Low-Temperature Sintering of K0.5Na0.5NbO3 Ceramics

The objective of this work was to lower the sintering temperature of K_0.5Na_0.5NbO₃ (KNN) without reducing its piezoelectric properties. The KNN was sintered using 0.5, 1, 2, and 4 mass% of (K, Na)-germanate. The influence of the novel sintering aid, based on alkaline germanate with a melting point near 700°C, on the sintering, density, and piezoelectric properties of KNN is presented. The alkaline-germanate-modified KNN ceramics reach up to 96% of theoretical density at sintering temperatures as low as 1000°C, which is approximately 100°C less than the sintering temperature of pure KNN. The relative dielectric permittivity (ɛ/ɛ₀) and losses (tanδ), measured at 10 kHz, the piezo d ₃₃ coefficient, the electromechanical coupling and mechanical quality factors ( k _p, k _t, Q _m) of KNN modified with 1 mass% of alkaline germanate are 397, 0.02, 120 pC/N, 0.40, 0.44, and 77, respectively. These values are comparable to the best values obtained for KNN ceramics sintered above 1100°C.     

Characteristics of Si3N4-SiO2-Ce2O3 Compositions Sintered in High-pressure Nitrogen

Full-density Si₃N₄-SiO₂-Ce₂O₃ compositions were prepared by sintering with 2.5 MPa nitrogen pressure at temperatures of 1900° and 2090°C. Room-temperature flexural strengths near 700 MPa for sintered material compared favorably with the strength of hot-pressed material. At 1370°C, where flexural strengths as high as 363 MPa were obtained, it was observed that the coarsest structure was the strongest and the finest structure was the weakest. One of the compositions tested, Si₃N₄-8.7 wt% SiO₂-8.3 wt%-Ce₂O₃, was found to have excellent 200-h oxidation resistance at 700°, 1000°, and 1370°C, without incidence of 700° to 1000°C phase instability and cracking.     

Double Precursor Solution Coating Approach for Low-Temperature Sintering of [Pb(Mg1/3Nb2/3)O3]0.63[PbTiO3]0.37 Solids

Lead-based piezoelectric ceramics typically require sintering temperatures higher than 1000°C at which significant lead loss can occur. Here, we report a double precursor solution coating (PSC) method for fabricating low-temperature sinterable polycrystalline [Pb(Mg_1/3Nb_2/3)O₃]_0.63-[PbTiO₃]_0.37 (PMN–PT) ceramics. In this method, submicrometer crystalline PMN powder was first obtained by dispersing Mg(OH)₂-coated Nb₂O₅ particles in a lead acetate/ethylene glycol solution (first PSC), followed by calcination at 800°C. The crystalline PMN powder was subsequently suspended in a PT precursor solution containing lead acetate and titanium isopropoxide in ethylene glycol to form the PMN–PT precursor powder (second PSC) that could be sintered at a temperature as low as 900°C. The resultant d ₃₃ for samples sintered at 900°, 1000°, and 1100°C for 2 h were 600, 620, and 700 pm/V, respectively, comparable with the known value. We attributed the low sintering temperature to the reactive sintering nature of the present PMN–PT precursor powder. The reaction between the nanosize PT and the submicrometer-size PMN occurred roughly in the same temperature range as the densification, 850°–900°C, thereby significantly accelerating the sintering process. The present PSC technique is very general and should be readily applicable to other multicomponent systems.     

Solid-State Sintering of YBa2Cu3Ox in Different Oxygen Partial Pressures

The effect of oxygen partial pressure, ranging from 0.001 to 1.0 atm, and temperature, in the range of 930°–955°C, on the solid-state sintering kinetics of the superconducting ceramic YBa₂Cu₃O_7-δ has been studied. Isothermal compaction rates between 930° and 955°C reached a maximum at some critical PO₂ (PO₂^CRIT), with decreasing rates both above and below this oxygen partial pressure. This behavior was not observed for YBa₂Cu₃O_7-δ sintered at 960°C, when a liquid phase is present. The activation energy for sintering above PO₂^CRIT has been estimated to be ∼190 kJ/mol, whereas below PO₂^CRIT it was found to be ∼130 kJ/mol. The oxygen ion diffusion was considered to be the rate-determining step above PO₂^CRIT, while it is hypothesized that lattice strain caused by the formation of oxygen ion vacancies below PO₂^CRIT affected the rate of sintering.     

