Microwave dielectric properties of B2O3 doped LaAlO3 ceramics at low sintering temperature

The microwave dielectric properties and the microstructures of LaAlO₃ ceramics with B₂O₃ additions (0.25–1 wt%) prepared with conventional solid-state route have been investigated. Doping with B₂O₃ (up to 0.5 wt%) can effectively promote the densification of LaAlO₃ ceramics. It is found that LaAlO₃ ceramics can be sintered at 1400°C due to the liquid phase effect of B₂O₃ addition. The Q × f value as well as the dielectric constant decreases at higher B₂O₃ doping level (1 wt%) due to the increase of boundary phases. At 1460°C, LaAlO₃ ceramics with 0.5 wt% B₂O₃ addition possesses a dielectric constant ( _r ) of 22.9, a Q × f value of 44700 (at 9 GHz) and a temperature coefficients of resonant frequency ( _f ) of –36 ppm/°C. The B₂O₃-doped LaAlO₃ ceramics can find applications in microwave devices requiring low sintering temperature.     

Corrosion of alumina ceramics in acidic aqueous solutions at high temperatures and pressures

Alumina is resistant against corrosive aqueous solutions and could be used as a reactor material in the Supercritical Water Oxidation (SCWO) process. For this reason, the corrosion resistance of alumina and zirconia toughened alumina (ZTA) ceramics was investigated in aqueous solutions containing 0.1 mol/kg H₂SO₄, H₃PO₄ or HCl at T = 240°C–500°C at p = 27 MPa. In sulfuric acid, the solubility of alumina and its corrosion products was high at temperatures of 240°C–290°C. The corrosion rate was still high at higher temperatures (340°C–500°C), but the corrosion products were less soluble and formed a non-protecting scale on the samples. Phosphoric acid was less corrosive due to the formation of berlinite (AlPO₄) on the surface of the specimens. In hydrochloric acid, the dissolution of the alumina grains was the predominant corrosion phenomenon at temperatures of 240°C–290°C. At higher temperatures, intergranular corrosion was observed, but a dissolution of the grains did not occur.     

Microwave dielectric properties and its compatibility with silver electrode of LiNb<Subscript>0.6</Subscript>Ti<Subscript>0.5</Subscript>O<Subscript>3</Subscript> with B<Subscript>2</Subscript>O<Subscript>3</Subscript> and CuO additions

The influences of B₂O₃ and CuO (BCu, B₂O₃: CuO = 1:1) additions on the sintering behavior and microwave dielectric properties of LiNb_0.6Ti_0.5O₃ (LNT) ceramics were investigated. LNT ceramics were prepared with conventional solid-state method and sintered at temperatures about 1,100 °C. The sintering temperature of LNT ceramics with BCu addition could be effectively reduced to 900 °C due to the liquid phase effects resulting from the additives. The addition of BCu does not induce much degradation in the microwave dielectric properties. Typically, the excellent microwave dielectric properties of ε_r = 66, Q × f = 6,210 GHz, and τ _f  = 25 ppm/^oC were obtained for the 2 wt% BCu-doped sample sintered at 900 °C. Chemical compatibility of silver electrodes and low-fired samples has also been investigated.     

Dielectric properties of B2O3-doped (1 − x)LaAlO3-xSrTiO3 ceramic system at microwave frequency

The effects of B₂O₃ addition on the microwave dielectric properties and the microstructures of (1−x)LaAlO₃-xSrTiO₃ ceramics prepared by conventional solid-state routes have been investigated. Doping with 0.25 wt.% B₂O₃ can effectively promote the densification and the microwave dielectric properties of (1−x)LaAlO₃-xSrTiO₃ ceramics. It is found that LaAlO₃-SrTiO₃ ceramics can be sintered at 1400°C due to the liquid phase effect of a B₂O₃ addition observed by scanning electronic microscopy (SEM). The dielectric constant as well as the Q×f value decreases with increasing B₂O₃ content. At 1460°C, 0.46LaAlO₃-0.54SrTiO₃ ceramics with 0.25 wt.% B₂O₃ addition possesses a dielectric constant (ε_r) of 35, a Q×f value of 38,000 (at 7 GHz) and a temperature coefficients of resonant frequency (τ_f) of −1 ppm/°C.     

