Effect of La/Sn co-substitution for Ba/Ta on crystal structure and dielectric properties of Ba5NdTi3Ta7O30 ceramics

The effects of La/Sn co-substitution for Ba/Ta were investigated for the modification of Ba₅NdTi₃Ta₇O₃₀ ceramics. The modified ceramics (Ba_5?x La _x )NdTi₃(Ta_7?x Sn _x )O₃₀ exhibited single tetragonal tungsten bronze phase for x<1.5, while a small amount of secondary phase BaTi₄O₉ was observed for x>1.5. The lattice constants decreased with increasing La/Sn content, while the axial ratio 10^1/2c/a decreased when x was below 1.5, then slightly increased. With increasing La and Sn content, the temperature coefficient of dielectric constant (at 1 MHz) was remarkably lowered from ?1560 ppm/ °C to ?286 ppm/ °C, while the dielectric constant gradually reduced, and the dielectric loss slightly increased. There were some clear relationships between the temperature coefficient and bond valence, tolerance factor: the temperature coefficient of dielectric constant linearly increased when the bond valence of the ions at B sites increased, while the same effect occurred when the tolerance factor decreased. In addition, the stability of the tetragonal tungsten bronze phase is discussed in relation to electronic difference and tolerance factor.     

Crystal structure and dielectric properties of La3+ substituted Ba5LaTi3Ta7O30 ceramics

The effects of La³⁺ partial substitution for Ba²⁺ on crystal structure and dielectric properties were investigated for modification of Ba₅LaTi₃Ta₇O₃₀ ceramics. When the substitution x > 0.6, the sintering temperature obviously decreased. Superstructure peaks were observed for more La³⁺ substitution (x ≥ 2.4). The stability of the tetragonal tungsten bronze phase was discussed with electronegativity difference and tolerance factor. Lattice parameters decreased at first, and then slightly increased; while axial ratio decreased with increasing La content. Dielectric constant slightly increased when x < 1.2, then gradually declined with the increase of La substitutions for Ba. Dielectric loss was very low except x = 2.4. Temperature coefficient of dielectric constant (at 1 MHz) was remarkably improved. The variation of dielectric properties was discussed from the change of crystal structure as well as the effects of vacancies at A-sites and superstructure.     

Effects of Ca and Zr substitution upon structure and properties of Ba5LaTi3Ta7O30 low-loss dielectric ceramics

Effects of Ca and Zr substitution upon the dielectric properties of Ba₅LaTi₃Ta₇O₃₀ ceramics were investigated together with the structural characterization. All the samples of Ba₅La(Zr_xTi_1−x)₃Ta₇O₃₀ formed a filled tungsten-bronze structures, whereas the solid solution limit was determined as x=0.25 in (Ca_xBa_1−x)₅LaTi₃Ta₇O₃₀. Beyond this limit secondary phase of CaTa₂O₆ was detected and it would become the major phase for the Ca-rich compositions. The temperature coefficient of dielectric constant was improved with increasing Zr content while the dielectric constant decreased and the low dielectric loss varied little (in the order of 10⁻⁴). In the case of (Ca_xBa_1−x)₅LaTi₃Ta₇O₃₀, small temperature coefficient of dielectric constant could be obtained with increasing Ca content while the dielectric constant decreased significantly, and a small amount substitution of Ca for Ba induced decrease in dielectric loss.     

Preparation, characterization and dielectric properties of Ba3M0.33Ta4.67O15 (M=Zn, Ni) ceramics

Two novel Ba₃M_0.33Ta_4.67O₁₅ (M=Zn, Ni) ceramics were prepared by conventional high temperature solid-state reactions and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and dielectric measurements methods. Both compounds are paraelectric phases of the filled tetragonal tungsten–bronze (TB) structure at room temperature. At 1 MHz Ba₃Zn_0.33Ta_4.67O₁₅ and Ba₃Ni_0.33Ta_4.67O₁₅ ceramics show high dielectric constants of 108 and 101 combined with low dielectric loss of 0.0052 and 0.0029, respectively.     

Temperature-stable and low loss Fe-containing dielectrics in BaO-Ln<Subscript>2</Subscript>O<Subscript>3</Subscript>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> system

Polycrystalline samples of Ba₄Ln₂Fe₂Ta₈O₃₀ (Ln = La and Nd) were prepared by a high temperature solid-state reaction technique. The formation, structure, dielectric and ferroelectric properties of the compounds were studied. Both compounds are found to be paraelectrics with filled tetragonal tungsten bronze (TB) structure at room temperature. Dielectric measurements revealed that the present ceramics have exceptional temperature stability, a relatively small temperature coefficient of dielectric constant (τ _ε ) of −25 and −58 ppm/°C, with a high dielectric constant of 118 and 96 together with a low dielectric loss of 1.2 × 10⁻³ and 2.8 × 10⁻³ (at 1 MHz) for Ba₄La₂Fe₂Ta₈O₃₀ and Ba₄Nd₂Fe₂Ta₈O₃₀, respectively. The measured dielectric properties indicate that both materials are possible candidates for the fabrication of discrete multilayer capacitors in microelectronic technology.     

