondly, the grain growth of this nanocrystalline phase occurred at high temperature. The annealing products are different for the milling products obtained at the initial stage (-TiAl + ₂-Ti₃Al) and final stage (-TiAl + Ti₂AlN), which is attributed to the different nitrogen contents in the milled products. The activation energies for the crystallization and grain growth are 251.9 and 296.9 kJ mol^–1, respectively.