Phase Equilibria in the System Na2O – Nb2O5

Phase equilibria data, obtained both by differential thermal analysis and by quenching, are presented for the system Na₂O-Nb₂O₅. Five compounds corresponding to the formulas 3Na₂O.1Nb₂0₆, lNa₂O. 1Nb₂O₅, lNa₂O 4Nb₂O₆, lNa_zO.7Nb₂O₅, and lNa₂O. 10Nb₂O₆ have been found. The compound 3Na_z0.lNb₂O₅ melts congruently at 992°C. The compounds 1Na₂O. 4Nb₂O₆, lNa₂O.7Nb₂O, and 1Na₂O. 1Onb₂O₅ melt incongruently at 1265°, 1275°, and 1290°C., respectively. The well-known perovskite structure phase NaNbO₃ was found to melt congruently at 1412°C. The transition temperatures in NaNbO₅ were checked by thermal analysis and only the major structural changes at 368° and 640°C. could be detected. A new disordered form of NaNbO₃ could be preserved to room temperature by very rapid quenching.     

Thermodynamic Characterization of the Eutectic Phase Mixture NaNbO3/Na3NbO4. II: Solid-State Electrochemical Investigation

The difference in the standard Gibbs-free energies of NaNbO₃ and Na₃NbO₄ has been determined by means of two electrochemical approaches based on potentiometric solid oxide electrolyte galvanic cells. The results are consistent with each other and prove the phase equilibrium between the niobates to be univariant. Quantitatively, the data can be represented by (525°–700°C)
The results are in between a wide range spanned by estimated literature data and are close to the findings of a previous manometric study.     

Tungsten Bronze Field and Melt Growth of Crystals in the Na2O-BaO-Nb2O5 System

The extents of the liquidus and solidus fields were determined for tungsten bronze-type solid solutions in the Na₂O-BaO-Nb₂O₅ system by DTA and melt crystal growth experiments. Bronze-type solid solutions exist to 7.1Na₂O-34.9BaO-58Nb₂O₅ in the Nb₂O₅-rich region and from 12Na₂O-38BaO-50Nb₂O₅ to 4.6Na₂O-45.4BaO-50Nb₂O₅ along the NaNbO₂-BaNb₂O₆ join, which includes NaBa₂Nb₅O₁₅=10Na₂O-40BaO-50Nb₂O₆. There is little, if any, solid solubility of compositions with a deficiency of Nb₂O₅. Curie temperatures decline rapidly and dielectric constant peaks broaden with Nb₂O₅ substitution because the Nb:O ratio becomes greater than the octahedral 1:3 ratio. Useful ferroelectrics exist along the NaNbO₃-BaNb₂O₆ join where the Nb:O ratio is 1:3. Large striae-free crystals, with less optical scattering than Czochralski-grown crystals, were grown from unseeded Na₂O-rich melts (e.g. 15Na₂O-37.5BaO-47.5Nb₂O₅) cooled from 1520° to 1300°C at 2°C/h. Annealing effects on these crystals whose compositions lie on the NaNbO₃-BaNb₂O₆ join are discussed.     

Ferroelectric and Piezoelectric Properties of Fine-Grained Na0.5K0.5NbO3 Lead-Free Piezoelectric Ceramics Prepared by Spark Plasma Sintering

Lead-free piezoelectric ceramics have received attention because of increasing interest in environmental protection. Niobate ceramics such as NaNbO₃ and KNbO₃ have been studied as promising Pb-free piezoelectric ceramics, but their sintering densification is fairly difficult. In the present study, highly dense Na_0.5K_0.5NbO₃ ceramics were prepared using spark plasma sintering (SPS). Although the SPS temperature was as low as 920°C, the density of the Na_0.5K_0.5NbO₃ solid solution ceramics was raised to 4.47 g/cm³ (>99% of the theoretical density). After post-annealing in air, reasonably good ferroelectric and piezoelectric properties were obtained in the Na_0.5K_0.5NbO₃ ceramics with submicron grains. The crystal phase of the Na_0.5K_0.5NbO₃ has an orthorhombic structure. The Curie temperature is 395°C and the piezoelectric parameter ( d ₃₃) of the Na_0.5K_0.5NbO₃ ceramics reached 148 pC/N.     

