Hiroyuki Ikawa

Products and microwave dielectric properties of ceramics with nominal compositions (Ba1−xCax)(B1/2B1/2′)O3 (B=Y3+, Nd3+, Gd3+; B′=Nb5+, Ta5+)

Abstract The products and microwave dielectric properties of ceramics were investigated which had nominal composition (Ba 1− x Ca x )( B 1/2 B 1/2 ′)O 3 ( B 3+ =Y 3+ , Nd 3+ , Gd 3+ ; B ′ 5+ =Nb 5+ , Ta 5+ ). The products versus x relationships of these systems were complex, owing to the small amounts of the third and fourth compounds in a wide range of intermediate x . Dielectric properties of (Ba 1− x Ca x )(Nd 1/2 Nb 1/2 )O 3 , (Ba 1− x Ca x )(Y 1/2 Nb 1/2 )O 3 and (Ba 1− x Ca x )(Y 1/2 Ta 1/2 )O 3 systems were evaluated. Relative permittivities ( e r ) of the first system changed linearly with x ; however, those of the last two systems increased sharply to have respective peaks at x of 0.2, and then decreased with x . Those sharp increases in e r were similar to those reported for (Ba 1− x Ca x )(Mg 1/3 Ta 2/3 )O 3 and other systems. However, the relative values of the increase in e r were different. Plausible mechanisms to explain those differences among systems are discussed by introducing an index of rattling of the Ca 2+ ion.

Effects of heat treatment on the phase transitions in BaTiO3 fine particles

Abstract Effects of heat treatment were investigated on the phase transitions in BaTiO3 fine particles synthesized by hydrothermal method. The virgin fine particles and the samples treated below 400°C showed no phase transition below room temperature, while the intrinsic successive phase transitions were detected for the samples treated above 600°C by the low temperature heat capacity measurements. The enthalpy and entropy of the orthorhombic-tetragonal phase transition increased with increasing temperature of heat treatment and approached to the values of the bulk sample obtained by melt-frozen method.

Structural and electrical changes in Ln1.9Sr1.1-xCaxCu2O6+δ (Ln=La, Pr) systems

Abstract Two series of La2SrCu2O6 type compounds, Ln1.9Sr1.1−xCaxCu2O6+δ (Ln = La and Pr), were prepared to study the effects of cation substitution on the electrical properties. The relation between lattice constants and Ca content x did not obey Vegards law: the curvature changed at x = 0.4−0.5 for Ln = La and x = 0.2−0.3 for Ln = Pr. The oxygen content 6 + δ decreased with the increase of x and became almost constant (slightly decreased) in the region of x ≧ 0.5 for the La system and x ≧ 0.2 for the Pr system. The electric resistivity for the La system increased with increasing x in x ≦ 0.4 and was almost constant in x > 0.4. Carrier concentration determined from Hall coefficients changed corresponding to the oxygen content. However, the Hall mobility indicated a discontinuous change at x = 0.4−0.5 for the La system and at x = 0.2−0.3 for the Pr system, suggesting that the electronic structure discontinuously changed between x = 0.4−0.5 for the La system and between x = 0.2−0.3 for the Pr system.

Magnetotransport property of layered manganites (La,Sr,Ca)3Mn2O7

Abstract We report the magnetotransport properties of two series of Sr 3 Ti 2 O 7 type manganites, La 2−2 x Sr 1+2 x Mn 2 O 7 and La 1·4 Sr 1·6− x Ca x Mn 2 O 7 . The former manganites show magnetotransport properties, which depend on x , that is, the carrier number. They show insulator to metal transition, when reducing temperature. The transition temperature increases with x up 0·4 and decreases with x exceeding 0·4. Around their transition temperature, they also show giant magnetoresistance, which decreases monotonically with increasing x . The latter manganites also show the electric transition temperature, which decreases with increasing x , that is, decreasing lattice constants. The giant magnetoresistance around transition temperature decreases monotonically with decreasing lattice constants. The magnetotransport properties of Sr 3 Ti 2 O 7 type manganites are sensitive to carrier number and lattice constants.

Properties of transition metal oxides with layered perovskite structure

Abstract The manganates of La n − n x Sr 1+ n x Mn n O 3 n +1 ( x =0.4; n =1, 2, ∞) were prepared to study the relationship between their electronic properties and their crystal structure; two-dimensional structure with n =1, pseudo-two-dimensional with n =2 and three-dimensional with n =∞. The n =1 manganate exhibits semiconducting behavior below room temperature. When the manganate has a higher dimensional structure with an increase in the value of n , the electric conduction changes to a more itinerant one. The n =2 manganate exhibits an electric transition from semiconductor to metal at 127 K when the temperature is reduced below room temperature. The n =∞ manganate exhibits metallic behavior below room temperature. Ferromagnetism was found when the metallic conduction occurred, i.e., in the n =2 manganate below the electric transition temperature, and in the n =∞ manganate below room temperature. From photoelectron spectroscopy, the valence band structure was found to consist of two features, due to the photoemission from the Mn 3d orbital (∼3 eV) and the O 2p orbital (∼5.5 eV). The change in the dimension of the crystal structure affects the electronic structure, that is, the ratio of the intensity of the photoemission from the Mn 3d orbital to that of the O 2p orbital increases with an increase in n .

