Uchida Hiroshi

Thin films of Nd3+-substituted bismuth titanate, (Bi4.00-y,Nd-y)Ti3.00O12 (BNT), Nd3+/V5+-cosubstituted bismuth titanate, (Bi4.00-y,Nd-y)(Ti3.00-xVx)O-12 (BNTV), and La3+-substituted bismuth titanate, (Bi-3.25,La-0.75)Ti3.00O12 (BLT) were fabricated on the (111)Pt/Ti/SiO2/(100)Si substrates by a chemical solution deposition technique. These films possessed random-oriented polycrystalline structure. The BNT film had larger remnant polarization (P-r) than the BLT film; P-r and coercive field (E-c) of the BNT film with y=0.50 were 32 muC/cm2 and 126 kV/cm, respectively. Furthermore, V5+ substitution improved the P-r value of the BNT film up to 37 muC/cm2 (BNTV film; y=0.50, x=0.02), while the BNTV film had an E-c value of approximately 119 kV/cm which was similar to that of the BNT film. Ferroelectric properties of the Pb-free polycrystalline BNT and BNTV films are comparable with those of conventional Pb-based ferroelectric films like a lead zirconate titanate. (C) 2002 American Institute of Physics.

By molecular dynamics(MD) simulation crystal growth from supercooled MgO melt has been studied, whose crystal has an NaCl type. The model was similar to that previously applied for NaCl, Crystal growth of the melt on the crystal layers has been studied; 1400 or 1800 ions were disposed in an MD basic cell. The Gilbert-Ida-type pair potentials whose parameters were presented by Kawamura and Akarnatsu were employed, with which the melting point simulated in a three-dimensional space was 2670 K, while the experimental m.p. is 3099 K. The cases were studied that the initial interfaces between crystalline and melt were the (100), (110) and (111) planes; crystal grew in the [100], <100> and [111] directions, respectively. For the (100) system the growth rate was studied at various temperatures and found to possess a maximum(ca. 90ms(-1))at about 2300K. The mean square displacements on the x-y plane and in the z-direction were calculated. The overall profile of the growth feature of MgO is very similar to that of NaCl. (C) 2002 Elsevier Science B.V. All rights reserved.

The effect of rare-earth oxide addition (1-5 mass% of Ln(2)O(3) addition; Ln = Y, La, Nd, Sm, Gd, Dy, Er, and Yb) on the properties of hot-pressed (1550 degreesC, 90 min, 31 MPa) magnesium silicon nitride (MgSiN2) compact (ceramic) has been investigated. The role of rare-earth oxide addition on the relative density was classified as follows: (i) Positive effect (Y2O3, La2O3, and Yb2O3 addition), (ii) no appreciable effect (Nd2O3 and Er2O3 addition), and (iii) negative effect (Sm2O3, Gd2O3, and Dy2O3 addition). The grain sizes of the MgSiN2 ceramics with rare-earth oxide addition were almost comparable to or slightly smaller than those of the pure MgSiN2 ceramic. The average Vickers hardness of the MgSiN2 ceramic with 1 mass% of Y2O3 addition showed the highest value (21.5 GPa) amongst the MgSiN2 ceramics with rare-earth oxide addition. The thermal conductivity of the MgSiN2 ceramic had a maximum for the case of 1 mass% of Yb2O3 addition (26.6 W(.)m(-1.)K(-1)) and was believed to be the highest value so far reported for MgSiN2 ceramic. It was concluded that the relative density and Vickers hardness were best enhanced through the use of Y2O3 addition, whereas the addition of Yb2O3 was most suitable for enhancing the thermal conductivity. (C) 2002 Elsevier Science Ltd. All rights reserved.

Bismuth titanate (Bi4Ti3O12; BIT)-based ferroelectric materials are proposed from the view of the "Site-engineering", where the Bi-site ions are substituted by lanthanoid ions (La3+ and Nd3+) and Ti-site ions by other ions with higher charge valence (V5+). In the present study, influences of vanadium (V) - substitution for (Bi,M)(4)Ti3O12 thin films [M = lanthanoid] on the ferroelectric properties are evaluated. V-substituted (Bi,M)(4)Ti3O12 films have been fabricated using a chemical solution deposition (CSD) technique on the (111)Pt/Ti/SiO2/(100)Si substrate. Remnant polarization of (Bi,La)(4)Ti3O12 and (Bi,Nd)(4)Ti3O12 films has been improved by the V-substitution independent of the coercive field. The processing temperature of BLT and BNT films could also be lowered by the V-substitution.

