Effect of Bi2O3 Addition on the Sintering Temperature and Microwave Dielectric Properties of Zn2SiO4 Ceramics

Bi₂O₃ was added to a nominal composition of Zn_1.8SiO_3.8 (ZS) ceramics to decrease their sintering temperature. When the Bi₂O₃ content was <8.0 mol%, a porous microstructure with Bi₄(SiO₄)₃ and SiO₂ second phases was developed in the specimen sintered at 885°C. However, when the Bi₂O₃ content exceeded 8.0 mol%, a liquid phase, which formed during sintering at temperatures below 900°C, assisted the densification of the ZS ceramics. Good microwave dielectric properties of Q × f =12,600 GHz, ɛ_r=7.6, and τ_f=−22 ppm/°C were obtained from the specimen with 8.0 mol% Bi₂O₃ sintered at 885°C for 2 h.     

Dielectric Properties of the Fluorite-like Bi2O3–Nb2O5 Solid Solution and the Tetragonal Bi3NbO7

Our analysis of the microwave dielectric properties of the δ-Bi₂O₃–Nb₂O₅ solid solution (δ-BN_ss) showed a continuous increase in permittivity and dielectric losses with an increasing concentration of Nb₂O₅. The only discontinuity was found for the temperature coefficient of resonant frequency, which is negative throughout the entire homogeneity range but reaches a minimum value for the sample with 20 mol% Nb₂O₅. At the same composition there is a discontinuity in the grain size of the δ-BN_ss ceramics. For the sample containing 25 mol% Nb₂O₅ two structural modifications were observed. A single-phase tetragonal Bi₃NbO₇, in the literature referred to as a Type-III phase, is formed in a very narrow temperature range from 850° to 880°C. A synthesis performed below or above this temperature range resulted in the formation of the end member of the δ-BN_ss homogeneity range. Compared with the δ-BN_ss the Bi₃NbO₇ ceramics exhibit lower microwave dielectric losses, an increased conductivity, and a positive temperature coefficient of resonant frequency.     

Microwave Dielectric Properties of Li2TiO3 Ceramics Doped with ZnO–B2O3 Frit

A type of new low sintering temperature ceramic, Li₂TiO₃ ceramic, has been found. Although it is difficult for the Li₂TiO₃ compound to be sintered compactly at temperatures above 1000°C for the volatilization of Li₂O, dense Li₂TiO₃ ceramics were obtained by conventional solid-state reaction method at the sintering temperature of 900°C with the addition of ZnO–B₂O₃ frit. The sintering behavior and microwave dielectric properties of Li₂TiO₃ ceramics with less ZnO–B₂O₃ frit (≤3.0 wt%) doping were investigated. The addition of ZnO–B₂O₃ frit can lower the sintering temperature of the Li₂TiO₃ ceramics, but it does not apparently degrade the microwave dielectric properties of the Li₂TiO₃ ceramics. Typically, the good microwave dielectric properties of ɛ_r=23.06, Q × f =32 275 GHz, τ_f = 35.79 ppm/°C were obtained for 2.5 wt% ZnO–B₂O₃ frit-doped Li₂TiO₃ ceramics sintered at 900°C for 2 h. The porosity was 0.08%. The Li₂TiO₃ ceramic system may be a promising candidate for low-temperature cofired ceramics applications.     

Phase Relations In The Pseudo-Ternary System La2O3–SrO–Fe2O3

The phase relations in the pseudo-ternary system La₂O₃–SrO–Fe₂O₃ have been investigated in air. Isothermal sections at 1100° and 1300°C are presented based on X-ray diffraction and thermal analysis of annealed samples. Extended solid solubility was observed for the compounds Sr _{n +1− v} La _v Fe _n O_{3 n +1−δ} ( n =1, 2, 3, and ∞) and Sr_{1− x} La _x Fe₁₂O₁₉, while only limited solubility of La in Sr_{4− z} La _z Fe₆O_13±δ was observed. At high Fe₂O₃ content, a liquid with low La₂O₃ content was stable at 1300°C.     

A New Method for Synthesizing Li2.06Nb0.18Ti0.76O3 Powder at Low Temperatures

Li_2.06Nb_0.18Ti_0.76O₃ powder has been successfully prepared at low temperatures via a facile and manageable, activated pretreatment on the inert raw Nb₂O₅. It is demonstrated that with triethanolamine, citric acid, and hydrogen peroxide, this simple pretreatment process could activate Nb₂O₅ efficiently. Pure Li₂TiO₃ solid solution phase was thus obtained by calcining the mixture of the activated Nb₂O₅, LiOH·H₂O, and Ti(C₄H₉O)₄ at temperatures as low as 650°C, which is about 200°C lower than that of the traditional solid-state method. To the best of our knowledge, this temperature is the lowest one for preparing Li₂TiO₃ solid solution. Additionally, the phase transformation and the morphology of the final powder are also discussed.     

