Structure of B2O3 and Binary Aluminoborate Melts at 1600°C

Viscosity and density data were obtained up to 1700°C for a series of binary aluminoborate melts that contained as much as 15 mole% (∼21 wt%) Al₂O₃ and up to 1620°C for pure molten B₂O₃. Large expansion coefficient decreases and a slight activation energy increase for B₂O₃ above 1400°C suggested a tightening of its structure. The addition of Al₂O₃ reduced viscosity and increased activation energy. The decreased compositional dependence of molar volume (compared to SiO₂ additions) and the increased expansion coefficients accompanying Al₂O₃ additions suggested a loosening of the O—B—O structure at 1600°C. Molar volume deviations from ideality were similar to but smaller than those for SiO₂ and GeO₂ additions at 1300°C. Microclustering of aluminum-bearing polyhedra appeared to occur at slightly higher boron atom contents than with SiO₂ and GeO₂ additions.     

Structure of Glasses and Melts in the Na2O·GeO2·B2O3 System

Density and viscosity results are presented for ternary Na₂O·GeO₂·B₂O₃ melts (∼600° to 1300°C) and glasses containing as much as 35 mole % Na₂O. Synthetic partial molar volume models indicate a fairly broad stability region for BO₄ tetrahedra in the B₂O₃-rich melts. Similar models for GeO₂-rich melts reveal a more limited stability region for GeO₆ octahedra. The expansion coefficient contours and viscosity isotherms confirm the volume-based conclusions for the liquid state. The high-temperature volume models were used to develop glass volume models that agree to within several percent of experiment. It has been concluded that the melts and glasses possess similar structures. The relatively greater compositional stability of GeO₆ octahedra in the presence of B₂O₃ (compared to Al₂O₃) can be related to the smaller average number of oxygens around boron (III), at a fixed O/Ge ratio, compared to aluminum (III). Evidence is presented for a slight decrease of the thermal stability of GeO₆ octahedra in the GeO₂-rich melts above about 1000°C.     

The System SiO2–P2O2

Phase relations in the binary system between SiO₂-P₂O₅ and SiO₂ were investigated by the quenching method using sealed platinum tubes to prevent the loss of P₂O₅. The compound Si0₂-P₂O₅ exists in two forms, the low-temperature β form inverting sluggishly but reversibly to the high-temperature β form at 1030°C. The β form melts congruently at 1290°C. The compound 2SiO₂-P₂O₅ melts incongruently at 1120°C to a silica-rich liquid and SiO_a-P₂O₅. In the region between 5 and 25 mole % PO₂, reactions were so sluggish that no data could be obtained by quenching.     

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.     

Immiscibility and the System Lanthanum Oxide–Boric Oxide

The phase equilibrium diagram for the system La₂O₃-B₂O₃ has been determined experimentally. The compounds La₂O₃-3B₂O₃and La₂O₃-B₂O₃ melt congruently at 1141°± 5°C. and 1660°± 15°C, respectively. At 1488°± 5°C, La₂O₃-B₂O₃ inverts from the aragonite-type structure to a high-temperature form. Trilanthanum borate, 3La₂O₃ B₂O₃, melts incongruently at 1386°± 5°C. to give liquid and La₂O₃. No solid solutions exist in the system. A region of liquid immiscibility exists in the system and extends at 1136°± 5°C. from almost pure B₂O₃ to 21.5 mole % La₂O₃. The experimental value for the extent of immiscibility agrees with that calculated from theoretical considerations. A second method for estimating immiscibility in the system is demonstrated, which requires experimentally only the determination of the index of refraction of the modifier-rich liquid. Principles governing immiscibility are discussed.     

Viscosity-Temperature Relation for Network Liquids

The viscosity-temperature relation for the network liquids is considered in the light of the results for associated liquids and polymers. A critical examination of all the data available indicates that for liquid B₂O₃, SiO₂, and BeF₂ the energy of activation for flow is temperature dependent. Liquid GeO₂ is possibly an anomaly. Empirical equations which satisfactorily describe the viscosity of associated liquids are not applicable to liquid B₂O₃ and SiO₂.     

