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1.
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. 相似文献
2.
Chang Hyunju Lee J. D. Rodrigues R. P. Ellis D. E. Dravid V. P. 《Journal of Materials Synthesis and Processing》1998,6(5):323-328
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. 相似文献
3.
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 Nd 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. 相似文献
4.
Antolii Belous Oleg V’yunov Maya Glinchuk Valentin Laguta Darko Makovez 《Journal of Materials Science》2008,43(9):3320-3326
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.
相似文献
Oleg V’yunovEmail: |
5.
B. Shri Prakash K. B. R. Varma 《Journal of Materials Science: Materials in Electronics》2006,17(11):899-907
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. 相似文献
6.
Shri Pal Prem Chand S. D. Pandey 《Journal of Materials Science: Materials in Electronics》1991,2(2):89-93
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. 相似文献
7.
Zunjie Wei Hongwei Wang Yunyue Jin Hu Zhang Songyan Zeng 《Journal of Materials Science》2002,37(9):1809-1812
TiAl alloys with higher C additions were fabricated using a XDTM 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. 相似文献
8.
A. J. Bourdillon S. J. Bull P. J. Burnett T. F. Page 《Journal of Materials Science》1986,21(5):1547-1552
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. 相似文献
9.
The effects of zirconium on (Ba, Sr)TiO3 solid solution are studied by preparing (Ba0.75Sr0.25) (Zry Ti1-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 (Ba1-xSrx (ZryTi1-y)O3 system can be estimated byT
c = 125 – 270x – 350y (°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. 相似文献
10.
Longhai Yang Xiaolian Chao Lingling Wei Nan Zhao Zupei Yang 《Journal of Materials Science: Materials in Electronics》2016,27(3):2221-2227
Na0.65Bi0.45Cu3Ti4O12 ceramics were successful prepared by the conventional solid-state reaction technique. Compared to Na0.50Bi0.50Cu3Ti4O12 (NBCTO), the composition of Na0.65Bi0.45Cu3Ti4O12 was designed in terms of changing the Na/Bi ratio. Colossal dielectric permittivity of ~1.2 × 104 at 1 kHz was obtained in Na0.65Bi0.45Cu3Ti4O12 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. 相似文献
11.
The real part of the dielectric constant () and the dielectric loss angle (tan ) as well as the ac conductivity of ferrite Mg1+x
Ti
x
Nd
y
Fe2–2x–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. 相似文献
12.
J. Cho C.-M. Wang H. M. Chan J. M. Rickman M. P. Harmer 《Journal of Materials Science》2002,37(1):59-64
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. 相似文献
13.
G. S. A. M. Theunissen A. J. A. Winnubst A. J. Burggraaf 《Journal of Materials Science》1992,27(18):5057-5066
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% YO1.5 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. 相似文献
14.
Ekaphan Swatsitang Krissana Prompa Thanin Putjuso 《Journal of Materials Science: Materials in Electronics》2018,29(15):12639-12651
In this work, the nominal CaCu3?xMgxTi4.2O12 (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 (Rgb) caused by the very high density of grain, resulting from the substitution of small ionic radius Mg2+ dopants in the structure. In addition, CaCu3?xMgxTi4.2O12 ceramics displayed non-linear characteristics with the significant enhancements of a non-linear coefficient (α) and a breakdown field (Eb) due to Mg2+doping. The high values of ε′ (14012), α (13.64) and Eb (5977.02 V/cm) with very low tanδ value (0.009) were obtained in a CaCu2.90Mg0.10Ti4.2O12 ceramic sintered at 1100 °C for 8 h. 相似文献
15.
The dielectric properties and electrical response of grain boundaries of Na1/2La1/2Cu3Ti4O12 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 Na1/2La1/2Cu3Ti4O12 ceramics. Through analyses using complex impedance and electric modulus, it was found that the dielectric properties of Na1/2La1/2Cu3Ti4O12 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 Na1/2La1/2Cu3Ti4O12 ceramics is due to the Schottky effect. The giant low frequency dielectric response in Na1/2La1/2Cu3Ti4O12 ceramics is attributed to Maxwell–Wagner polarization at the grain boundaries. 相似文献
16.
Ying Yuan Shuren Zhang Changmin Li 《Journal of Materials Science: Materials in Electronics》2004,15(9):601-606
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. 相似文献
17.
Oxidation-induced microstructural changes in reduced yttrium-doped barium titanate (Ba1 – xYxTi1 – x4+Tix3+
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. 相似文献
18.
Prasit Thongbai Bundit Putasaeng Teerapon Yamwong Santi Maensiri 《Materials Research Bulletin》2012,47(9):2257-2263
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 Ca0.925Sm0.05Cu3Ti4O12 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. 相似文献
19.
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 MPam1/2 using a straightforward graphical procedure involving an empirical R-curve equation. This value is almost 30% higher than that of similar material (2.1 MPam1/2) with a larger grain size, suggesting that the fracture toughness of this material, which fractures intergranularly, might be grain-size dependent. 相似文献
20.
K. Y. Wang 《Journal of Materials Science》1995,30(21):5427-5432
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. 相似文献