Ceramic joining

A method of ceramic-ceramic joining that exploits a thin layer of a transient liquid phase to join alumina to alumina has been developed, and the results of its application to joining alumina are reported. Through the use of microdesigned multilayer Cu/Pt interlayers, transient liquid-phase joining has been achieved at 1150°C, yielding an interlayer that is platinum-rich at temperatures substantially lower than those required for solid-state diffusion bonding with pure platinum interlayers. Flexure tests indicate that ceramic/metal interface strengths exceeding those of the ceramic can be achieved. Post-bonding anneals of 10 h duration in air and gettered argon at 1000 °C had discernibly different effects on room-temperature joint strength. The microstructure and chemistry of fracture surfaces were examined using SEM and EDS in an effort to identify the nature of strength-limiting flaws in both as-bonded and postbonding annealed specimens. Topics requiring further study are identified. Opportunities for extensions of the method to other systems are discussed.     

Calculation and experimental determinations of the residual stress distribution in alumina/Ni/alumina and alumina/Ni/nickel alloy systems

Residual stress problems encountered in joining ceramics–ceramics or ceramics–metals systems for high-temperature applications >1000 °C have been studied. A solid-state bonding technique under hot-pressing via metallic foils sheet of Ni was used for joining alumina–alumina and alumina–nickel alloy (HAYNES^? 214™). The residual stresses expected in the specimen were predicted by finite-element method (FEM) calculations using an elastic–plastic-creep model (EPC). Stress distributions in the specimen were characterized experimentally using X-ray diffraction (XRD) and Vickers Indentation Fracture (VIF) techniques. The tensile and shear stress profiles have been determined along selected lines perpendicular to the bonding interface. The results of the FEM calculation of residual stresses have been compared experimentally with the results of classical XRD and indentation methods. It was found that the tensile stress concentration showed higher values at the edge of the boundary. The residual stresses caused by the thermal expansion mismatch between alumina (Al₂O₃) and Ni-based super-alloy (HAYNES^? 214™) severely deteriorated the joints compared to Al₂O₃–Al₂O₃ joint with the same solid-state bonding parameters. The correlations between the FEM calculations and experimental results obtained by XRD and VIF method were discussed.     

Microwave dielectric properties and microstructures of the 11Li2O-3Nb2O5-12TiO2 ceramics with B2O3 addition

The effects of B₂O₃ addition, as a sintering agent, on the sintering behavior, microstructure and microwave dielectric properties of the 11Li₂O-3Nb₂O₅-12TiO₂ (LNT) ceramics have been investigated. With the low-level doping of B₂O₃ (≤2 wt.%), the sintering temperature of the LNT ceramic could be effectively reduced to 900 °C. The B₂O₃-doped LNT ceramics are also composed of Li₂TiO₃ss and “M-phase” phases. No other phase could be observed in the 0.5-2 wt.% B₂O₃-doped ceramics sintered at 840-920 °C. The addition of B₂O₃ induced no obvious degradation in the microwave dielectric properties but increased the τ_f values. Typically, the 0.5 wt.% B₂O₃-doped ceramics sintered at 900 °C have better microwave dielectric properties of ?_r = 49.2, Q × f = 8839 GHz, τ_f = 57.6 ppm/°C, which suggest that the ceramics could be applied in multilayer microwave devices requiring low sintering temperatures.     

Low temperature sintering and microwave dielectric properties of ZnTiNb2O8 ceramics with BaCu(B2O5) additions

The phases, microstructure and microwave dielectric properties of ZnTiNb₂O₈ ceramics with BaCu(B₂O₅) additions prepared by solid-state reaction method have been investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The pure ZnTiNb₂O₈ ceramic shows a high sintering temperature of about 1250 °C. However, it was found that the addition of BaCu(B₂O₅) lowered the sintering temperature of ZnTiNb₂O₈ ceramics from above 1250 °C to 950 °C due to the BCB liquid-phase. The results showed that the microwave dielectric properties were strongly dependent on densification, crystalline phases and grain size. Addition of 3 wt% BCB in ZnTiNb₂O₈ ceramics sintered at 950 °C afforded excellent dielectric properties of ?_r = 32.56, Q × f = 20,100 GHz (f = 5.128 GHz) and τ_f = −64.87 ppm/°C. These represent very promising candidates for LTCC dielectric materials.     