Revised Phase Diagram for the System Al2O3—SiO2

On the basis of 190 runs made up to 1860°C in sealed noble-metal containers the following revisions have been made in the equilibrium diagram for the system A1₂O₃–SiO₂. Mullite melts congruently at 1850°C. The extent of equilibrium solid solution in mullite at solidus temperature is from approximately 60 mole % Al₂O₃ (3/2 ratio) to 63 mole % A1₂O₃. Metastable solid solutions can be prepared up to about 67 mole % Al₂O₃. There is no evidence for stable solubility of excess SiO₂ beyond the 3/2 composition at pressures below 3 kbars. Refractive indices are presented for glasses containing up to 60 mole % Al₂O₃ and from them the composition of the eutectic is confirmed at 5 mole % SiO₂. The variation in lattice constants of the mullite solid solution is not an unequivocal guide to composition since mullites at one composition produced at different temperatures show differences in spacing, no doubt reflecting Al-Si ordering phenomena. The possibility of quartz and corundum being the stable assemblage at some low temperatures and pressures cannot be ruled out. A new anhydrous phase in the system is described, which was previously thought to be synthetic andalusite; it is probably a new polymorph of the Al₂SiO₅ composition with ortho-rhombic unit-cell dimensions a =7.55 A, b =8.27 A, and c = 5.66 A.     

A New High-pressure Form of B2O3 and Inferences on Cation Coordination from Infrared Spectroscopy

A new crystalline variety of B₂O₃ has been prepared at pressures above 22,000 atmospheres and above 400°C. The properties are listed. These moderate pressures also are sufficient to catalyze the crystallization of the more common hexagonal form from B₂O₃ glass. Pressures, however, of up to 50,000 atmospheres will not convert GeO₂ quartz to GeO₂ rutile at room temperature. From the infrared absorption patterns of appropriate phases it can be shown that in coesite the tetrahedral coordination of Si⁴⁺ (or Be²⁺ in the model BeF₂-coesite phase) must be unchanged. In the new form of B₂O₃, however, the coordination number must be substantially different from that in the common hexagonal B₂O₃.     

Low-Temperature Sintering and Microwave Dielectric Properties of the Zn2SiO4 Ceramics

B₂O₃ was added to nominal composition Zn_1.8SiO_3.8 (ZS) ceramics to decrease their sintering temperature for application to low-temperature cofired ceramic (LTCC) devices. B₂O₃ reacted with SiO₂ to form a liquid phase containing SiO₂ and B₂O₃. The composition and melting temperature of the liquid phase depended on the sintering temperature and the B₂O₃ content. The specimen containing 20.0 mol% of B₂O₃ sintered at 900°C exhibited high microwave dielectric properties of Q × f =53 000 GHz, ɛ _r=5.7, and τ_f=−16 ppm/°C, confirming the promising potential of the B₂O₃-added ZS ceramics as candidate materials for the LTCC devices.     

Unusual Immiscibility Structures in Tellurite Glasses

Unusual droplet microaggregates are observed in TeO₂-rich glasses of the systems TeO₂–B₂O₃, TeO₂–GeO₂, TeO₂–B₂O₃–GeO₂, and TeO₂–GeO₂–V₂O₅. A decrease of the TeO₂ content is established in the aggregates in comparison with the matrix. Their appearance is related to the process of metastable liquid-phase separation at high viscosity of the melts.     

Sintering and Microwave Dielectric Properties of the LiNb0.63Ti0.4625O3 Ceramics with the B2O3–SiO2 Liquid-Phase Additives

The effect of B₂O₃–SiO₂ liquid-phase additives on the sintering, microstructure, and microwave dielectric properties of LiNb_0.63Ti_0.4625O₃ ceramics was investigated. It was found that the sintering temperature could be lowered easily, and the densification and dielectric properties of LiNb_0.63Ti_0.4625O₃ ceramics could be greatly improved by adding a small amount of B₂O₃–SiO₂ solution additives. No secondary phase was observed for the ceramics with B₂O₃–SiO₂ additives. With the addition of 0.10 wt% B₂O₃–SiO₂, the ceramics sintered at 900°C showed favorable microwave dielectric properties with ɛ_r=71.7, Q × f =4950 GHz, and τ_f=−2.1 ppm/°C. The energy dispersive spectra analysis showed an excellent co-firing interfacial behavior between the LiNb_0.63Ti_0.4625O₃ ceramic and the Ag electrode. It indicated that LiNb_0.63Ti_0.4625O₃ ceramics with B₂O₃–SiO₂ solution additives have a number of potential applications on passive integrated devices based on the low-temperature co-fired ceramics technology.     

