Grain Growth Control and Solid-State Crystal Growth by Li2O/PbO Addition and Dislocation Introduction in the PMN–35PT System

Grain growth behavior and solid-state single crystal growth (SSCG) in the Pb(Mg_1/3Nb_2/3)O₃–35 mol% PbTiO₃ (PMN–35PT) system have been investigated with varying Li₂O/PbO ratios. The effect of dislocation density on crystal growth has also been studied. For SSCG, a BaTiO₃ single-crystal seed was embedded in a polycrystalline PMN–PT matrix. During annealing, a PMN–PT single crystal grew from the seed at the cost of the small matrix grains. Addition of Li₂O dopant first enhanced and then reduced abnormal grain growth in the matrix. In the 2 mol% Li₂O and 6 mol% PbO excess PMN–PT samples annealed at 1200°C, considerable single-crystal growth occurred without formation of abnormally large grains in the matrix. Increasing the dislocation density in the BaTiO₃ seed crystal resulted in enhanced growth of single crystals. These results were explained in terms of interface reaction-controlled nucleation and growth, based on crystal growth theories.     

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.     

Microstructures and Piezoelectric Properties in the Li2O-Excess 0.95(Na0.5K0.5)NbO3–0.05LiTaO3 Ceramics

As a candidate for lead-free piezoelectric materials, Li₂O-excess 0.95(Na_0.5K_0.5)NbO₃–0.05LiTaO₃ (NKN–5LT) ceramics were developed by a conventional sintering process. The sintering temperature was lowered by adding Li₂O as a sintering aid. Abnormal grain growth in NKN–5LT ceramics was observed with varying Li₂O content. This grain-growth behavior was explained in terms of interface reaction-controlled nucleation and growth. In the 1 mol% Li₂O excess NKN–5LT samples sintered at 1000°C for 4 h in air, the electromechanical coupling factor and the piezoelectric constant of NKN–5LT ceramics were found to reach the highest values of 0.37 and 250 pC/N, respectively.     

Abnormal Grain Growth of Pb(Mg1/3Nb2/3)O3-35 mol% PbTiO3 Ceramics Induced by the Penetration Twin

Pb(Mg_1/3Nb_2/3)O₃-35 mol% PbTiO₃ (PMN-35PT) specimens with a 5 mol% excess PbO were prepared by excessive heat treatment at 1150°C to induce abnormal grain growth. Through electron backscatter diffraction analysis and the observation of a three-dimensional morphology, the abnormally grown PMN-35PT grains were found to be twinned crystals with penetration characteristics. The morphology of the PMN-35PT twinned crystal was crystallographically analyzed. The abnormal grain growth of PMN-35PT is suggested to be due to preferential growth at the reentrant angles formed by twins.     

Pseudobinary Phase Relations of Li2Ti3O7

The Li₂O-TiO₂ pseudobinary phase diagram was determined from 50 to 100 mol% TiO₂ by DTA, microscopy, and X-ray analysis; Li₂Ti₃O₇ effectively melts congruently at 1300° and decomposes eutectoidally at 940°C. A solid solution based on Li₂TlO₃ from 50 to ∼65 mol% TiO₃ was observed to exist at >930°C. A new metastable phase was discovered with a composition of ∼75 mol% TiO₂ and with a hexagonal unit cell (8.78 by 69.86 × 10⁻¹nm). Discrepancies in the literature regarding some of these phase equilibria are reconciled.     

The Role of Li2CO3 and PbO in the Low-Temperature Sintering of Sr, K, Nb (SKN)-Doped PZT

It is demonstrated that the use of ∼0.9 mol% Li₂CO₃ (LC) as a sintering aid for Sr, K, Nb modified Pb(Zr_{1− x} ,Ti _x )O₃ (PZT) ceramics is effective only in the presence of excess PbO (∼2 mol%). It is shown that LC and PbO react to form the compound, Li₂PbO₃ (LPO) which has a melting temperature of ∼836°C. Using dilatometry, we were able to correlate shrinkage during heating of a green ceramic to the melting of the LPO. Consequently, complete densification and sizeable grain growth are achieved by solution-precipitation of the ceramic through the liquid phase. Importantly, sintering is not particularly effective with such small additions of either LC or PbO alone. In confirmation of this model, the LPO compound was presynthesized and used as the only sintering aid in the same PZT composition. The densification behavior of this mixture compared well with the case of separate additions.     