The relationship between the microstructure and microwave dielectric properties of zirconium titanate ceramics

Zirconium titanate (ZrTiO₄) ceramics have been prepared by the mixed oxide route using small additions of ZnO, Y₂O₃ or CuO. Specimens were sintered mainly at 1400 °C and cooled at various rates: water-quench, air-quench, 300 °C h^–1, 120 °C h^–1, 6 °C h^–1 and 1 °C h^–1. Products prepared with additives exhibited densities of at least 93% of the theoretical value. As the cooling rate after sintering was decreased, the length of the lattice parameter in the b direction was reduced and transmission electron diffraction revealed superlattice reflections associated with cation ordering. For specimens cooled at 1 °C h^–1, electron diffraction patterns exhibited features consistent with an incommensurate superstructure in the a direction. The dielectricQ value of rapidly cooled (air-quenched) ceramics was 2000 at 5 GHz. With an increase in the degree of cation ordering theQ value increased to a maximum of 4400 for specimens cooled at 6 °C h^–1. For specimens cooled at the slowest rate (1 °C h^–1) theQ value fell to 2000 due in part to the presence of microcracks and exsolved ZrO₂. Diffusion of trivalent impurities (yttria) into the host ZrTiO₄ grains also led to a lowering of theQ values.The microwave dielectric properties of zirconium titanate ceramics are sensitive to processing conditions and mircrostructural features. The highestQ values (lowest loss) should be achieved in homogeneous specimens, free of trivalent impurities and lattice defects, in which lowQ-value second phases, microcracks and pores are eliminated.     

Simultaneous synthesis and densification of transparent,photoluminescent polycrystalline YAG by current activated pressure assisted densification (CAPAD)

We report a method for the synthesis and processing of transparent bulk polycrystalline yttrium aluminum garnet (YAG) and photoluminescent Ce-doped YAG ceramics via solid-state reactive-current activated pressure assisted densification (CAPAD). The process uses commercially available γ-Al₂O₃, Y₂O₃, and CeO₂ nanopowders. The nanopowders were reacted and densified simultaneously at temperatures between 850 °C and 1550 °C and at a maximum pressure of 105 MPa. The solid-state reaction to phase pure YAG occurs in under 4 min at processing temperatures 1100 °C which is significantly faster (on the order of tens of hours) and occurs at much lower temperatures (∼600 °C) compared to conventional reaction sintering. We found that the reaction significantly improves densification – the shrinkage rate of reaction-produced YAG was three times higher than that of YAG using pre-reacted powder. The Ce additions were found to retard the reaction driven shrinkage kinetics by a factor ∼3, but are still faster (by a factor ∼1.6) than those associated with direct densification (no synthesis). Densities >99% were achieved in both pure YAG and Ce doped YAG (Ce:YAG). Results of optical measurements show good transparency in the visible and photoluminescence (PL) in the Ce:YAG. The PL peak is broad and appears white when excited using blue light confirming that the ceramics can be used in solid state lighting to produce white light.     

Improved microwave dielectric properties of B2O3-doped Nd(Co1/2Ti1/2)O3 ceramics with near zero temperature coefficient of resonant frequency

The microwave dielectric properties and the microstructures of Nd(Co_1/2Ti_1/2)O₃ ceramics prepared by conventional solid-state route have been studied. The prepared Nd(Co_1/2Ti_1/2)O₃ exhibited a mixture of Co and Ti showing 1:1 order in the B-site. It is found that low-level doping of B₂O₃ (up to 0.75 wt.%) can significantly improve the density and dielectric properties of Nd(Co_1/2Ti_1/2)O₃ ceramics. Nd(Co_1/2Ti_1/2)O₃ ceramics with additives could be sintered to a theoretical density higher than 98.5% at 1320 °C. Second phases were not observed at the level of 0.25-0.75 wt.% B₂O₃ addition. The temperature coefficient of resonant frequency (τ_f) was not significantly affected, while the dielectric constants (?_r) and the unloaded quality factors Q were effectively promoted by B₂O₃ addition. At 1320 °C/4 h, Nd(Co_1/2Ti_1/2)O₃ ceramics with 0.75 wt.% B₂O₃ addition possesses a dielectric constant (?_r) of 27.2, a Q × f value of 153,000 GHz (at 9 GHz) and a temperature coefficient of resonant frequency (τ_f) of 0 ppm/°C. The B₂O₃-doped Nd(Co_1/2Ti_1/2)O₃ ceramics can find applications in microwave devices requiring low sintering temperature.     

Effect of sintering aid on microwave dielectric behaviors of Ni0.5Ti0.5NbO4 ceramics for LTCC applications

The effects of ZnO glass addition on the microwave dielectric properties of Ni_0.5Ti_0.5NbO₄ (NTN) ceramics prepared by solid-state reaction method have been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The pure NTN ceramics have ε_r of 60.6, Q × f value of 70,100 GHz, and τ_f value of 76.6 ppm ^°C⁻¹ sintered at 1140 °C for 6 h. The results indicate that the addition of ZnO can effectively benefit the densification and further improve the dielectric constant. Moreover, the lower sintering temperature of NTN ceramics from 1140 to 930 °C is obtained by the addition of ZnO glass. However, an excess of ZnO suppresses the grain growth and decreases the Q × f value of NTN ceramics. The NTN ceramics with 2 wt% ZnO sintered at 930 °C for 6 h possess promising microwave dielectric properties: ε_r of 56.3, Q × f value of 67,000 GHz, and τ_f value of 78.6 ppm °C⁻¹, which shows that the materials are suitable for low-temperature co-fired ceramics applications.     