Nucleation and Crystallization of NaNbO3 from Glasses in the Na2O-Nb2O5-SiO2 System

Transparent glass-ceramics, in which the major phase was NaNbO₃, were obtained by heat treatment of glasses in the Na₂O-Nb₂O₅-SiO₂ system. The structure of the glass and the changes occurring during crystallization as a function of temperature and heating rate were examined by X-ray diffraction, transmission and replication electron microscopy, density, and other measurements. On heating, a rather abrupt formation of uniformly dispersed particles was observed. In the early stages of crystallization, these particles contained NaNbO₃ as loose, radially grown dendrites of identical crystal orientation which became dense during later stages of crystallization. The particle sizes ranged from 200 to 10,000 A, depending on the SiO₂ content of the glass. Transparency of the crystallized material was dependent on the particle size rather than on the amount of NaNbO₃ formed. The temperature at which crystallization occurred increased with the heating rate whereas the viscosity at crystallization decreased. For a given value of the rate of crystal formation per °C of temperature increase, the product (viscosity)ⁿ× (heating rate) was constant. The nucleation and growth phenomena which occurred in these glasses was attributed to microheterogeneities of higher Nb₂O₅ content which formed part of the glass structure.     

Crystallization of Beta NaFeO2 from a Glass Along the Na2SiO3-Fe2O3 Join

Fine-particle beta sodium ferrite (β-NaFeO₂), rather than α-Fe₂O₃, may be responsible for superparamagnetic behavior in a glass of composition (in mole fractions) 0.37Na₂O-0.26Fe₂O₃-0.37SiO₂. The 700°C isothermal section of the phase diagram of the Na₂O-Fe₂O₃-SiO₂ system is given, showing a three-phase field bounded by Na₂SiO₃-NaFeO₂-Fe₂O₃; there is no evidence for the existence (at 700°C) of compounds of molar composition 6Na₂O-4Fe₂O₃-5SiO₂ or 2Na₂O-Fe₂O₃-SiO₂. The Moessbauer spectrum of β-NaFeO₂ has an internal magnetic field of 487 kOe at room temperature.     

Subsolidus Equilibria in the System LiF-AlF3-Na3AIF6

Subsolidus equilibrium relations in the system LiF-AlF₃-Na₃AlF₆ were investigated in air by solid-state reaction techniques. The system contains 2 ternary compounds, Na₂LiAlF₆ and Na₃Li₃Al₂F₁₂, and extensive cryolite solid solution on the Na₃AlF₆ - Li₃AlF₆ binary join. Both ternary compounds decompose between 500° and 600°C. At and below 500°C, seven 3-phase regions and two 2-phase regions exist; the 2-phase regions result from the extensive cryolite solid-solution series. Phase relations for the 500°C isothermal section are presented, and relations above and below this temperature are described.     

Viscosity of Molten Na2O─B2O3 Slags

The viscosity of sodium borate slags at high Na₂O concentrations (37.3 to 49.4 mol%) and high temperatures (1000° to 1300°C) follows an Arrhenius-type relationship. This relationship was also observed for sodium borate slags (mass% Na₂O/mass% B₂O₃= 0.86) containing CaO and CaF₂ for the same temperature range. There has been a reduction in viscosity of the sodium borate slags (mass% Na₂O₃mass% B₂O = 0.53 to 0.86) with increase in Na₂O concentration. On adding CaO (10 to 50 mass%) to the sodium borate slag (mass% Na₂O/mass% B₂O₃= 0.86), the viscosity increased considerably, while an addition of CaF₂ (S to 15 mass%) to the slag (30.9 mass% Na₂O₃ 35.8 mass% B₂O₃, 33.3 mass% CaO) decreased the viscosity. The average activation energies of Na₂O─B₂O₃, Na₂O─B₂O₃─CaO₃ and Na₂O─B₂O₃─CaO─CaF₂ slag systems have been estimated as 14.6, 124.7, and 41.4 kJ/mol, respectively, for the given composition ranges and 1000° to 1300°C temperature range.     

Mineralizing Effect of Li2B4O7 and Na2B4O7 on Magnesium Aluminate Spinel Formation

Lithium borate (Li₂B₄O₇) and sodium borate (Na₂B₄O₇) mineralize spinel formation from stoichiometric MgO and Al₂O₃ between 1000° and 1100°C. Mineralization with both compounds is shown to be mediated by B-containing liquids which form glass on cooling. However, the liquid compositions depend on the type of mineralizer and temperature, suggesting that templated grain growth or dissolution–precipitation mechanisms are operating, one dominating over the other under certain conditions. Na₂B₄O₇-mineralized compositions show predominantly templated grain growth at 1000°C, which changes to dissolution–precipitation at 1100°C, whereas Li₂B₄O₇-mineralized compositions show dissolution–precipitation from 1000°C. Li₂B₄O₇ is a stronger mineralizer as spinel formation is complete with 3 wt% Li₂B₄O₇ at 1000°C and with ≥1.5 wt% addition at 1100°C, whereas Na₂B₄O₇-mineralized compositions are found to retain some unreacted corundum even at 1100°C.     