Structural and Dielectric Properties of Ag(Nb0.8Ta0.2)1−xSbxO3 (x≤0.08) Ceramics

Sb₂O₅ were selected to substitute (Nb_0.8Ta_0.2)₂O₅ and the effects of Sb substitution on the dielectric properties of Ag(Nb_0.8Ta_0.2)O₃ ceramics were studied. The perovskite Ag(Nb_0.8Ta_0.2)_{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(Nb_0.8Ta_0.2)_{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(Nb_0.8Ta_0.2)_{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.     

The Bi2O3–Nb2O5–NiO Phase Diagram

The Bi₂O₃–Nb₂O₅–NiO phase diagram at 1100°C was determined by means of solid-state synthesis, X-ray diffraction, and scanning electron microscopy. A ternary eutectic with a melting point below 1100°C was found to exist in the field between NiO, Bi₂O₃, and the end-member of the δBi₂O₃–Nb₂O₅ solid solution. The existence of the previously reported Bi₃Ni₂NbO₉ phase was disproved. A pyrochlore homogeneity range around Bi_1.5Ni_0.67Nb_1.33O_6.25 was determined together with all the phase relations in this phase diagram.     

The Li2O–Nb2O5–TiO2 Composite Microwave Dielectric Ceramics with Adjustable Permittivities

A group of new y M-phase/(1− y ) Li_{2+ x} Ti_{1−4 x} Nb_{3 x} O₃ composite ceramics with adjustable permittivities for low-temperature co-fired ceramic applications was initially investigated in the study. The 0.5 M-phase/0.5 Li_{2+ x} Ti_{1−4 x} Nb_{3 x} O₃ ( x =0.01, 0.02, 0.04, 0.06, 0.081) composite ceramics were first investigated to find the appropriate "Li₂TiO₃ss" composition ( x value). The best dielectric properties of ɛ_r=40.1, Q × f values up to 9318 GHz, τ_f=25 ppm/°C, were obtained for the ceramics composites at x =0.02. Based on the good dielectric properties, the suitable "Li₂TiO₃ss" composition with x =0.02 was mixed with the Li_1.0Nb_0.6Ti_0.5O₃ powder as the ratio of y "M-phase"/(1− y ) "Li₂TiO₃ss" ( y =0.2, 0.4, 0.5, 0.6, 0.8). By adjusting the y values, the group of composite ceramics could exhibit largely are adjustable permittivities varying from ∼20 to ∼60, while Q × f and τ_f values relatively good. Nevertheless, in this study, because there are interactions between the M-phase and Li₂TiO₃ss during sintering process, their microwave dielectric properties could not be predicted precisely by the empirical model.     

Phase Relations in the System Bi2O3-WO3

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.     

Congruent Composition and Li-Rich Phase Boundary of LiNbO3

The congruent composition of LiNbO₃ was determined precisely by correlating crystal growth compositions with the Curie temperatures of samples quenched from a melt before and after crystal growth and of various sections of crystals grown from that melt. The initial melt composition was determined by control of Li₂CO₃ and Nb₂O₅ contents. The melt compositions were varied from 47 to 49 mol% Li₂O. The variation of T_e with melt composition was found to follow T_e = 9095.2 − 369.05C + 4.228C², where C is mol% Li₂O. High-temperature DTA was used to determine T_e with a precision of ±2°C. Above 1000°C the Li-rich phase boundary was found to be a function of temperature by determining the weight gain after complete lithiation of congruently grown crystals. This boundary curves slightly toward lower Li₂O content as temperature increases. The congruent composition of LiNbO₃ contains 48.45 mol% Li₂O and has a measured Curie temperature of 1138°± 2°C.     

Effect of Li2CO3 Addition on the Sintering Temperature and Microwave Dielectric Properties of Mg2V2O7 Ceramics

Li₂CO₃ was added to Mg₂V₂O₇ ceramics in order to reduce the sintering temperature to below 900°C. At temperatures below 900°C, a liquid phase was formed during sintering, which assisted the densification of the specimens. The addition of Li₂CO₃ changed the crystal structure of Mg₂V₂O₇ ceramics from triclinic to monoclinic. The 6.0 mol% Li₂CO₃-added Mg₂V₂O₇ ceramic was well sintered at 800°C with a high density and good microwave dielectric properties of ɛ _r=8.2, Q × f =70 621 GHz, and τ_f=−35.2 ppm/°C. Silver did not react with the 6.0 mol% Li₂CO₃-added Mg₂V₂O₇ ceramic at 800°C. Therefore, this ceramic is a good candidate material in low-temperature co-fired ceramic multilayer devices.     