Relationships among in-plane residual stress and some dielectric properties (remanent polarization, coercive field, dielectric permittivity, etc.) were investigated on ferroelectric thin films. Epitaxial PbTiO_3 thin films were deposited on Pt/(100)SrTiO_3 substrate by MOCVD. Residual stresses of the films were determined by a modified sin^2ψ method. The external stress in this study could be varied from +2.0 GPa up to +6.0 GPa. We first observed the change of the dielectric properties together with the tensile stress application in actual experiments; decrease of the remanent polarization from 11 μC・cm^&lt-2> down to 3 μC・cm^<-2> and of the coercive field form 115 kV・cm^<-1> down to 95 kV・cm^<-1>. The Curie temperature of the film was decreased from 450℃ to 437℃ with increasing the residual stress, while the dielectric permittivity at room temperature was measured to be approximately 210, irrespectively of the residual stress. Further, the tensile stress caused deformation of PbTiO_3 crystal lattice, which was described as the decrease of the c/a ratio. We conclude that the residual stress varied the dielectric properties by deforming the crystal lattice.

An analytical technique to determine residual stress in eqitaxial thin film by asymmetric X-ray diffraction (XRD) was studied. The residual stresses of PbTiO_3 films were determied by XRD technique and displace measurement technique. Measurement results by these techniques were compared with each other to consider the respective advantages of each technique. The stresses measured by XRD technique were not consistent with those by displacement measurement technique, because the latter technique involves errors which origined from the shape and the size of the substrate. The stress in a polycrystalline film measured by modified sin^2ψ method was in good agreement with that measured by normal sin^2ψ method. This result suggests that modified sin^2ψ method can be applied to stress measurement not only in epitaxial thin films, but also in polycrystalline thin films. We further discuss the precision of the stress determination technique. Residual stress of epitaxial PbTiO_3 film on (100) SrTiO_3 substrate measured by modified sin^2ψ method was comparable to the theoretical stress estimated from the difference in thermal expansion coefficients between the film and the substrate. Stress values measured by modified sin^2ψ method may thus have the precision required for actual application.

The effect of yttrium oxide (Y₂O₃) addition on the sintering of magnesium silicon nitride (MgSiN₂) powder has been examined. The sintering conditions examined in this paper were firing temperature (1400 and 1500°C), firing time (1 to 7 h), and the amount of Y₂O₃ (1 to 7 mass%). The phases present, microstructures, flexural strength, and thermal conductivity of the resulting sintered compacts were evaluated using X-ray diffractometry, scanning electron microscopy, flexural testing, and atomic force microscopy. When the MgSiN₂ compact without Y₂O₃ addition was fired at 1500°C for 1 h or more, it partly decomposed to form α- and β-silicon nitride (Si₃N₄). The formation of α- and β-Si₃N₄ upon firing at 1500°C for 3 h could be restricted by the addition of Y₂O₃ as a sintering aid; no α- and β-Si₃N₄ were detected with increasing Y₂O₃ amount up to 3 mass%; further increases in the amount of Y₂O₃ up to 7 mass%, however, produced Y2Si₃O3N₄. Maximum relative density (-90%) was attained when a MgSiN₂ compact with 3 mass% of Y₂O₃ addition was fired at 1500°C for 3 h. The flexural strength of MgSiN₂ compacts with 3 to 7 mass% of Y₂O₃ addition fired at 1500°C for 3 h attained 160 to 200 MPa. Thermal conductivities of the sintered MgSiN₂ compacts with 1 to 7 mass% of Y₂O₃ addition were in the approximate range of 15 to 18 W·m^-1·K^-1.

Magnesium silicon nitride (MgSiN2) powder was prepared by carbothermally reducing a magnesium metasilicate whose chemical composition corresponded to MgO . SiO2 or MgSiO3. About 0.2 g of the powder mixture of magnesium metasilicate and carbon (C) with the molar ratio of C to MgO . SiO2 equal to 6.0 was heated at 1250 degrees C for 7 h in nitrogen atmosphere. The crystalline phase of the carbothermally reduced powder was only MgSiN2. The residual carbon could be removed by heating the powder at 600 degrees C for 1 h in air. The yield of MgSiN2 powder was similar to 70%. The resulting powder contained 3.22 mol% oxygen. The primary particle sizes were ranging from 0.1 to 0.5 mu m.

Magnesium silicon nitride (MgSiN_2) was synthesized by the nitridation of powders in the magnesium (Mg) -silicon (Si) system. Although an Mg and Si powder with an Mg/Si ratio of 1.0 ( = stoichiometric ratio of MgSiN_2) was heated up to 1400℃ in nitrogen (N_2) , a single phase of MgSiN_2 could not be obtained, due to evaporation of Mg during heating. On the basis of the information that Mg_2Si formed as an intermediate phase during the synthesis of MgSiN_2, an Mg_2Si powder was heated at a temperature between 600 and 1600℃ for 1h in N_2. Although MgSiN_2 and other phases such as Si and amorphous Mg_3N_2 were present in the temperature range of 900℃ to 1300℃, the powder heated at 1400℃ consisted of only MgSiN_2 phase. The chemical composition of the powder heated at 1400℃ for 1h was almost stoichiometric; the content of the oxygen was as low as 0.76 mol%. The powder consisted of agglomerates with sizes of 〜5μm; these agglomerates were furthermore composed of primary particles with an average size of 0.54 μm. The oxidation resistant temperature of MgSiN_2 was estimated to be 〜830℃. The MgSiN_2 powder had good resistivity to hydration.