Studies in the System Li2O–Al2O3–Fe2O3-H2O

Subsolidus equilibrium relations in a portion of the system Li2O-Fe₂O₃-Al₂O3 in the temperature range 500° to 1400°C. have been determined near po₂ = 0.21. Of particular interest in this system is the LiFe₅O₈-LiAl₅O₈ join, which shows complete solid solution above 1180°C. Below this temperature the solid solution exsolves into two spinel phases. At 600°C. approximately 15 mole % of each compound is soluble in the other. The high-temperature solid solution and the low-temperature exsolution dome extend into the ternary system from the 1:5 join. There is no appreciable crystalline solubility of LiFeO₂ or of α-Fe₂O₃ in LiFe₅O₈. An attempt to confirm HFe₅O₈ as the correct formulation of the magnetic ferric oxide "γ-Fe₂O₃" was inconclusive, but in the absence of positive evidence, the retention of γ-Fe₂O₃ is recommended. All the metallic oxides of the Group IV elements increase the temperature of the monotropic conversion of -γ-Fe₂O₃ to α-Fe₂O₃. Silica and thoria have a greater effect on this conversion than does titania or zirconia.     

Subsolidus Equilibria in the System Li2O-Al2O3-Cr2O3

On the join LiCrO₂-LiAlO₂, solid solutions of formulas LiCri- _x Al _x O₂ (0< x <0.6) form. On the spinel join LiAl₅O₈-"LiCr₅O₈," solid solutions of formulas LiAl_{5- x} Al _x O₈ (0< x <3) form and the order-disorder transition, observed in LiAl₅O₈ at 1295°C, occurs at increasingly lower temperatures with increasing Cr content. No evidence was found for the metastable polymorphs of LiAl₅O₈ reported by Datta and Roy. Ternary subsolidus equilibria at 1200° to 1300°C were determined for compositions containing <50% Li₂O.     

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.     

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.     

Studies in Lithium Oxide Systems: XI, Li2O–B2O3–P2O5

The glassforming region in the system was roughly outlined and liquidus data were obtained for the three joins LiPO₃-BPO₄, Li₄P₂O₇-BPO₄, and Li₃PO₄-Li₂B₄O₇. Compatibility relations for the ternary subsystems Li₄P₂O₇-BPO₄-P₂O₅ and Li₂O-Li₃PO₄-Li₂B₈O₁₃ were established. Two ternary compounds with the probable compositions 22Li₂O - 11B₂O₃ - 13P₂O₅ and 2Li₂O 3B₂O₃ P₂O₅ were detected.     

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.     

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.     

Mechanical Activation of Heterogeneous Sol–Gel Precursors for Synthesis of MgAl2O4 Spinel

MgAl₂O₄ spinel precursor was prepared using a heterogeneous sol–gel process. The effect of high-energy milling on the precursor decomposition and spinel formation was investigated. The milling decreased the Al(OH)₃ dehydroxylation temperature from 190° to about 130°C. The activation energy for spinel formation decreased from 688 kJ/mol for the as-prepared precursors to 468 kJ/mol for the precursors milled for 5 h. Milling of the precursor lowered the incipient temperature of spinel formation from 900° to 800°C, and the temperature of complete MgAl₂O₄ spinel formation from >1280° to ∼900°C.     

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.     

Studies in Lithium Oxide Systems: II, Li2O Al2O3–Al2 O 3

Solid-state reactions between Li₂O and Al₂ O₃ were studied in the region between Li₂O.Al₂ O ₃ and Al₂ O ₃. The compound Li₂ O Al₂ O ₃ melts at 1610°± 15°C. and undergoes a rapid reversible inversion between 1200° and 1300°C. Vaporization of Li₂ O from compositions in the system proceeds at an appreciable rate at 1400°C, as shown by fluorescence. Lithium spinel, Li₂ O -5Al₂O₃, was the only other compound observed. The effect of Li₂ O on the sintering of alumina was investigated.     

Phase Transformation Kinetics in the Doped System LiAl5O8—LiFe5O8

Solid solutions of various compositions in the system LiAl₅O₈–LiFe₅O₈ were prepared by calcining mixtures of Li₂CO₃, Al₂O₃, and Fe₂O₃. Lattice parameters of the solid solutions were determined by X-ray diffraction (XRD). Samples of equimolar composition were also prepared with titanium and magnesium as dopants. The doped and undoped samples were annealed at 1050°C for up to 192 h in air. Annealing at this temperature, which is inside the miscibility gap for an equimolar composition, led to the decomposition of the originally single-phase solid solution. The extent of phase transformation was followed by XRD. The fraction transformed was estimated using integrated peak intensities. The kinetic data were analyzed in light of the conventional kinetic equation and the previously derived equation which takes into account the diffusive and the interface transfer processes. It was observed that titanium enhances and magnesium suppresses the kinetics of phase separation. These results are rationalized on the premise that the predominant point defects are Schottky defects in this spinel system.     

