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1.
Sb2O5 were selected to substitute (Nb0.8Ta0.2)2O5 and the effects of Sb substitution on the dielectric properties of Ag(Nb0.8Ta0.2)O3 ceramics were studied. The perovskite Ag(Nb0.8Ta0.2)1− x Sb x O3 ceramics showed no obvious change with x value being no more than 0.08, and the pseudoperovskite unit cell parameters a = c , b and monoclinic angle β decrease with Sb concentration increasing. The dielectric properties of Ag(Nb0.8Ta0.2)1− x Sb x O3 ceramics were found to be affected greatly by the substitution of Sb for Nb/Ta. The ɛ value of Ag(Nb0.8Ta0.2)1− x Sb x O3 ceramics sintered at their densified temperature increased from 480 to 825 with x from 0 to 0.08, the tan δ value decreased sharply from 0.0065 to 0.0023 (at 1 MHz) with x increasing from 0 to 0.04, and then kept a stable lower tan δ value ∼0.0024 with x to 0.08. The temperature coefficient of capacitance values continuously decreased from a positive value of 1450 ppm/°C for x =0 to a negative value of −38.52 ppm/°C for x =0.08.  相似文献   

2.
Structural Characterization of Aluminum Phosphate Binder   总被引:1,自引:0,他引:1  
The chemical structure of aluminum phosphate binder, which is used as a sealant for thermally sprayed coatings, was studied. Powder X-ray diffraction (XRD) analysis showed that the binder was a mixture of several phases. None of these phases could be identified from the powder XRD data only. Electron diffraction revealed several distinct microcrystalline phases. The predominated phase was analyzed in more detail. The three-dimensional reciprocal lattice was reconstructed from a tilt series of electron-diffraction patterns that indicated that the main phase was the cyclohexaphosphate Al2P6O18. The similarity of the intensity distribution in the observed and simulated electron-diffraction patterns of Al2P6O18 confirmed this observation. The following unit-cell parameters were determined from the reconstructed reciprocal lattice: a = 8.8(7) Å, b = 15.5(6) Å, and c = 6.3(9) Å, β= 108°, and Z = 2. The reflection conditions indicated that the space group was P 21/ a .  相似文献   

3.
Two kinds of solid solution systems of Ta-doped MgTiO3 were identified by X-ray diffraction, which can be represented by the formulae MgTi1− x (Mg1/3Ta2/3) x O3 (0≤ x <0.5) and MgTi1− x Ta x O3 (0≤ x <0.05). The conductivity and microwave dielectric loss for the two solid solution systems were examined by AC impedance and microwave resonator measurements, respectively. In the system MgTi1− x (Mg1/3Ta2/3) x O3, the mechanism for the solid solution formation is the isovalent substitution of for Ti4+. In the system MgTi1− x Ta x O3, the doping mechanism is the aliovalent substitution of Ta5+ for Ti4+, where for a small amount Ta doping, the oxygen vacancies formed during the high-temperature preparation are filled by an extra oxygen introduced from Ta2O5 and further Ta doping leads to an increase in the contents of and electrons, which was consistent with conductivity measurements. In both systems, the Q × f values improved, e.g., ∼17% for the isovalent substitution at x =0.08 and ∼10% for the aliovalent substitution at x =0.02. The filling oxygen vacancy and the substitution of Ta/Mg for Ti may contribute to the improvement of Q × f values for both systems.  相似文献   

4.
The "subsolidus" phase relations at room temperature in the system CaO-B2O3-BaO are investigated. Specimens of various compositions were prepared from appropriate ratios of CaCO3, B2O3, and BaCO3, and fired from 780° to 1040°C according to their melting points. There are three ternary compounds in this system. The crystal structures of these compounds were determined by X-ray diffraction (XRD). CaBa2(BO3)2 and Ca5Ba2B10O22 are monoclinic structures. The lattice constants a = 14.221 Å, b = 4.569 Å, c = 11.926 A, β= 99.947°, and V = 763.4 å3 for CaBa2(BO3)2 and a = 15.714 å, b = 6.184 å, c = 10.204 å, β= 93.954°, and V = 989.29 å3 for Ca5Ba2B10O22 are obtained. The third compound, CaBa2(B3O6)2, is isostructural with the high form of BaB2O4 with lattice constants a = 7.167 å and c = 35.298 å. Powder second harmonic generation efficiencies of these ternary compounds were measured using a homemade apparatus.  相似文献   