FORMATION OF C-AXIS ORIENTED BARIUM FERRITE THIN FILMS BY ALTERNATE DEPOSITION OF A FE3O4 LAYER AND BAO.3FE2O3 LAYER

Deposition of c-axis perpendicularly oriented barium ferrite (BaM:BaFe12O19) thin films was attempted by means of an alternate periodic deposition of S (spinel Fe3O4) and R (BaO⋅3Fe2O3) layers. The period of the layers was fixed at 1.15 nm, which corresponds to the period of hexagonal barium ferrite crystals in the c-axis direction. Films 115 nm thick were deposited on a c-axis oriented ZnO underlayer by using a dc sputtering system with three facing target sputtering sources. This layer deposition method was expected to deposit a film with much better crystallite orientation at a lower substrate temperature than needed to deposit a film when using a conventional sputter deposition method (i.e., sputter deposition using a stoichiometric BaM sintered target). The films deposited at temperatures above 540 °C had a hexagonal crystal structure and showed c-axis orientation, but this deposition method did not result in a marked reduction of substrate temperature needed for the deposition of a BaM film with hex...

Products and microwave dielectric properties of ceramics with nominal composition (Ba0.9Ca0.1)(YxB′1/2)O(3x+4.5)/2 (B′=Nb5+, Ta5+)

Abstract The products and microwave dielectric properties of ceramics with nominal composition (Ba 0.9 Ca 0.1 )(Y x B ′ 1/2 )O (3 x +4.5)/2 ( B ′=Nb 5+ , Ta 5+ ) are investigated. When x =0.5, i.e. (Ba 0.9 Ca 0.1 )(Y 1/2 B ′ 1/2 )O 3 ( B′ =Nb 5+ , Ta 5+ ), the product contains a considerable amount of Y 2 O 3 as well as the main perovskite phase. When x =0.3 the product is single phase, equivalent to Ba(Ca 1/9 Y 3/9 Nb 5/9 )O 3 or Ba(Ca 1/9 Y 3/9 Ta 5/9 )O 3 . The lattice parameters of these new compounds are smaller than those of Ba(Y 1/2 Nb 1/2 )O 3 and Ba(Y 1/2 Ta 1/2 )O 3 . The relative permittivities ( e r ) of these new compounds are larger than those of Ba(Y 1/2 B ′ 1/2 )O 3 ( B ′=Nb 5+ , Ta 5+ ). The increase in e r of the Nb-system is about 4 times larger than that of the Ta-system. The Q f values of the present ceramics are larger than the Ca-containing perovskite in the (Ba 1− x Ca x )(Mg 1/3 Ta 2/3 )O 3 system. The sharp increase of e r in this study cannot be explained by the Ca 2+ rattling ion model at the A -site, which applies to the case of the (Ba 1− x Ca x )(Mg 1/3 Ta 2/3 )O 3 system. A new method to explain the increase in e r is discussed.

Preparation of c-axis-oriented barium ferrite thin films with small crystallite size

BaM thin films were deposited on thermally oxidized silicon wafer substrates at temperatures near 580 °C by using a dc facing-target sputtering system. The crystallite size of films 200 nm thick was greater than 200 nm, and it decreased monotonically with decreasing film thickness, reaching a value below 30 nm in films 30 nm thick. Further decreases in film thickness led to a degradation of the crystallinity of the film, causing the saturation magnetization and coercive force of the film to decrease significantly. Growth of BaM crystallites in films deposited on a SiO2/Si substrate was significantly suppressed when the films were deposited on a thin initial layer deposited beforehand under a substrate bias of −50 V.

DC electrorheology of fluid suspending barium titanate in the range of ferroelectric size effects

Abstract Hydrothermally synthesized barium titanate, which was paraelectric, was heated at various temperatures. The powder was mixed with silicone oil at 10 vol% to measure rheological properties of the suspension without and with dc field of 2kV/mm. The electrorheological effect (ER effect), the increase in shear stress by the dc field, showed a clear dependence to the heating temperature. The ER effect took a clear peak on a fluid dispersing powder heated at 700°C. The splitting of 200 and 002 X-ray diffractions of that barium titanate was uncertain, although this diffraction peak became clearly unsymmetrical.

Optical absorption spectra and the nature of conduction carriers of hole-doped Sr1+xLa1−xFeO4

Abstract The optical absorption as calculated from the diffuse reflectivity in the range of 0.5–5 eV and the temperature dependence of d.c. electrical conductivity of Sr1+xLa1−xFeO4 (0 ≦ x ≦ 0.3) with a K2NiF4 structure, were measured. The results obtained were analyzed based on the small polaron model, and discussed with reference to the changes in the physical properties and the electronic structure previously reported. The Mott type and charge-transfer type excitations were observed at ∼2.2 and ∼2.7 eV, respectively, in the optical absorption spectrum of the undoped SrLaFeO4. An additional broad absorption band below 1 eV was observed, and its intensity was directly proportional to the d.c. electrical conductivity at room temperature. Thus the absorption band below 1 eV was assigned to the optically induced polaron hopping. Based on the small polaron theory, the hopping energies of small polarons from 0.14 to 0.24 eV were obtained. These values were close to the activation energies of d.c. electrical conductivity obtained from the σT 3 2 vs 1 T plots. The results, in which the Fermi-level was fixed in the middle of the energy band gap of the photoemission spectroscopy previously reported could not be explained by the rigid-band model, but were consistent with the small polaron model.