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.     

Densification of Cr2O3-ZrO2 Ceramics by Sintering

Cr₂O₃ and ZrO₂ were mixed in various ratios and pressed to form compacts, which were then sintered in carbon powder. Compacts with >30 wt% Cr₂O₃ were sintered to densities >98% of true density at 1500°C. This method of sintering in carbon powder can be used to prepare very dense Cr₂O₃-ZrO₂ ceramics at a relatively low temperature, (∼1500°C) without additives.     

Low-Temperature Sintering and Microwave Dielectric Properties of Li2MgSiO4 Ceramics

Development of a low-temperature sintered dielectric material derived from Li₂MgSiO₄ (LMS) for low-temperature cofired ceramic (LTCC) application is discussed in this paper. The LMS ceramics were prepared by the solid-state ceramic route. The calcination and sintering temperatures of LMS were optimized at 850°C/4 h and 1250°C/2 h, respectively, for the best density and dielectric properties. The crystal structure and microstructure of the ceramic were studied by the X-ray diffraction and scanning electron microscopic methods. The microwave dielectric properties of the ceramic were measured by the cavity perturbation method. The LMS sintered at 1250°C/2 h had ɛ_r=5.1 and tan δ=5.2 × 10⁻⁴ at 8 GHz. The sintering temperature of LMS is lowered from 1250°C/2 h to 850°C/2 h by the addition of both lithium borosilicate (LBS) and lithium magnesium zinc borosilicate (LMZBS) glasses. LMS mixed with 1 wt% LBS sintered at 925°C/2 h had ɛ_r=5.5 and tan δ=7 × 10⁻⁵ at 8 GHz. Two weight percent LMZBS mixed with LMS sintered at 875°C/2 h had ɛ_r=5.9 and tan δ=6.7 × 10⁻⁵ at 8 GHz.     

Densification and Mechanical Behavior of HfC and HfB2 Fabricated by Spark Plasma Sintering

Hafnium diboride (HfB₂)- and hafnium carbide (HfC)-based materials containing MoSi₂ as sintering aid in the volumetric range 1%–9% were densified by spark plasma sintering at temperatures between 1750° and 1950°C. Fully dense samples were obtained with an initial MoSi₂ content of 3 and 9 vol% at 1750°–1800°C. When the doping level was reduced, it was necessary to raise the sintering temperature in order to obtain samples with densities higher than 97%. Undoped powders had to be sintered at 2100°–2200°C. For doped materials, fine microstructures were obtained when the thermal treatment was lower than 1850°C. Silicon carbide formation was observed in both carbide- and boride-based materials. Nanoindentation hardness values were in the range of 25–28 GPa and were independent of the starting composition. The nanoindentation Young's modulus and the fracture toughness of the HfB₂-based materials were higher than those of the HfC-based materials. The flexural strength of the HfB₂-based material with 9 vol% of MoSi₂ was higher at 1500°C than at room temperature.     

Sintering and Crystallization of CaO–Al2O3–ZrO2–SiO2 Glasses Containing Different Amount of Al2O3

In this work several complementary techniques have been employed to carefully characterize the sintering and crystallization behavior of CaO–Al₂O₃–ZrO₂–SiO₂ glass powder compacts after different heat treatments. The research started from a new base glass 33.69 CaO–1.00 Al₂O₃–7.68 ZrO₂–55.43SiO₂ (mol%) to which 5 and 10 mol% Al₂O₃ were added. The glasses with higher amounts of alumina sintered at higher temperatures (953°C [lower amount] vs. 987°C [higher amount]). A combination of the linear shrinkage and viscosity data allowed to easily find the viscosity values corresponding to the beginning and the end of the sintering process. Anorthite and wollastonite crystals formed in the sintered samples, especially at lower temperatures. At higher temperatures, a new crystalline phase containing ZrO₂ (2CaO·4SiO₂·ZrO₂) appeared in all studied specimens.     

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.     