5.
The hexagonal perovskite, Ba8ZnTa6O24, was prepared in single-phase form and was found to be a stable secondary phase, formed as a result of the loss of ZnO from Ba(Zn1/3Ta2/3)O3 microwave dielectrics. The experimental and calculated X-ray patterns of Ba8ZnTa6O24 indicate it is isostructural with Ba8Ta6NiO24 with an 8H (cchc)2 close-packed BaO3 stacking sequence and the lattice parameters, a =10.0825(14), c =19.0587(38)Å. High-density ceramics of Ba8ZnTa6O24 could be prepared at temperatures considerably lower (1400°C) than those used to sinter pure Ba(Zn1/3Ta2/3)O3, and exhibit very good microwave dielectric properties with ɛ=30.5, Q f=62 300, and τf=+36 ppm/°C at 8.9 GHz.  相似文献   

6.
Compositional inhomogeneity is examined in Li- and Ta-modified potassium sodium niobate (K,Na)NbO3 perovskite ceramics. The inhomogeneous distribution of the A-site (K and Na) and B-site (Nb and Ta) cations is found to be correlative. The inhomogeneity could not be eliminated by prolonged high-temperature annealing in samples prepared by direct mixing of alkaline carbonates and Ta and Nb oxides. In contrast, the precursor method, where a (Nb x Ta1− x )2O5 solid solution was first formed, led to a considerably improved compositional homogeneity and appreciably enhanced dielectric, ferroelectric, and piezoelectric properties. Our results suggest that more attention needs to be paid toward controlling the compositional fluctuation of this complex solid solution system.  相似文献   

7.
Zinc Vanadates in Vanadium Oxide-Doped Zinc Oxide Varistors   总被引:1,自引:0,他引:1  
Convergent-beam electron diffraction has been used to determine the space groups of β- and γ-Zn3(VO4)2 particles in vanadium oxide-doped zinc oxide varistors. The crystal structure of β-Zn3(VO4)2 has been determined to be monoclinic with space group P 21 and lattice parameters of a = 9.80 Å, b = 8.34 Å, c = 10.27 Å, and β= 115.8°, whereas that of γ-Zn3(VO4)2 is monoclinic with space group Cm and a = 10.40 Å, b = 8.59 Å, c = 9.44 Å, and β= 98.8°. Energy-dispersive X-ray microanalysis of these two phases shows significant deviations from their expected stoichiometry. It is apparent that the β-phase is, in fact, the metastable Zn4V2O9 phase, whereas the γ-phase either is a new oxide that consists of zinc, vanadium, and manganese or, more likely, is a zinc vanadate phase with a Zn:V atomic ratio of 1:1 that has the ability to go into solid solution with manganese.  相似文献   

8.
Phase relations in the system Bi2O3-WO3 were studied from 500° to 1100°C. Four intermediate phases, 7Bi2O3· WO3, 7Bi2O3· 2WO3, Bi2O3· WO3, and Bi2O3· 2WO3, were found. The 7B2O · WO3 phase is tetragonal with a 0= 5.52 Å and c 0= 17.39 Å and transforms to the fcc structure at 784°C; 7Bi2O3· 2WO3 has the fcc structure and forms an extensive range of solid solutions in the system. Both Bi2O3· WO3 and Bi2O3· 2WO3 are orthorhombic with (in Å) a 0= 5.45, b 0=5.46, c 0= 16.42 and a 0= 5.42, b 0= 5.41, c 0= 23.7, respectively. Two eutectic points and one peritectic exist in the system at, respectively, 905°± 3°C and 64 mol% WO3, 907°± 3°C and 70 mol% WO3, and 965°± 5°C and 10 mol% WO3.  相似文献   

9.
Two cubic pyrochlore phases exist in the system ZnO–Bi2O3–Sb2O5. Neither has the supposed "ideal" stoichiometry, Zn2Bi3Sb3O14. One, P 1, is a solid solution phase, Zn2+ x Bi2.96−( x − y )Sb3.04− y O14.04+δ where 0< x <0.13(1), 0< y <0.017(2) and a =10.4285(9)−10.451(1) Å. The other, P 2, is a line phase, Zn2Bi3.08Sb2.92O13.92 with a =10.462(2) Å. Subsolidus phase relations at 950°C involving phases P 1 and P 2 in the ZnO–Bi2O3–Sb2O5 phase diagram have been determined.  相似文献   

10.
A La(OH)3 gel solvothermal process has been developed for the preparation of nanocrystalline lanthanide oxysulfide (La2O2S) in polar solvents at 300°C through a reaction between a La(OH)3 gel and K2S. X-ray powder diffraction (XRD) indicated that the product was hexagonal La2O2S with cell parameters a = 4.046 Å and c = 6.951 Å. Transmission electronic microscopy (TEM) showed that different morphology nanocrystallites were formed, including particles with diameters of about 10 nm and nanorods about 10 nm in diameter and 300 nm in length, depending on the solvent.  相似文献   