Joining of silicon nitride ceramics by hot pressing

The joining of hot-pressed silicon nitride ceramics, containing Al₂O₃ and Y₂O₃ as sintering aids, has been carried out in a nitrogen atmosphere. Uniaxial pressure was applied at high temperature during the joining process. Polyethylene was used as a joining agent. Joining strength was measured by four-point bending tests. The effects of joining conditions such as temperature (from 1400 to 1600°C), joining pressure (from 0.1 to 40 MPa), holding time (from 0.5 to 8 h) and surface roughness (R _max) of the joining couple (about 0.12, 0.22 and 1.2m) on the joining strength were examined. The joining strength was increased with increases in joining temperature, joining pressure and holding time. Larger surface roughness caused lower joining strength. The higher joining strength was attributed to a larger true contact area. The area was increased through plastic deformation of the joined couple at elevated temperatures. The highest joining strength attained was 567 MPa at room temperature, which was about half the value of the average flexural strength of the original body. The high temperature strength measured at 1200° C did not differ very much from the room-temperature value.     

Effect of thermal residual stresses on the strength for both alumina/Ni/alumina and alumina/Ni/nickel alloy bimaterials

This paper describes some technical limitations encountered in joining ceramics–ceramics or ceramics–metals, and how, to some extent, they have been practically overcome. The effect of the residual stresses on the strength of joints fabricated between alumina–alumina or alumina and the nickel base alloy HAYNES^? 214™ using a solid-state bonding technique with Ni interlayer was studied. Finite element analyses (FEA) for the elastic–plastic and elastic–plastic–creep behavior have also been used to better design the joints and to predict their performance. It was found that the residual stresses caused by the thermal expansion mismatch between alumina (Al₂O₃) and the Ni-based superalloy (HAYNES^? 214™) have severely deteriorated the joints compared to Al₂O₃–Al₂O₃ joint fabricated with the same solid-state bonding parameters. The high residual stresses zones obtained through the FEA simulation fitted well with the fractographic observations of the Al₂O₃/Ni/HAYNES^? 214™ joints. Also, in order to use the joint material as a structural material, the study about the effect of geometrical parameters has been performed. Optimal geometries have been determined.     

The properties low-temperature fixed piezoelectric ceramics

Chemicals having relatively Iow melting points have been used to reduce the required sintering temperature of MnO₂-doped Pb_0.9875((Zr_0.52Ti_0.48)_0.975Nb_0.025. Among these compounds Li₂CO₃, Na₂CO₃, B₂O₃, Bi₂O₃ and V₂O₅ are suitable additives for this purpose. The substitution and interstitial properties of the doped ions are discussed in connection with their influences on piezoelectricity and dielectric properties. HighQ _m materials and materials with temperature-stability ofQ _m could be obtained by Li⁺ and B³⁺ or Bi³⁺ doping. Compensation pair effects are proposed to explain the results. Materials exhibiting aQ _m of about 500 with a deviation of less than 10% at temperatures up to 300° C have been prepared for filter use. The sintering temperature is as low as 1070° C.     

Improved high-Q microwave dielectric resonator using CuO-doped MgNb2O6 ceramics

The microwave dielectric properties and the microstructures of MgNb₂O₆ ceramics with CuO additions (1-4 wt.%) prepared with conventional solid-state route have been investigated. The sintered samples exhibit excellent microwave dielectric properties, which depend upon the liquid phase and the sintering temperature. It is found that MgNb₂O₆ ceramics can be sintered at 1140 °C due to the liquid phase effect of CuO addition. At 1170 °C, MgNb₂O₆ ceramics with 2 wt.% CuO addition possesses a dielectric constant (ε_r) of 19.9, a Q×f value of 110,000 (at 10 GHz) and a temperature coefficient of resonant frequency (τ_f) of −44 ppm/°C. The CuO-doped MgNb₂O₆ ceramics can find applications in microwave devices requiring low sintering temperature.     