Studies in Lithium Oxide Systems: V, Li2O-Li20 B2O3

Nine compositions containing 40 to 68% B₂O₃ were used to study the high-lithia portion of the system Li₂O-B₂O₃ by quenching and differential thermal analysis methods. The compounds 3Li₂O 2B₂O₃ and 3Li₂O B₂O₃ melted incongruently at 700°± 6°C, and 715°± 15°C., respectively. The compound 2Li₂O B₂O₃ is assumed to dissociate slightly below 650°± 15° C., although the data could also be interpreted as in-congruent melting. Below 600°± 6°C. it does dissociate to the 3:2 and 3:1 compounds. In this narrow temperature interval the 2:1 compound had an inversion at 618°± 6°C. Both forms of the 2:1 compound could be quenched to room temperature. X-ray diffraction data for the compounds are tabulated, and the complete phase diagram for the system Li₂O-B₂O₃ is presented.     

Phase Relations in the System Li2O.B2O3-B2O3-NiO

Phase equilibria were determined by standard quench methods in binary systems NIO-B₂O₃, the binary join Li₂O.B₂O₃-NiO, and three other sections through the Li₂O.B₂O₃-B₂O₃-NiO system. The only new compound observed was 2NiO.B₂O₃, which is stable from 1303° to 1480°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.     

Influence of V2O5 Additions to 5Li2O–1Nb2O5–5TiO2 Ceramics on Sintering Temperature and Microwave Dielectric Properties

The effects of the addition of V₂O₅ on the sintering behavior, microstructure, and microwave dielectric properties of 5Li₂O–1Nb₂O₅–5TiO₂ (LNT) ceramics have been investigated. With low-level doping of V₂O₅ (≤3 wt%), the microstructure of the LNT ceramic changed from a special two-level intergrowth structure into a two-phase composite structure with separate grains. And the sintering temperature of the LNT ceramics could be lowered to around 900°C by adding a small amount of V₂O₅ without much degradation in microwave dielectric properties. Typically, better microwave dielectric properties of ɛ_r=41.7, Q × f =7820 GHz, and τ _f =45 ppm/°C could be obtained for the 1 wt% V₂O₅-doped ceramics sintered at 900°C.     

Studies in Lithium Oxide Systems: XII, Li2O-B2O3-Al2O3

Tentative phase relations in the binary system BnOa-A1₂O₃ are presented as a prerequisite to the understanding of the system Li₂O-B₂O₃-Al₂O₃. Two binary compounds, 2A1₂O₃.B₂O₃ and 9A1₂O₃.-2B₂O₃, melted incongruently at 1030° f 7°C and about 144°C, respectively. Two ternary compounds were isolated, 2Li₂O.A1₂O₃.B₂O₃ and 2Li₂O. 2AI₂O₃. 3B₂0₃. The 2:1:1 compound gave a melting reaction by differential thermal analysis at 870°± 20° C, but the exact nature of the melting behavior was not determined. The 2:2:-3 compound melted at 790°± 20° C to Li_zO.-5Al₂O₃ and liquid. X-ray diffraction data for the compounds are presented and compatibility triangles are shown.     

Studies in Lithium Oxide Systems: IX, Li2o-Al2O3-TiO2

Compatible phases in the system Li₂O-Al₂O₃-TiO₂ at various temperature levels were determined mainly by solid-state reactions for the portion of the ternary system bounded by Li₂O Al₂O₂, Li₂O.TiO₂, Al₂O, and TiO₂. The existence of a ternary compound, Li₂O.Al₂O₃.4TiO₂, and nine joins was established. The ternary compound has a lower limit of stability at 1090°± 15°C. and dissociates and recombines rapidly at 1380°± 15°C.     

Subsolidus Phase Relations in the Ga2O3-In2O3-SnO2 System

Subsolidus phase relationships in the Ga₂O₃–In₂O₃–SnO₂ system were studied by X-ray diffraction over the temperature range 1250–1400°C. At 1250°C, several phases are stable in the ternary system, including Ga₂O₃( ss ), In₂O₃( ss ), SnO₂, Ga_{3− x} In_{5+ x} Sn₂O₁₆, and several intergrowth phases that can be expressed as Ga_{4−4 x} In_{4 x} Sn _{n −4}O_{2 n −2} where n is an integer. An In₂O₃–SnO₂ phase and Ga₄SnO₈ form at 1375°C but are not stable at 1250°C. GaInO₃ did not form over the temperature range 1000–1400°C.     

Phase Diagram for a Portion of the System Li2O-Nd2O3-P2O5

In the ternary system Li₂O-Nd₂O₃-P₂0₅, part of the phase diagram relevant to the growth of single LindP₄O₁₂ (LNP) crystals was examined. LNP melts incongruently and decomposes into NdP₃O₉ and liquid at the peritectic temperature of 970°C. For the crystal growth, an Li₂O-P₂O₅ mixture should be used as a flux. The melt compositions from which LNP nucleates were clarified.     

Molten Salt Synthesis of Magnesium Aluminate (MgAl2O4) Spinel on Ti3AlC2 Substrate

A MgAl₂O₄ (MA) spinel layer was synthesized on Ti₃AlC₂ substrate through the molten salt synthesis (MSS) method. The Ti₃AlC₂ substrate was immersed in MgCl₂·6H₂O powders and treated at 800°, 850°, and 900°C for 4 h in air. A continuous and 10-μm-thick MgAl₂O₄ layer was obtained at 900°C, by which the surface hardness of Ti₃AlC₂ can be effectively improved. The combined scanning electron microscopy observations and crystal morphology simulation further revealed that the as-formed MgAl₂O₄ presents tetragonal bipyramids morphology with (400)-orientation.