11.
Pb(Mg1/3Ta2/3)0.7Ti0.3O3 thin films of single perovskite phase were successfully synthesized by using the RF sputtering deposition technique, followed by post-thermal annealing. While the perovskite structure of Pb(Mg1/3Ta2/3)0.7Ti0.3O3 is rather unstable, phase evolution in the thin films was manipulated by controlling both working pressure during the sputtering process and post-thermal annealing temperature. The desirable perovskite phase was promoted by increasing the working pressure in the range of 10–25 mTorr, followed by thermal annealing at 600°C. The ferroelectric, dielectric, and polarization behaviors of Pb(Mg1/3Ta2/3)0.7Ti0.3O3 films were characterized over a wide range of frequencies. They are strongly affected by the film thickness, where the relative permittivity and remanent polarization increase, while the coercive field decreases with increasing film thickness in the range of 115–360 nm.  相似文献   

12.
Subsolidus phase relations of the oxides in the system Fe-Ta-O were experimentally determined at 1200°C, 1 atm total pressure, and variable partial pressures of oxygen. Tantalum pentoxide reacts readily with either ferrous or ferric oxide at subsolidus temperatures and the following ternary compounds have been synthesized: Fe4Ta209, Fe3Ta2O8-1, FeTaO4, FeTa2O6, solid solutions between the latter two compounds, and tantalian magnetites. The compositions of solid solutions between FeTaO4 and FeTa2O6 were very sensitive to variation of oxygen pressure. This sensitivity explains the oxidation behavior of tapiolite. The incorporation into magnetite of up to 7 at.% Ta was demonstrated, with a resulting increase in the size of the unit cell and decrease in magnetic permeability.  相似文献   

13.
Lithium metal oxides with the nominal composition Li5La3M2O12 (M = Nb, Ta), possessing a garnetlike structure, have been investigated with regard to their electrical properties. These compounds form a new class of solid-state lithium ion conductors with a different crystal structure compared with all those known so far. The materials are prepared by solid-state reaction and characterized by powder XRD and ac impedance to determine their lithium ionic conductivity. Both the niobium and tantalum members exhibit the same order of magnitude of bulk conductivity (∼10−6 S/cm at 25°C). The activation energies for ionic conductivity (<300°C) are 0.43 and 0.56 eV for Li5La3Nb2O12 and Li5La3Ta2O12, respectively, which are comparable to those of other solid lithium conductors, such as Lisicon, Li14ZnGe4O16. Among the investigated materials, the tantalum compound Li5La3Ta2O12 is stable against reaction with molten lithium. Further tailoring of the compositions by appropriate chemical substitutions and improved synthesizing methods, especially with regard to minimizing grain-boundary resistance, are important issues in view of the potential use of the new class of compounds as electrolytes in practical lithium ion batteries.  相似文献   

14.
Complex perovskite-type compounds with the general formula Pb(B+1/4B5+3/4)O3, where B+= Li+ and B5+= Nb5+ or Ta5+, were synthesized using a high-pressure technique and studied by X-ray powder diffraction. The X-ray patterns were indexed on the basis of a cubic cell with a 0= 4.071 Å for Pb(Li1/4 Nb3/4)O3 and a 0= 4.052 Å for Pb(Li1/4Ta3/4)O3. Electrical properties of the new perovskites were also studied.  相似文献   

15.
The ternary compound Ho2Mn x Ta2− x O7 was synthesized in air at 1523 K. Structural characterization of the compound was taken by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The compound possessed a trigonal super cell derived from a rhombohedral basic cell with the a /2 of the trigonal super cell. The super cell was caused by the ordering of Mn and Ta ions and the derived structure model was in agreement with that of the zirkelite structure with the space group P 3121. The compound had stacking disorder with different stacking sequences along the c -axis. An orientation anomaly in the super cell reflections revealed that the super cell had an incommensurate nature.  相似文献   

16.
Dielectric properties of A(B3+1/2B5+1/2)O3 (A = Ba, Ca, Sr; B3+= La, Nd, Sm, Yb; B5+= Nb, Ta) ceramics have been investigated at microwave frequencies. Sr(B3+1/2B5+1/2)O3 and Ca(B3+1/2B5+1/2)O3 ceramics have relative dielectric constants (ε r ) above 20 and negative temperature coefficients of resonant frequency (T f ). In the group of Ba(B3+1/2B5+1/2)O3 ceramics, T f changes from + 118 ppm/° to nearly zero according to the kinds of B-site ions. Among the ceramics investigated, Sr(Sm1/2Ta1/2)O3 ceramics have the highest Q values at microwave frequencies. For Sr(Sm1/2Ta1/2)O3 ceramics Q = 7000, ε r = 27.7, and T f =−62.5 ppm/° at 8.5 GHz. The microstructure of Sr(Sm1/2Ta1/2)O3 ceramics is composed of a matrix of the ternary compound (Sr-Sm-Ta-O system) and secondary phase grains of the binary compound (Sm-Ta-O system).  相似文献   