Compositional Dependence of Piezoelectric Properties in NaxK1−xNbO3 Lead-Free Ceramics Prepared by Spark Plasma Sintering

Lead-free piezoelectric Na _x K_{1− x} NbO₃ ( x =20–80 mol%) ceramics were fabricated using spark plasma sintering at a low temperature (920°C). All the Na _x K_{1− x} NbO₃ ceramics showed a similar orthorhombic phase structure, while the corresponding lattice parameters decreased from the KNbO₃ side to the NaNbO₃ side with increasing Na content. A discontinuous change in lattice parameter close to composition of 60 mol% Na indicated the presence of a transitional area that is similar to the morphotropic phase boundary (MPB) in Na _x K_{1− x} NbO₃ ceramics. The sintered density of the Na _x K_{1− x} NbO₃ ceramics decreased with increasing Na content, from a relative density of 99% for the K-rich side to 92% for the Na-rich side. The piezoelectric constant d ₃₃ and planar mode electromechanical coupling coefficient k _p showed a maximum value of 148 pC/N and 38.9%, respectively, due to the similar MPB effects in the PZT system.     

Phase Relations in the Pseudobinary System Na2TiSi4O11-Na2Ti2Si2O9

The quenching technique was used to study subliquidus and subsolidus phase relations in the pseudobinary system Na₂ Ti₂Si₂ O₁₁-Na₂ Ti₂ Si₂ O₉. Both narsarukite (Na₂TiSi₄O₁₁) and lorenzenite (Na₂Ti₂Si₂O₉) melt incongruently. Narsarsukite melts at 911°±°C to SiO₂+liquid, with the liquidus at 1016°C. Lorenzenite melts at 910°±5°C to Na₂ Ti₆ O₁₃+liquid; Na₂ Ti₆ O₁₃ reacts with liquid to form TiO₂ and is thus consumed by 985°±5°C. The liquidus occurs at 1252°C.     

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 Equilibria in the System CaF2-AlF3-Na3AlF6 and Part of the System CaF2-AlF3-Na3AlF6-Al2O3

The complete phase-equilibrium diagram of the system CaF₂-AlF₃-Na₃AlF₆ and the subsolidus portion of the system CaF₂-AlF₃-Na₃AlF₆-Al₂O₃ were established from microscopic, powder X-ray diffraction, quench, and DTA data obtained from samples encapsulated in sealed tubes and either reacted in the solid state or melted and recrystallized. The system Na₃AlF₆-CaF₂ contains a simple eutectic with no compound formation or solid solution. The system CaF₂-AlF₃-Na₃AlF₆ contains two ternary compounds, NaCaAlF₆ and NaCaAl₂F₉, which melt incongruently at 735° and 712°C, respectively; NaCaAlF₆ exists in three polymorphic forms with transitions at 610° and 722°C and NaCaAl₂F₉ is body-centered cubic with a₀= 10.765 Å. The three binary and two ternary compounds divide the system into eight compatibility triangles. Along the NaCaAlF₆-Na₃AlF₆ join, 7 mol% NaCaAlF₆ is soluble in α-cryolite at 525° and 42% in β-cryolite at 731°C. The quaternary system Na₃AlF₆-AlF₃-CaF₂-Al₂O₃ contains eight compatibility tetrahedra.     

Crystalline Compounds and Glasses in the System B2O3-NaF-NaBF4

The system B₂O₃-NaF-NaBF, has been studied by subjecting selected compositions to thermal treatment in the range 400° to 600°C. Weight losses, chemical analyses, ir, Raman, and X-ray diffraction techniques were used to define the composition of the crystalline phases and the structural units being formed in the system. The stoichiometry of the BF₃ evolved from NaBF₄-B₂O₃ mixtures indicated that a composition corresponding to Na₂B₃F₅O₃ was formed in mixtures containing up to 33.3 mol% B₂O₃. At higher boron oxide concentrations, Na₂B₃F₅O₃ was consumed, yielding 2NaF.3B₂O₃. The crystalline compounds Na₃B₃F₆O₃, 2NaF.3B₂O₃, and phase B (apparently NaF.B₂O₃) were formed in the system. The compound Na₃B₃F₆O₃ appeared as the stable oxygen-containing species in NaBF₄-NaF mixtures of low oxide content. The main fluorine-containing structural units of the system are BF₄, (–O)₃BF, (–O)₂BF₂, (–O)₂BF, whereas the main structure for binary NaF-B₂O₃ mixtures is (–O)₃BF.     