Sintering of Si3N4 with the Addition of Rare-Earth Oxides

The effect of rare-earth oxide additives on the densification of silicon nitride by pressureless sintering at 1600° to 1700°C and by gas pressure sintering under 10 MPa of N₂ at 1800° to 2000°C was studied. When a single-component oxide, such as CeO₂, Nd₂O₃, La₂O₃, Sm₂O₃, or Y₂O₃, was used as an additive, the sintering temperature required to reach approximate theoretical density became higher as the melting temperature of the oxide increased. When a mixed oxide additive, such as Y₂O₃–Ln₂O₃ (Ln=Ce, Nd, La, Sm), was used, higher densification was achieved below 2000°C because of a lower liquid formation temperature. The sinterability of silicon nitride ceramics with the addition of rare-earth oxides is discussed in relation to the additive compositions.     

Fabrication of Hard and Strong α/β-Si3N4 Composites With MgSiN2 as Addivitives

α/β-Si₃N₄ composites with various α/β phase ratios were prepared by hot pressing at 1600°–1650°C with MgSiN₂ as sintering additives. An excellent combination of mechanical properties (Vickers indentation hardness of 23.1 GPa, fracture strength of about 1000MPa, and toughness of 6.3 MPa·m^1/2) could be obtained. Compared with conventional Si₃N₄-based ceramics, this new material has obvious advantages. It is as hard as typical in-situ-reinforced α-Sialon, but much stronger than the latter (700 MPa). It has comparable fracture strength and toughness, but is much harder than β-Si₃N₄ ceramics (16 GPa). The microstructures and mechanical properties can be tailored by choosing the additive and controlling the heating schedule.     

Preparation of Dense BaTiO3 Ceramics with Submicrometer Grains by Spark Plasma Sintering

Dense BaTiO₃ ceramics consisting of submicrometer grains were prepared using the spark plasma sintering (SPS) method. Hydrothermally prepared BaTiO₃ (0.1 and 0.5 µm) was used as starting powders. The powders were densified to more than similar/congruent95% of the theoretical X-ray density by the SPS process. The average grain size of the SPS pellets was less than similar/congruent1 µm, even by sintering at 1000-1200°C, because of the short sintering period (5 min). Cubic-phase BaTiO₃ coexisted with tetragonal BaTiO₃ at room temperature in the SPS pellets, even when well-defined tetragonal-phase BaTiO₃ powder was sintered at 1100° and 1200°C and annealed at 1000°C, signifying that the SPS process is effective for stabilizing metastable cubic phase. The measured permittivity was similar/congruent7000 at 1 kHz at room temperature for samples sintered at 1100°C and showed almost no dependence on frequency within similar/congruent10⁰-10⁶ Hz; the permittivity at 1 MHz was 95% of that at 1 kHz.     

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.     

Influence of Sintering Temperature on Grain Growth and Phase Structure of Compositionally Optimized High-Performance Li/Ta-Modified (Na,K)NbO3 Ceramics

Microstructure characteristics, phase transition, and electrical properties of (Na_0.535K_0.485)_0.926Li_0.074(Nb_0.942Ta_0.058)O₃ (NKN-LT) lead-free piezoelectric ceramics prepared by normal sintering are investigated with an emphasis on the influence of sintering temperature. Some abnormal coarse grains of 20–30 μm in diameter are formed in a matrix consisting of about 2 μm fine grains when the sintering temperature was relatively low (980°C). However, only normally grown grains were observed when the sintering temperature was increased to 1020°C. On the other hand, orthorhombic and tetragonal phases coexisted in the ceramics sintered at 980°–1000°C, whereas the tetragonal phase becomes dominant when sintered above 1020°C. For the ceramics sintered at 1000°C, the piezoelectric constant d ₃₃ is enhanced to 276 pC/N, which is a high value for the Li- and Ta-modified (Na,K)NbO₃ ceramics system. The other piezoelectric and ferroelectric properties are as follows: planar electromechanical coupling factor k _p=46.2%, thickness electromechanical coupling factor k _t=36%, mechanical quality factor Q _m=18, remnant polarization P _r=21.1 μC/cm², and coercive field E _c=1.85 kV/mm.     