Crystallization of Li2O · Al2O3· 6SiO2 Glasses Containing Niobium Pentoxide as Nucleating Dopant

Niobium pentoxide (T form, orthorhombic system) was utilized to promote devitrification in Li₂O · Al₂O₃· 6SiO₂ glasses. Two or more mole percentage of this nucleating dopant enhanced crystallization in these glasses. Glasses containing 4.0 and 8.0 mol% T-Nb₂O₅ exhibited a high tendency to form dispersed TT-Nb₂O₅ (monoclinic system) precipitates during the glass quenching process. The crystallization process in glasses containing 2.0 or 4.0 mol% T-Nb₂O₅ occurred as microphase separation, followed by the formation of dispersed TT-Nb₂O₅ crystalline precipitates (760°C), followed by β-quartz solid-solution ( ss ) formation (850° to 900°C) heterogeneously nucleated from the precipitates. β-quartz( ss ) transformed to β-spodumene( ss ), along with a polymorphic transition from the TT-Nb₂O₅ to M-Nb₂O₅ (tetragonal system) crystalline phase.     

Cassiterite Solubility in High-Silica K2O-Al2O3-SiO2 Liquids

The saturation surface of cassiterite, SnO₂, was determined for liquids in the system K₂O–Al₂O₃–SiO₂ as a function of bulk composition and temperature. At fixed K₂O/Al₂O₃ cassiterite solubility varies weakly with SiO₂ concentration (76 to 84 mol%), temperature (1350° to 1550°C), and log ( f _O2) (−0.7 to −5.3). Cassiterite solubility is also approximately independent of composition in liquids with molar ratios of K₂O/Al₂O₃ lessthan equal to 1 (peraluminous liquids). As K₂O/Al₂O₃ increases from 1 (peralkaline liquids), however, cassiterite solubility increases steeply and approximately linearly with K₂O in excess of Al₂O₃. It is proposed that potassium in excess of aluminum combines with Sn⁴⁺ to form quasi-molecular complexes with an effective stoichiometry of K₄SnO₄.     

Crystal Chemistry and Phase Relations in the System Li2O-Fe2O3-TiO2

Above 755°C, compounds along the spinel join LiFe₅O₈-Li₄Ti₅O₁₂ form a complete solid solution and below that temperature a two-phase region separates the ordered LiFe₅O₈ and the disordered spinel phase. At 800° and 900°C, cubic LiFeO₂ ( ss ) and monoclinic Li_zTi0₃ ( ss ) exist on the monoxide join LiFeO₂-Li₂TiO₃. The distributions of cations in both the spinel and monoxide structures were calculated as a function of equilibrium temperature and composition. Sub-solidus equilibria in the system Li₂O-Fe₂O₃-TiO₂ at 800° and 900°C were determined for compositions containing ∼50 mol% Li₂O.     

Spectroscopic Properties of Er3+-Doped Na2O–La2O3–Al2O3–SiO2 Glasses

Er³⁺-doped sodium lanthanum aluminosilicate glasses with compositions of (90− x )(0.7SiO₂·0.3Al₂O₃)· x Na₂O·8.2La₂O₃· 0.6Er₂O₃·0.2Yb₂O₃·1Sb₂O₃ (in mol%) ( x = 12, 20, 24, 40, 60 mol%) were prepared and their spectroscopic properties were investigated. Judd–Ofelt analysis was used to calculate spectroscopic properties of all glasses. The Judd–Ofelt intensity parameter Ω _t ( t = 2, 4, 6) decreases with increasing Na₂O. Ω₂ decreases rapidly with increasing Na₂O while Ω₄ and Ω₆ decrease slowly. Both the fluorescent lifetime and the radiative transition rate increase with increasing Na₂O. Fluorescence spectra of the ⁴ I _13/2 to ⁴ I _15/2 transition have been measured and the change with Na₂O content is discussed. It is found that the full width at half-maximum decreases with increasing Na₂O.     

Phase Diagram and Oxygen-Ion Conductivity in the Y2O3-Nb2O5 System

The phase equilibria in the Y₂O₃-Nb₂O₅ system have been studied at temperatures of 1500° and 1700°C in the compositional region of 0-50 mol% Nb₂O₅. The solubility limits of the C-type Y₂O₃ cubic phase and the YNbO₄ monoclinic phase are 2.5 (±1.0) mol% Nb₂O₅ and 0.2 (±0.4) mol% Y₂O₃, respectively, at 1700°C. The fluorite (F) single phase exists in the region of 20.1-27.7 mol% Nb₂O₅ at 1700°C, and in the region of 21.1-27.0 mol% Nb₂O₅ at 1500°C, respectively. Conductivity of the Y₂O₃- x mol% Nb₂O₅ system increases as the value of x increases, to a maximum at x = 20 in the compositional region of 0 ≤ x ≤ 20, as a result of the increase in the fraction of F phase. In the F single-phase region, the conductivity decreases in the region of 20-25 mol% Nb₂O₅, because of the decrease in the content of oxygen vacancies, whereas the conductivity at x = 27 is larger than that at x = 25. The conductivity decreases as the value of x increases in the region of 27.5 ≤ x ≤ 50, because of the decrease in the fraction of F. The 20 mol% Nb₂O₅ sample exhibits the highest conductivity and a very wide range of ionic domain, at least up to log p _O2=−20 (where p _O2 is given in units of atm), which indicates practical usefulness as an ionic conductor.     