Structures and dielectric properties of La/Sn co-substituted Ba<Subscript>4</Subscript>Nd<Subscript>2</Subscript>{T}i<Subscript>4</Subscript>Ta<Subscript>6</Subscript>O<Subscript>30</Subscript> ceramics

The effects of La/Sn co-substitution at A and B sites of Ba₄Nd₂Ti₄Ta₆O₃₀ ceramics were investigated. With La/Sn co-substitution, the temperature coefficient was significantly improved to near zero, at the same time, dielectric constant reduced and dielectric loss increased. In particular, remarkable annealing effects on dielectric properties were observed for these modified ceramics. After annealing, dielectric constant considerably increased, and temperature coefficient shifted to more negative.     

Investigations of Bi substitution in NdBa2Cu3Oy

Attempts to substitute Bi for Nd in orthorhombic NdBa₂Cu₃O _y , prepared in air or oxygen at about 950°C led instead to formation of Ba₂NdBiO₆, a new cubic compound witha=0.8703 nm. The possibility was then explored of preparing superconducting (Nd_1–x Bi _x )Ba₂Cu₃O _y , by first forming the tetragonal phase at 880–950°C in nitrogen or argon followed by reheating in oxygen or air at 250–500°C in order to insert the additional oxygen required to yield the orthorhombic form while avoiding oxidation of Bi³⁺ to Bi⁵⁺. X-ray diffraction studies, electrical conductivity measurements, and thermogravimetric analysis of products indicate that Bi does not enter the NdBa₂Cu₃O _y , lattice in either the tetragonal or the orthorhombic phase. Ba₂NdBiO₆ clearly forms on reheating in oxygen or air even at low temperatures, and evidence is presented that a poorly crystallized oxygen-deficient form of this compound is already present prior to the reheating.     

Dielectric properties of Pb[(1?x)(Zr1/2Ti1/2)?x(Zn1/3Ta2/3)]O3 ceramics prepared by columbite and wolframite methods

Polycrystalline samples of Pb[(1 − x)(Zr_1/2Ti_1/2) − x(Zn_1/3Ta_2/3)]O ₃ , where x = 0.1–0.5 were prepared by the columbite and wolframite methods. The crystal structure, microstructure, and dielectric properties of the sintered ceramics were investigated as a function of composition via X-ray diffraction (XRD), scanning electron microscopy (SEM), and dielectric spectroscopy. The results indicated that the presence of Pb(Zn_1/3Ta_2/3)O₃ (PZnTa) in the solid solution decreased the structural stability of overall perovskite phase. A transition from tetragonal to pseudo-cubic symmetry was observed as the PZnTa content increased and a co-existence of tetragonal and pseudo-cubic phases was observed at a composition close to x = 0.1. Examination of the dielectric spectra indicated that PZT–PZnTa exhibited an extremely high relative permittivity at the MPB composition. The permittivity showed a ferroelectric to paraelectric phase transition at 330 °C with a maximum value of 19,600 at 100 Hz at the MPB composition.     

Properties of sol-gel derived strontium barium niobate ceramics and the effect of V2O5 additive

Strontium barium niobate, Sr _x Ba_1–x Nb₂O₆, (SBN) ceramics with a range of Sr/Ba ratios were fabricated using a solution chemistry approach with alkoxides dissolved in acetic acid. Powders obtained from dried gels were calcined at 800°C for three hours, a heat treatment that produced fully crystalline powders. After preparing bulk ceramics from these powders by sintering at 1200°C for 1–24 hours, the tetragonal tungsten bronze (TTB) phase was present in all compositions between 0 x 1, although not predicted by the phase diagram for BaNb₂O₆and SrNb₂O₆. Vanadium pentoxide (V₂O₅) additions increased the densification, the amount of tungsten bronze phase, and the dielectric constant for all compositions except for Sr_0.65Ba_0.35Nb₂O₆(SBN65) ceramics.     

Microwave dielectric and elastic properties of glass-added Ba6−3xR8+2xTi18O54 (x = 2/3)

Microwave dielectric properties of Ba_6−3xSm_8+2xTi₁₈O₅₄ (x = 2/3) [BST] ceramics with the addition of 0–3 wt.% of various glasses have been studied. It has been found that the addition of 0.5 wt.% of the glasses decreases the sintering temperature by about 150 °C. In general, addition of 0.5 wt.% of Zn, Mg and Pb-based glasses deteriorate the quality factor, whereas aluminum and barium borosilicates do not decrease it considerably. The quality factor and dielectric constant decrease with increasing amount of glass. The temperature coefficient of resonant frequency shifts towards positive or negative depending on the composition of the glass. A glass–ceramic composite with a dielectric constant 64, Q × f nearly 8500 GHz and near to zero τ_f could be obtained at a sintering temperature of 1175 °C when 3–4 wt.% Al₂O₃–B₂O₃–SiO₂ glass was added to BST ceramic. The Young's modulus decreases with increasing amount of glass, irrespective of the composition of glass.     