Low-Temperature Sintering of La(Ca)CrO3 Powder Prepared through the Combustion Process

Ultrafine La(Ca)CrO₃ (LCC) powder was prepared through the glycine–nitrate gel combustion process. It was shown for the first time that the use of relatively inexpensive CrO₃ as a starting material for chromium has potential for the bulk preparation of sinter-active LCC powder. As-prepared powder, when calcined at 700°C, resulted in LCC along with a small amount of CaCrO₄. The calcined powder was found to be composed of soft agglomerates with a particle size of ≈70–290 nm. The cold pressing and sintering of the calcined powder at 1200°C resulted in the mono-phasic La_0.7Ca_0.3CrO₃ with density ≈98% of its theoretical value. This is the lowest sintering temperature ever reported for La_0.7Ca_0.3CrO₃. The conductivity of the sintered La_0.7Ca_0.3CrO₃ at 1000°C was found to be ≈57 S/cm in air. The sintering and electrical behavior achieved for La_0.7Ca_0.3CrO₃ may find application as an interconnect material for high-temperature solid oxide fuel cells if problems with chemical expansion and poor conductivity in fuel can be overcome.     

Synthesis of BaCu(B2O5) Ceramics and their Effect on the Sintering Temperature and Microwave Dielectric Properties of Ba(Zn1/3Nb2/3)O3 Ceramics

BaCu(B₂O₅) ceramics were synthesized and their microwave dielectric properties were investigated. BaCu(B₂O₅) phase was formed at 700°C and melted above 850°C. The BaCu(B₂O₅) ceramic sintered at 810°C had a dielectric constant (ɛ_r) of 7.4, a quality factor ( Q × f ) of 50 000 GHz and a temperature coefficient of resonance frequency (τ_f) of −32 ppm/°C. As the BaCu(B₂O₅) ceramic had a low melting temperature and good microwave dielectric properties, it can be used as a low-temperature sintering aid for microwave dielectric materials for low temperature co-fired ceramic application. When BaCu(B₂O₅) was added to the Ba(Zn_1/3Nb_2/3)O₃ (BZN) ceramic, BZN ceramics were well sintered even at 850°C. BaCu(B₂O₅) existed as a liquid phase during the sintering and assisted the densification of the BZN ceramic. Good microwave dielectric properties of Q × f =16 000 GHz, ɛ_r=35, and τ_f=22.1 ppm/°C were obtained for the BZN+6.0 mol% BaCu(B₂O₅) ceramic sintered at 875°C for 2 h.     

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.     

Change in Oxygen Content of Y2O3-Nd2O3-Doped Silicon Nitride During Firing

The oxygen content of silicon nitride with 1 mol% Y₂O₃—Nd₂O₃ additive was measured after firing to determine the compositional change during gas-pressure sintering. Oxygen content decreases from 2.5 to 0.94 wt% during firing for 4 h at 1900°C and 10-MPa pressure in N₂. This decrease in oxygen results from the release of SiO gas generated by a thermaldecomposition reaction between Si₃N₄ and SiO₂. The resultant sintered silicon nitride material contains less than 1 wt% oxygen.     

Sintering and Properties of Sialons without Externally Added Liquid Phase

β'-Sialon has the general formula Si_6-ZAl_ZO_ZN₈-Z The synthesis of pure β'-sialon with three different Z values of 0.5, 1.0, and 2.0 in the system Si₃N₄-AlN-Al₂O₃-SiO₂ has been reported without the addition of a foreign sintering aid. A small shift in the composition toward the SiO₂ corner has been made in each case. The Z=1.0 and 2.0 sialon can be sintered almost to full density while that with Z=0.5 sialon is difficult even with a higher amount of excess oxide addition. A packing bed of Si₃N₄ and SiO₂ in a weight ratio of 7:3 was found to be most useful. The coefficient of linear thermal expansion of Z=1.0 sialon is 2.2°10^-6/°C (25° to 1000°C). The room-temperature modulus of rupture value can be retained up to 90% of the value at 1400°C. A similar trend has also been observed in the K_lC value with temperature. The steady-state flexural creep rate varies from 0.5°10^-6 to 2.5°10^-6 h^-1 in the temperature and load ranges of 1200° to 1300°C and 100 to 250 MPa.