Phase Relations and Glass Formation in the System PbO-GeO,

Phase relations in the system PbO-GeO₂ were determined using the quenching technique. The five compounds detected were: 4PbO-GeO₂, 3PbO-2GeO₂, PbO-GeO₂, and PbO-4GeO₂. The 3:2 and 1:1 compounds melt congruently at 744° and 799°, respectively. The 4:1 compound melts incongruently at 726°C to PbO plus liquid, whereas the 1:4 compound melts incongruently to GeO₂ plus liquid at 790°C. The 1:2 compound has a temperature range of stability between 707° and 730°. The data indicate that no liquid immiscibility gap exists in the system. Indices of refraction for glasses in the system were compared with lead silicate glasses. An addition of ∼65%PbO to GeO₂ is required to prepare a glass with an index near 2.0 whereas with SiO₂, ∼85% PbO is required. It appears that the lead germanate glasses have higher indices than all other two-component oxide glasses. The addition of PbO to GeO₂ decreases the rutile-to-quartz transformation temperature from 1000°C for pure GeO₂ to 990°C. Infrared spectra of lead germanate glasses (∼60w% PbO) show that transmission is good up to 5.5μ but decreases drastically between 5.5 and 6.5μ.     

Phase Equilibria in the System MgO-B2O3

Phase equilibria in the system MgO-B₂O₃ were investigated using DTA and quenching techniques. The system contains 4 invariant points. The compounds MgO·2B₂O₃ and 2MgO·B₂O₃ melt incongruently at 995° and 1312°C, respectively, whereas 3MgO·B₂O₃ melts congruently at 1410°C. A eutectic occurs at 1333°C and 71% MgO.     

Studies in Lithium Oxide Systems: VII, Li2O–B2O2–SiO2

Phase relations in the system Li₂O–B₂O₃–SiO₂ were studied by quenching and solid-state reactions. No ternary compounds were detected in the portion of the system containing less than 53% Li₂O. Compatibility triangles were formed from the binary borate and silicate compounds. Liquidus data obtained by quenching are reported for four joins, Li₂O·2SiO₂–Li₂O·2B₂O₃, Li₂O·SiO₂-Li₂O·2B₂O₃, Li₂O·SiO₂-Li₂O·B₂O₃, and Li₂O·2B₂O₃-SiO₂. The last join cuts across the two-liquid region and is not a true binary system. Some probable ternary invariant points were located in the portion of the system which was quenchable to glass and adjacent to the two-liquid region. Further data on the previously reported immiscible liquid formation are given and the significance is discussed. Data on the thermal expansion behavior of certain glasses are presented.     

Study of Ordering in High-Quartz Solid Solutions by Substitutions Affecting Superlattice Reflections

Ordering of the interstitial and network atoms in stuffed high-quartz solid-solution ( ss ) phases was studied by correlating suitable substitutions in the structure with the resultant changes in the superlattice reflections arising from these phases. In an MgO·Al₂O₃·3SiO₂ high-quartz ss , the special ordering of the interstitial Mg atoms, which produces satellite reflections around superlattice positions, was disturbed by replacement of ∼20 mol% of the MgO by Li₂O or ZnO. The interstitial atoms were further disordered by larger substitutions. The streaked superlattice reflections arising from an Li₂O·Al₂O₃·2SiO₂ high-quartz ss disappeared on replacement of ∼40 mol% of the Li₂O by MgO. This result implies that the interstitial Li atoms are ordered and were disordered by the substitution. The superlattice reflections were much intensified when ∼25 mol% of the SiO₂ was replaced by GeO₂. However, these reflections were intensified to a lesser degree when presumably both SiO₂ and Al₂O₃ were partially replaced by GeO₂ and Ga₂O₃, respectively. These observations strongly suggest an ordering of the network Si and Al atoms, which, together with the ordering of the interstitial Li atoms, contributed to the original streaked superlattice reflections.     