Lithium Ion-Conducting Glass–Ceramics of Li1.5Al0.5Ge1.5(PO4)3–xLi2O (x=0.0–0.20) with Good Electrical and Electrochemical Properties

NASICON-type structured Li_1.5Al_0.5Ge_1.5(PO₄)₃– x Li₂O Li-ion-conducting glass–ceramics were successfully prepared from as-prepared glasses. The differential scanning calorimetry, X-ray diffraction, nuclear magnetic resonance, and field emission scanning electron microscope results reveal that the excess Li₂O is not only incorporated into the crystal lattice of the NASICON-type structure but also exists as a secondary phase and acts as a nucleating agent to considerably promote the crystallization of the as-prepared glasses during heat treatment, leading to an improvement in the connection between the glass–ceramic grains and hence a dense microstructure with a uniform grain size. These beneficial effects enhance both the bulk and total ionic conductivities at room temperature, which reach 1.18 × 10⁻³ and 7.25 × 10⁻⁴ S/cm, respectively. In addition, the Li_1.5Al_0.5Ge_1.5(PO₄)₃–0.05Li₂O glass–ceramics display favorable electrochemical stability against lithium metal with an electrochemical window of about 6 V. The high ionic conductivity, good electrochemical stability, and wide electrochemical window of LAGP–0.05LO glass–ceramics suggest that they are promising solid-state electrolytes for all solid-state lithium batteries with high power density.     

Electrical Properties of Sr0.7Ba0.3TiO3 Ceramics Doped with Nb2O5, 3Li2O · 2SiO2, and Bi2O3

The electrical properties of Sr_0.5Ba_0.3TiO₃ in the presence of Nb₂O₅ as a donor, 3Li₂O · 2SiO₂ as a sintering agent, and Bi₂O₃ as a dopant have been studied. When the compositions of the ceramics were 1 mol Sr_0.7Ba_0.3TiO₃+ 0.5 mol% Nb₂O₅+ 2 mol% 3Li₂O · 2SiO₂+ 0.2 mol% Bi₂O₃, the ceramics were sintered at 1100°C and exhibited the following characteristics: apparent dielectric constant ɛ, 25000; loss factor tan δ, 2%; insulating resistivity ρ_j, 10¹⁰Ω· cm; variation of dielectric constant with temperature Δɛ/ɛ (−25° to +85°C), +10%, −14%. ɛ and tan δ show only small changes with frequency. The study shows this ceramic can be used in multilayer technology.     

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: 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.     

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.     

Low-Temperature Sintering of (Ba0.6Sr0.4)TiO3

An addition of just 0.4 wt% Li₂O to (Ba_0.6Sr_0.4)TiO₃ powder was able to reduce the sintering temperature to ≤900°C and produce ceramics with a relative density of 97%. Small amounts of two secondary phases were formed during this process: Li₂TiO₃ and Ba₂TiO₄. The addition of Li₂O depresses the ferroelectric character of the (Ba_0.6Sr_0.4)TiO₃ and, as a result, reduces the permittivity, improves the temperature coefficient of permittivity, and reduces the dielectric losses. The tunability shows no significant variation with Li₂O concentration and remains between 16.5% and 13.5%. A low-temperature sintering mechanism was proposed. The mechanism involves the intermediate formation of BaCO₃, its melting and the incorporation of Li⁺ into the BST. The sintering mechanism can be characterized as reactive liquid-phase sintering.     

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.     

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.     

Densification of Si3N4 with LiYO2 Additive

This paper deals with the densification and phase transformation during pressureless sintering of Si₃N₄ with LiYO₂ as the sintering additive. The dilatometric shrinkage data show that the first Li₂O- rich liquid forms as low as 1250°C, resulting in a significant reduction of sintering temperature. On sintering at 1500°C the bulk density increases to more than 90% of the theoretical density with only minor phase transformation from α-Si₃N₄ to β-Si₃N₄ taking place. At 1600°C the secondary phase has been completely converted into a glassy phase and total conversion of α-Si₃N₄ to β-Si₃N₄ takes place. The grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. Li₂O evaporates during sintering. Thus, the liquid phase is transient and the final material might have promising mechanical properties as well as promising high-temperature properties despite the low sintering temperature. The results show that the Li₂O−Y₂O₃ system can provide very effective low-temperature sintering additives for silicon nitride.     