Growth of Ferroelectric Pb(Ta1 – xNbx)2O6 Single Crystals

The liquidus relations in the Pb₂V₂O₇–PbNb₂O₆–PbTa₂O₆ system were studied by direct visual observation of melting and x-ray diffraction analysis. Pb(Ta_{1 – x} Nb _x )₂O₆ crystals, both ferroelectric (x= 0.3, 0.5, 0.7; tetragonal potassium tungsten bronze structure type) and nonferroelectric (rhombohedral structure), were grown by precipitation from Pb₂V₂O₇ fluxes. The Curie temperature and peak permittivity of the crystals were found to increase systematically with Nb content.     

Structural and dielectric properties of the phase Pb1−2xKxM3+xNb2O6, M = La or Bi

The Pb_1?2xK_xM³⁺_xNb₂O₆, M = La or Bi, solid solutions have been prepared by solid state reaction and their structural and ferroelectric properties have been established. Complete crystalline solid solubility exists in both systems; in addition, three structurally related phases, namely, the ferroelectric orthorhombic and tetragonal tungsten bronze phases and the paraelectric tetragonal K.₅La.₅Nb₂O₆ type phase, have been identified at room temperature. The composition ranges for three phases are 0.0 ≤ x ≤ 0.47, 0.48 ≤ x ≤ 0.85, 0.86 ≤ x ≤ 1.0, respectively. The ferroelectric phase transition temperature, T_c, decreases with increasing concentration of K⁺ and La³⁺ or Bi³⁺ in the orthorhombic tungsten bronze phase. A few compositions from each system exhibit excellent dielectric and piezoelectric characteristics, indicating that they could be future materials for piezoelectric and high frequency dielectric studies.     

Structure, dielectric and magnetic properties of Ba6FeNb9O30 tungsten bronze ceramics

Ba₆FeNb₉O₃₀ ceramics were synthesized by a standard solid-state sintering process. X-ray powder diffraction (XRD) refinements were carried out to analyze the crystal structure, and the dielectric, ferroelectric and magnetic properties were investigated and discussed. The tetragonal tungsten bronze structure in space group P4bm was determined with the lattice parameters: a = 12.597(6) ?, b = 12.597(6) ?, c = 3.989(1) ?. An extremely high dielectric constant was indicated at higher temperatures, and it dropped quickly when the sample was cooled down through a critical temperature, and this critical temperature showed strong frequency dependence. This dielectric relaxation was more obviously observed in the dielectric loss curve. The nonlinear magnetic hysteresis curve was observed in the present ceramics at 5 K, which was related to the magnetic ions (Fe³⁺) in tungsten bronze structure.     

Modification of Ba4Sm2Ti4Ta6O30 dielectric ceramics

Modification of promising tungsten bronze-type dielectric ceramic Ba₄Sm₂Ti₄Ta₆O₃₀ has been investigated by adjusting the composition in Ba_4-3xSm_2+2xTi₄Ta₆O₃₀. The temperature coefficient of dielectric constant has been significantly improved by such modification together with the suppressed dielectric loss, but the dielectric constant decreased significantly. © 2000 Kluwer Academic Publishers     

Aging effects on dielectric properties of barium neodymium titanium tantalate ceramics

The aging effects on crystal structures and dielectric properties were investigated in Ba_pNd_6−pTi_8−pTa_2+pO₃₀ (p = 3, 4 and 5) ceramics with tungsten bronze structure. From XRD analyses on fresh and aged specimens, the slight change of crystal constant was concluded in Ba_pNd_6−pTi_8−pTa_2+pO₃₀ ceramics. Strong aging effects on the dielectric properties were observed in the present ceramics. After aging, a slightly decreased dielectric constant was observed combined with a significantly reduced dielectric loss in all the three compositions. These results had been discussed in term of the disorder-order state of ions at A- and B-sites. From tolerance factor and electronegativity difference, the stability was discussed for the filled type tungsten bronze phase.     