Liquid Immiscibility in the System K2O-B2O3-SiO2

The subliquidus miscibility gap in the system K₂O-B₂O₃-SiO₂ has been determined for compositions with molar ratios SiO₂/B₂O₃<2 and T≥550°C. The shape of the miscibility gap is an elongated dome similar in form to, but less extensive than those in the lithium and sodium borosilicate systems. The consolute composition (molar) and temperature are estimated to be 4 ± 1 K₂O -30±8 B₂O₃-66±8 SiO₂ and 629±5°C, respectively .     

Surface Tension of Some Liquid Oxides and Their Temperature Coefficients

Surface-tension data are reported for liquid Al₂ O₃, B₂ O₃, GeO₂, P₂ O₅, and SiO₂. Abnormal positive temperature coefficients for B₂ O₃, GeO₂, and SiO₂ are shown to be due mainly to changes in the liquid structure (dissociation) with temperature.     

Phase Relations in the System Boron Oxide–Silica

The suggested phase diagrams for the system B₂O₃-SiO₂ as drawn from the findings of previous investigators are presented. These diagrams show the wide controversy regarding this system. The principal difficulties in obtaining reliable equilibrium data are the volatilization of B₂O₃, the hydration of B₂O₃-rich glasses, and the great viscosity of B₂O_rSiO_z melts. In this investigation firings were made in sealed platinum capsules. Where necessary, they were opened and studied under xylene to eliminate hydration. Phases were identified by optical and X-ray methods. The liquidus was obtained by determination of the temperatures at which the stable phases of SiO₂ completely dissolved in the borosilicate melts. The criteria used for equilibrium are discussed.     

Energetics of La2O3–HfO2–SiO2 Glasses

A series of La₂O₃–HfO₂–SiO₂ glasses, approximately along the join 0.73SiO₂–0.27( x HfO₂–(1− x )La₂O₃), 0< x <0.3), was prepared using containerless processing techniques (aerodynamic levitation combined with laser heating in oxygen). The enthalpy of formation and enthalpy of vitrification at 25°C were obtained from drop solution calorimetry of these glasses and appropriate crystalline compounds in a molten lead borate (2PbO–B₂O₃) solvent at 702°C. The enthalpy of formation from crystalline oxides was exothermic and became less exothermic with increasing HfO₂ content. Heat contents were measured by transposed temperature drop calorimetry and depended linearly on the HfO₂ content. Differential scanning calorimetry showed that both the onset glass transition and the onset crystallization temperature of these glasses increased with increasing HfO₂ content. Upon slow cooling in air, the glasses crystallized to a mixture of baddeleyite, cristobalite, lanthanum disilicate, and hafnon.     

Density, Surface Tension, and Viscosity of PbO-B2O3-SiO2 Glass Melts

The density, surface tension, and viscosity of the melts from the PbO-B₂O₃-SiO₂ system have been measured at temperatures in the range 1073–1473 K. The effect of composition on these properties was also investigated. The density of the melt was found to increase linearly with increasing PbO content. Molar volume was derived from the density data, and its deviation from the additivity of partial molar volumes was calculated. These deviations in molar volume from those obtained from additivity rules have been used along with the ratio of various coordination numbers of boron (as reported by Bray) to discuss the structure of the melts. The surface tension was found to decrease with decreasing SiO₂/B₂O₃ ratio, and to increase in the range of the PbO content between 30 and 60 mol%, showing a maximum at ∼60 mol% PbO, and then decreased with further additions. This result suggested that the surface tension would be affected primarily by the B₂O₃ content in the range of the PbO content between 30–60 mol%, and mainly by the PbO content in the range of the PbO content >60 mol%, respectively. The viscosity of the melt was found to decrease linearly with increasing PbO content. The results obtained indicate that the increase in viscosity with B₂O₃ was half that of SiO₂ (on a molar basis), and an empirical equation has been proposed for the viscosity as a function of mole fraction.