Microwave Dielectric Properties of CaTiO3-CaAl1/2Nb1/2O3 Ceramics Doped with Li3NbO4

The microwave dielectric properties of CaTi_{1− x} (Al_1/2Nb_1/2) _x O₃ solid solutions (0.3 ≤ x ≤ 0.7) have been investigated. The sintered samples had perovskite structures similar to CaTiO₃. The substitution of Ti⁴⁺ by Al³⁺/Nb⁵⁺ improved the quality factor Q of the sintered specimens. A small addition of Li₃NbO₄ (about 1 wt%) was found to be very effective for lowering sintering temperature of ceramics from 1450–1500° to 1300°C. The composition with x = 0.5 sintered at 1300°C for 5 h revealed excellent dielectric properties, namely, a dielectric constant (ɛ_r) of 48, a Q × f value of 32 100 GHz, and a temperature coefficient of the resonant frequency (τ_f) of −2 ppm/K. Li₃NbO₄ as a sintering additive had no harmful influence on τ_f of ceramics.     

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.     

Effect of Grain Coalescence on the Abnormal Grain Growth of Pb(Mg1/3Nb2/3)O3-35 mol% PbTiO3 Ceramics

Abnormal grain growth (AGG), which occurred during the heat treatment of Pb(Mg_1/3Nb_2/3)O₃-35 mol% PbTiO₃ (PMN-35PT) with excess PbO, was investigated. AGG has been suggested to be the consequence of grain coalescence that results in the formation of Σ3 coincidence site lattice and low angle grain boundaries. Because of reentrant edges appearing at the ends of these boundaries, the coarsening rate of grains was significantly enhanced and AGG occurred.     

Studies in Germanium Oxide Systems: I, Phase Equilibria in the System Li2O—GeO2

Phase equilibrium relations in the system Li₂O-GeO₂ were determined using standard quenching techniques. In contrast to published literature five congruently melting compounds were found to exist. They are Li₂O·7GeO₂, 3Li₂O O·8GeO₂, Li₂O O·GeO₂, 3Li₂O O·2GeO₂, and 2Li₂O.-GeO₂. The melting points, respectively, are 1033°± 5°C, 953°± 5°C, 1245°± 15°C, 1125°± 15°C, and 1280°± 15°C. Simple binary eutectic relations exist among the compounds. The eutectic temperature between 1:7 and GeO₂ is 1025°± 1h0°C at about 96.8 wt% GeO₂; the eutectic temperature between the 1:7 and 3:8 compounds is 935°± 10°C at about 90.9 wt% GeO₂; the eutectic temperature between the 3:8 and 1:1 compounds is 930°± 10 °C at about 89.8 wt% GeO₂. Liquidus data for compositions richer in lithia than the 1:1 compound are only approximate because of the difficulty of quenching them; the phase relations between the 1:1 and 3:2 and between the 3:2 and 2:l compounds, however, are found to be of the simple binary eutectic type. The glass–forming region was also determined. Melts allowed to cool in air crystallized. When, however, the melts were quenched, glasses containing as much as 8 wt% GeO₂ could be prepared in 5–g quantities. Both the refractive index–composition and density–composition curves for the glasses showed maxi–mums at about 6 to 8 wt% Li₂O.     

Sintering and Crystallization of a Glass Powder in the Li2O–ZrO2–SiO2 System

Sintering and crystallization of a 23.12 mol% Li₂O, 11.10 mol% ZrO₂, 65.78 mol% SiO₂ glass powder was investigated. By means of thermal shrinkage measurements, sintering was found to start at about 650°C and completed in a very short temperature interval (Δ T similar/congruent 100°C) in less than 30 min. Crystallization took place just after completion of sintering and was almost complete at about 900°C in 20 min. Secondary porosity prevailed over the primary porosity during the crystallization stage. The glass powder compacts first crystallized into lithium metasilicate (Li₂SiO₃), which transformed into lithium disilicate (Li₂Si₂O₅), zircon (ZrSiO₄), and tridymite (SiO₂) after the crystallization process was essentially complete. The microstructure was characterized by fine crystals uniformly distributed and arbitrarily oriented throughout the residual glass phase.