Phase Diagram of a Portion of the System Na3AlF6-AlF3-Al2O3.

The system Na₃AlF₆-AlF₃-Al₂O₃ was investigated by a combination of quenching, optical microscopy, and X-ray powder diffraction techniques in order to define liquidus temperatures, univariant lines, and invariant points. Phase fields for the primary crystallization of cryolite, chiolite, aluminum fluoride, α-alumina, and η-alumina were located. A ternary peritectic point contained 28.3% AlF₃-4.4% Al₂O₃-67.3% Na₃AlF₆ at 723°C. A eutectic point of composition 37.3% AlF₃-3.2% Al₂O₃-59.5% Na₃AlF₆ occurs at 684°C.     

Volume Relations in Na2O·B2O3 and Na2O·SiO2 B2O3, Melts at 1300·C

Density (and some viscosity) data are presented for binary sodium borate melts containing as much as 60 mole % Na₂O and for ternary sodium silicoborate melts with B/Si <2.0 between 1000°C and 1300°C. The high-temperature partial molar volume analysis of the binary sodium borate melts reveals about 50% BO₄ tetrahedra at the 40 mole % Na₂O composition, in agreement with recent NMR estimates for the binary glasses. No "boron anomaly" was found near 18 mole % Na₂O at high temperature. The synthetic partial molar volume model that agrees best with experiment for all ternary melts studied involves the presence of some BO₄ tetrahedra, the percentage of which varies with composition. This ternary model involves a high degree of internal consistency. No tendency toward extensive micro-immiscibility was observed for ternary melts near the SiO₂·B₂O₃ binary.     

Studies in Germanium Oxide Systems: II, Phase Equilibria in the System Na2O—GeO2

Phase equilibrium relations in the system Na₂O-GeO₂ have been determined using standard quenching techniques supplemented by differential thermal analysis. Two congruently melting compounds, Na₂O·GeO₂ and 2Na₂O·9GeO₂, exist; the melting points are 1103°± 15°C and 1073°± 3°C, respectively. The eutectic temperature between GeO₂ and 2Na₂O·9GeO₂ is 950°±f 10°C at 94.5 wt GeO₂. The eutectic temperature between 2Na₂O · 9GeO₂ and Na₂O·GeO₂ is 790° f 10°C at about 75 wt% GeO₂. Both the refractive index and the density of glasses in the system Na₂O-GeO₂ exhibit maximum values at about 16 to 18 mole % Na₂O. The Ge-O-Ge absorption band at 890 cm⁻¹ shifts toward lower wave numbers with the addition of Na₂O.     

Vaporization Kinetics of Na2SO4 from 900° to 1200°C

Continuous weight-change measurements were used to determine the vaporization kinetics of Na₂SO₄ from 900° to 1200°C in 150 torr O₂. Simple evaporation of the sulfate was indicated. An activation energy for vaporization of 71 kcal/mol was calculated. Vaporization results obtained for Na₂SO₄ on oxide substrates indicated an Na₂SO₄-Cr₂O₃ reaction; however, no Na₂SO₄-ZrO₂ reaction was observed.     

Static-Fatigue Life of Ce-TZP and Si3N4 in a Corrosive Environment

Stress rupture testing was performed in four-point flexure at 1000°C to determine the effects of Na₂SO₄-induced corrosion on the static-fatigue life of a Ce-TZP and a MgO-doped Si₃N₄. The results showed that the static-fatigue life of the Ce-TZP was unaffected by this corrosive environment. However, the static-fatigue life of a MgO-doped Si₃N₄ was reduced by the introduction of Na₂SO₄.     

Sol–Gel Synthesis of λ–Na2O·xAl2O3 Films

Na₂O· x Al₂O₃ ( x = 9, 11)films have been obtained by sol–gel method. Crystallization processes during heat treatments have been investigated by X–ray diffraction analysis. A metastable phase with the mullite structure, λ–Na₂O· x Al₂O₃, has been observed starting from 800°C. Films remained stable after a heat treatment at 1000°C for 250 h. Impedance spectroscopy measurements showed that the films of λ-Na₂O· x Al₂O₃ possess a large three–dimensional ionic conductivity at 400°C.