Phase Equilibria in the Systems SrO-CuO and SrO-1/2Bi2O3

The phase equilibria of the systems SrO-CuO and SrO-1/2Bi₂O₃ were studied by X-ray diffraction analysis of quenched powder samples. The compounds SrCuO₂ and Sr₂CuO₃ melt incongruently at 1085° and 1225°C, respectively. The newly found compound Sr₆Bi₂O₉ decomposes at 965°C into SrO and Sr₃Bi₂O₆ melts incongruently into SrO and liquid at 1210°C. SrBi₂O₄ undergoes a phase transition at ∼825°C, and although both are nonstoichiometric, the low-temperature phase is slightly poorer in SrO with 33.5 mol% SrO than the high-temperature phase.     

Phase Formation in the System Na2O · Al2O3-CaO · A12O3-Al2O3 at 1200°C in Air

Phase relations in the system Na₂O_· Al₂O₃-CaO· Al₂O₃-Al₂O₃ at 1200°C in air were determined using the quenching method and high-temperature X-ray diffraction. The compound 2Na₂O · 3CaO · 5Al₂O₃, known from the literature, was reformulated as Na₂O · CaO · 2Al₂O₃. A new compound with the probable composition Na₂O · 3CaO · 8Al₂O₃ was found. Cell parameters of both compounds were determined. The compound Na₂O · CaO-2Al₂O₃ is tetragonal with a = 1.04348(24) and c = 0.72539(31) nm; it forms solid solutions with Na₂O · Al₂O₃ up to 38 mol% Na₂O at 1200°C. The compound Na₂O · 3CaO · 8Al₂O₃ is hexagonal with) a = 0.98436(4) and c = 0.69415(4) nm. The compound CaO · 6Al₂O₃ is not initially formed from oxide components at 1200°C but behaves as an equilibrium phase when it is formed separately at higher temperatures. The very slow transformation kinetics between β and β "-Al₂O₃ make it very difficult to determine equilibrium phase relations in the high-Al₂O₃ part of the diagram. Conclusions as to lifetime processes in high-pressure sodium discharge lamps can be drawn from the phase diagram.     

Phase Relations in the Pyrochlore-Rich Part of the Bi2O3−TiO2−Nd2O3 System

In this study we used solid-state synthesis to determine the phase relations in the pyrochlore-rich part of the Bi₂O₃−TiO₂−Nd₂O₃ system at 1100°C. The samples were analyzed using X-ray powder diffraction and scanning electron microscopy with energy- and wavelength-dispersive spectroscopy. A single-phase pyrochlore ceramic was obtained with the addition of 4.5 mol% of Nd₂O₃. We determined the solubility limits for the three solid solutions: (i) the pyrochlore solid solution Bi_{(1.6–1.08 x )}Nd _x Ti₂O_{(6.4+0.3 x )}, where 0.25< x <0.96; (ii) the solid solution Bi_{4− x} Nd _x Ti₃O₁₂, where 0< x <2.6; and (iii) the Nd_{2− x} Bi _x Ti₂O₇ solid solution, where 0< x <0.35. The determined phase relations in the pyrochlore-rich part are presented in a partial phase diagram of the Bi₂O₃−TiO₂−Nd₂O₃ system in air at 1100°C.     

Reactions in the System Li2O–MgO–Al2O3–SiO2: I, The Cordierite-Spodumene Join

Phase equilibria along the nonbinary join between cordierite (2MgO · 2Al₂O₃· 5SiO₂) and spodumene (Li₂O · Al₂O₃· 4SiO₂) were investigated in the temperature range 800° to 1550°C. using the quench technique on fourteen compositions. The phase diagram at high temperatures is characterized by a very small region of solid solution on the cordierite side, appreciable solid solution on the spodumene side, and regions of three and four phases toward the center of the system, including liquid, α-cordierite, mullite, spinel, corundum, and β-spodumene and its solid solutions. The liquidus has a flat minimum between 40 and 50% cordierite at 1347°, and rises on one side to the congruent melting point of β-spodumene (1421°) and on the other side to the temperature of complete melting of cordierite (1530°). The lowest temperature at which liquid appears is 1325°. At low temperatures a complete series of metastable solid solutions exists between μ-cordierite and β-spodumene. The significance of the data in the preparation of thermal-shock-resisting bodies is discussed.