Synthesis and dielectric characterization of Ba0.6Sr0.4TiO3 ferroelectric ceramics

Ferroelectric ceramics Ba_0.6Sr_0.4TiO₃ (BST 40) were prepared, by solid-state reaction in the temperature range 1210-1450 °C. Maximum values of the ceramic densities were around 94% of their theoretical value. X-ray diffraction techniques (XRD) and scanning electron spectroscopy (SEM) were used to analyze the structure and the surface morphology of ceramics. Rounded, well defined or abnormal granular growth was observed in the SEM images, vs. sintering conditions and purity of the raw materials. In all samples, BST 40 ceramic is the major phase, but there are also present small amounts of secondary phases, as revealed in XRD diffraction patterns. Permittivity and dielectric loss measurements were performed in the temperature range − 150 to + 150 °C, and 150 Hz-5 MHz frequency values. Permittivity values rising from 1200 to 12,500, with increasing sintering temperatures, were recorded. Narrow and well defined transition peaks were noticed at higher sintering temperatures. Curie temperature was around 2 °C, for samples with the mentioned composition. Permittivity and losses vs. frequency show different behavior whether BST ceramics are in polar or non-polar state and with the distance toward phase transition. Microwave measurements performed at room temperature have shown lower values of permittivity, compared with similar data at low frequency, and dielectric losses lower than 1% at 0.7 GHz. The sintering conditions (temperatures, sintering time, etc.) and purity of the raw materials lead to important changes of transition temperatures in the polymorphic diagram, which we have built—for the other Ba1−xSrxTiO3 compositions (x = 0.25-0.90) sintered at 1260 °C for 2 h.     

The direct bonding between copper and MgO-doped Si3N4

An application of direct bonding method for copper to silicon nitride (Si₃N₄) joining was investigated. Si₃N₄ was sintered with 5wt% MgO at 1700 ° C for 30 min in nitrogen atmosphere, and oxidized at various temperatures. The bonding was performed at 1075 ° C in nitrogen atmosphere with low oxygen partial pressure. The direct bonding was not achieved for the Si₃N₄ oxidized below 1200 ° C or nonoxidized. During oxidation, magnesium ion added as sintering aids, diffused out to the surface of Si₃N₄ and formed MgSiO₃, which seemed to have an important role in the bonding. Fracture of the bonded specimen under tensile stress took place within the oxide layer of Si₃N₄. The bonding strength was decreased with oxidation temperature and time. Maximum strength was found to be 106 kg cm^–2 for the Si₃N₄ oxidized at 1200 ° C for 1 h.     

Processing, dielectric properties and impedance characteristics of Na0.5Bi0.5Cu3Ti4O12 ceramics

Na_0.5Bi_0.5Cu₃Ti₄O₁₂ (NBCTO) ceramics were prepared by conventional solid-state reaction method. The phase structure, microstructure and dielectric properties of NBCTO ceramics sintered at various temperatures with different soaking time were investigated. Pure NBCTO phase could be obtained with increasing the temperature and prolonging the soaking time. High dielectric permittivity (13,495) and low dielectric loss (0.031) could be obtained when the ceramics were sintered at 1000 °C for 7.5 h. The ceramics sintered at 1000 °C for 7.5 h also showed good temperature stability (−4.00 to −0.69%) over a large temperature range from −50 to 150 °C. Complex impedances results revealed that the grain was semiconducting and the grain boundaries was insulating. The grain resistance (R_g) was 12.10 Ω cm and the grain boundary resistance (R_gb) was 2.009 × 10⁵ Ω cm when the ceramics were sintered at 1000 °C for 7.5 h.     

Microwave dielectric properties of Li2TiO3 ceramics sintered at low temperatures

Li₂TiO₃ ceramics were prepared at the sintering temperatures from 1050 to 1250 °C. The optimal microwave dielectric properties were ?_r = 23.29, Q × f = 15,525 GHz (5.9 GHz), and τ_f = 35.05 ppm/ °C for the sample sintered at 1200 °C. The microwave dielectric properties were improved obviously when the Li₂TiO₃ ceramics were sintered at low temperatures with small additions of H₃BO₃ (B₂O₃ in the form of H₃BO₃). Only monoclinic Li₂TiO₃ was found in the pure or H₃BO₃-doped Li₂TiO₃ ceramics. About 1.0 wt.% H₃BO₃ addition aided the sintering of Li₂TiO₃ ceramics effectively while excessive H₃BO₃ (≥2.5 wt.%) was not favorable. Typically the best microwave dielectric properties were ?_r = 23.28, Q × f = 37,110 GHz (6.3 GHz), and τ_f = 30.43 ppm/ °C for the 1.0 wt.% H₃BO₃-doped Li₂TiO₃ ceramic sintered at 920 for 3 h, which is promising for LTCC applications.