17.
New Aurivillius phases in the Bi–Ag–Ti–O system were investigated by means of a solid-state reaction and X-ray diffraction. We found that the oxygen partial pressure has a significant influence on the synthesis of the Aurivillius phases. The mixed-layer Aurivillius phase Ag0.5Bi8.5Ti7O27 was observed after firing in an O2 flow, but a single-phase material is difficult to obtain. A single-phase compound of the four-layer Aurivillius phase Ag0.5Bi4.5Ti4O15 was obtained on firing in an oxygen partial pressure of 10 bar (1 × 106 Pa). The dielectric properties (at 1 MHz) of the Ag0.5Bi4.5Ti4O15 compound were as follows: T max=687°C, ɛ r =166 (∼20°C), and tan δ=0.004 (∼20°C).  相似文献   

18.
A novel, simple, and feasible route for synthesizing air-stable, water-soluble tantalum precursors has been developed using moisture-insensitive Ta2O5 as a starting source of the Ta element, based on the conventional basic flux method. Various analytical techniques have been used to characterize the formation mechanism, purity, and thermal decomposition features of these Ta precursors including tantalum oxalate, tantalum peroxo-tartarate, and tantalum peroxo-citrate. These Ta precursor solutions have a higher Ta ion purity over 99.0 wt% and a lone shelf life. Using home-made Ta precursors, photocatalyst powders of 1 mol% Ta-doped ZnO and ferroelectric films of SrBi2Ta2O9 on Pt/TiO2/SiO2/Si substrates have been prepared by a modified polymerizable complex (PC) route. Ta-doped ZnO powders from polymeric precursors show hexagonal wurtzite structures with uniform smaller grain sizes of 25 nm and a larger specific surface area of 32 m2/g. Moreover, they exhibit excellent photocatalytical activity under visible light irradiation compared with pure ZnO powders. PC-derived SrBi2Ta2O9 films also show comparable ferroelectric and dielectric properties with those from the metalorganic decomposition method. All these qualities indicate that water-soluble Ta precursors are potential and competitive candidates for photocatalyst and ferroelectric applications.  相似文献   

19.
A procedure for the formation of A12O3 coatings as diffusion barriers between ductile reinforcements (e.g., Nb and Ta) and intermetallic matrices (e.g., MoSi2 and NiAl) is described. The coating technique involved sol-gel processing of alumina -forming sols with the addition of submicrometer-sized A12O3 particles. Cracking in the coatings, a typical shortcoming of alumina sol-gel coating, was overcome by the addition of the fine particles into the sols. The surface charge of the A12O3 particles was adjusted to be the same as the AIO(OH) colloids in the sols and electrophoresis was used to codeposit A12O3 and AIO(OH) onto the surfaces of the reinforcements. The alumina gel derived from the sols acted as binder for the alumina particles, while the particles reduced the shrinkage of the sol-gel coatings and promoted the formation of dense coatings. The thickness of the coatings could be easily controlled without cracking and the effectiveness of the coatings as diffusion barriers was improved substantially.  相似文献   

20.
The phase stabilities in the(1−x)Ba(Zn1/3Ta2/3)O3 (BZT)-xBaZrO3(BZ)system have been investigated using samples prepared by the mixed oxide method. The substitution of Zr4+destabilizes the 1:2 cation ordering in BZT and pro-motes the formation of a cubic, 1:1 ordered structure with a doubled perovskite repeat. The homogeneity range of the 1:1 phase extends from x = 0.04 to approximately x = 0.25; substitutions beyond this range stabilize a disordered perovskite. The limits of stability of the 1:1 ordering coin-cide with compositions previously found to exhibit anoma-lies in their dielectric loss. The range of homogeneity is consistent with a "random layer" model for the 1:1 ordered "Ba{β';1/2β1/2}O3" structure. In this model the B× positions are assumed to be occupied exclusively by Ta5+, and the b× sites by a random distribution of Zn2+, Zr4+, and the remaining Ta 5+ cations. The validity of the model, where the ordered solid solutions can be represented by Ba{[Zn2− y /3Ta(1−2 y )/3Zr y ]1/2[Ta]1/2}O3(y =2x)was con-firmed by Rietveld refinements conducted using data col-lected with a synchrotron X-ray source.  相似文献   

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