Incorporation of bismuth into Ba6− x R8+2/3x Ti18O54(R = Nd, Gd)

Microstructural investigations and microanalyses of a series of Ba_6–x R _8+2/3x Ti₁₈O₅₄ ceramics (R = Nd, Gd) revealed that the solid-solubility limit for the isovalent substitution of R ³⁺ by Bi³⁺ depends on the composition of the Ba_6–x R _8+2/3x Ti₁₈O₅₄ phase. For the Nd analogue the solid-solubility limit (y in Ba_6–x (Nd_1–y Bi _y )_8+2/3x Ti₁₈O₅₄) decreases with a decrease in x from y = 0.16 for x = 2.0 to y = 0.10 for x = 0.8. An even lower solid-solubility limit (y = 0.06) was found for the Ba_4.5(Gd_1–y Bi _y )₉Ti₁₈O₅₄ compound (x = 1.5). All substituted Nd compositions exhibit higher permittivities (93–99), lower temperature coefficients of permittivity (11–15 ppm/K) and higher dielectric losses (Q · f = 1300–5500 GHz) than the parent compositions. By exceeding the solid-solubility limit, abrupt changes in the microstructural and dielectric characteristics are induced.     

Structural and dielectric properties of ferroelectric Sr1−xBaxBi2(Nb0.5Ta0.5)2O9 and Sr0.5Ba0.5Bi2(Nb1−yTay)2O9 ceramics

Ferroelectric Sr_1−xBa_xBi₂(Nb_0.5Ta_0.5)₂O₉ and Sr_0.5Ba_0.5Bi₂(Nb_1−yTa_y)₂O₉ were synthesized by solid state reaction route. X-ray diffraction studies confirm the formation of single phase layered perovskite solid solutions over a wide range of compositions (x=y=0.0, 0.25, 0.50, 0.75 and 1). The lattice parameters and the Curie temperature (T_c) have been found to have linear dependence on x and y. Transmission electron microscopy (TEM) suggest the lowering of orthorhombic distortion with increasing Ba²⁺ substitution. Variations in microstructural features as a function of x and y were monitored by scanning electron microscopy (SEM). The dielectric constant at room temperature increases with increase in both x and y. Interestingly Ba²⁺ substitution on Sr²⁺ site induces diffused phase transition and diffuseness increases with increasing Ba²⁺ concentration.     

Preparation and characterization of new dielectric ceramics Ba<Subscript>5</Subscript>LnTi<Subscript>2</Subscript>Nb<Subscript>3</Subscript>O<Subscript>18</Subscript> (Ln = La,Nd)

Two new A₆B₅O₁₈ type cation-deficient perovskites Ba₅LnTi₂Nb₃O₁₈ (Ln = La, Nd) were prepared by the conventional solid-state reaction route. The phase and structure of the ceramics were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM). Both compounds crystallize in the trigonal system. Ba₅LaTi₂Nb₃O₁₈ has a dielectric constant of 56.6, a high quality factors (Q_u × f > 16,700 at 4.3331 GHz), and a relatively large temperature coefficient of resonant frequency (_f) + 142 ppm°C^–1 at room temperature; Ba₅NdTi₂Nb₃O₁₈ has a higher dielectric constant of 47.3 with high quality factors Q_u × f > 15,000 at 4.6830 GHz, and _f + 128 ppm°C^–1.     

Microwave dielectric properties of Ba<Subscript>4</Subscript>LaTiNb<Subscript>3−<Emphasis Type="Italic">x</Emphasis></Subscript>Ta<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>15</Subscript> ceramics

High dielectric constant and low loss ceramics in the system Ba₄LaTiNb_3−x Ta _x O₁₅ (x = 0–3) have been prepared by conventional solid-state ceramic route. Ba₄LaTiNb_3−x Ta _x O₁₅ solid solutions adopted A₅B₄O₁₅ cation-deficient hexagonal perovskite structure for all compositions. The materials were characterized at microwave frequencies. They show a linear variation of dielectric properties with the value of x. Their dielectric constant varies from 53.1 to 42.3, quality factor Q_u × f from 18,790 to 28,070 GHz and temperature variation of resonant frequency from +94.3 to +33.1 ppm/°C as the value of x increases.     

Microwave dielectric properties of (Pb1−3x/2Lax)(Mg1/2W1/2)O3

La modified Pb(Mg_1/2W_1/2)O₃ were prepared by solid-state reaction process, and the sintering behavior, microstructure and microwave dielectric properties were investigated by X-ray powder diffraction (XRD), Raman scattering and HP network analyzer in this paper. A series of single phase perovskite type solid solutions with A-site vacancies (Pb_1−3x/2La_x(Mg_1/2W_1/2)O₃ (0 ≤ x ≤ 2/3)) were formed. The solid solution took cubic perovskite type structure (Fm3m) with random distribution of A-site vacancies when 0 < x < 0.5, and tetragonal or orthorhombic structure with the ordering of A-site vacancies when 0.5 ≤ x ≤ 2/3. The dielectric constant and temperature coefficient of resonant frequency decrease with increasing La content. Relatively good combination microwave dielectric properties were obtained for x = 0.56: ?_r = 28.7; Q × f = 18098; and τ_f = −5.8 ppm/°C.