Microwave Dielectric Properties of Low Firing Temperature Bi2W2O9 Ceramics

Phase stability, sinterability, and microwave dielectric properties of Bi₂W₂O₉ ceramics and their cofireability with Ag, Cu, and Au electrodes have been investigated. Single-phase Bi₂W₂O₉ powder was synthesized by solid-state reaction in air at 800°C for 3 days. X-ray powder diffraction data show Bi₂W₂O₉ to have an orthorhombic crystal structure described by the noncentrosymmetric space group Pna 2₁, with lattice parameters a =5.4401(8), b =5.4191(8), c =23.713(4) Å. Ceramics fired at temperatures up to 865°C remain single-phase but above this temperature ferroelectric Bi₂WO₆ appears as a secondary phase. The measured relative permittivity of Bi₂W₂O₉ ceramics increases continuously from 28.6 to 40.7 for compacts fired between 860° and 885°C. The bulk relative permittivity of Bi₂W₂O₉ corrected for porosity was calculated as 41.3. Bi₂W₂O₉ ceramics fired up to 875°C exhibit moderate quality factors, Q × f _r, ∼7500–7700 GHz and negative temperature coefficient of resonant frequency, ∼−54 to −63 ppm/°C. Chemical compatibility experiments show Bi₂W₂O₉ ceramics to react with both Ag and Cu electrodes, but to form good contacts with Au electrodes.     

Pressureless Sintering of α-Si3N4 Porous Ceramics Using a H3PO4 Pore-Forming Agent

A new method for preparing high bending strength porous silicon nitride (Si₃N₄) ceramics with controlled porosity has been developed by using pressureless sintering techniques and phosphoric acid (H₃PO₄) as the pore-forming agent. The fabrication process is described in detail and the sintering mechanism of porous ceramics is analyzed by the X-ray diffraction method and thermal analysis. The microstructure and mechanical properties of the porous Si₃N₄ ceramics are investigated, as a function of the content of H₃PO₄. The resultant high porous Si₃N₄ ceramics sintered at 1000°–1200°C show a fine porous structure and a relative high bending strength. The porous structure is caused mainly by the volatilization of the H₃PO₄ and by the continous reaction of SiP₂O₇ binder, which could bond on to the Si₃N₄ grains. Porous Si₃N₄ ceramics with a porosity of 42%–63%, the bending strength of 50–120 MPa are obtained.     

Crystal Chemistry and Dielectric Properties in the Systems BaTiO3-UO2 and BaTiO3-BaUO3

Polycrystalline barium titanate fired in nitrogen at 1300° to 1400°C accommodates up to 3 mole % UO₂ in solid solution; its structure is then cubic at room temperature. With BaUO₃ additions the structure becomes disordered and quasi-cubic. In air, about 1 mole % UO₂ goes into solid solution in BaTiO₃ but the structure remains tetragonal. Diffraction peaks of a new phase, possibly a ternary oxide of barium, uranium, and titanium, appear in patterns of specimens containing more than 2 mole % UO₂. The dielectric constant of BaTiO₃ ceramics fired in air, steam, or oxygen increases with up to about 0.5 mole % UO₂ but declines rapidly above this level. The dielectric constant of BaUO₃ is about two orders of magnitude lower than that of BaTiO₃, and additions of BaUO₃ invariably lower the dielectric constant of BaTiO₃.     

Formation and Microwave Dielectric Properties of the Mg2V2O7 Ceramics

The formation process and microwave dielectric properties of the Mg₂V₂O₇ ceramics were investigated. The MgV₂O₆ phase that was formed at around 450°C interacted with remnant MgO above 590°C to form a homogeneous monoclinic Mg₂V₂O₇ phase. Finally, this monoclinic Mg₂V₂O₇ phase was changed to a triclinic Mg₂V₂O₇ phase for the specimen fired at 800°C. Sintering at 950°C for more than 5 h produced high-density triclinic Mg₂V₂O₇ ceramics. In particular, the Mg₂V₂O₇ ceramics sintered at 950°C for 10 h exhibited the good microwave dielectric properties of ɛ_r=10.5, Q × f =58 275 GHz, and τ_f=−26.9 ppm/°C.     

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.     

Synthesis and Microwave Dielectric Properties of MgSiO3 Ceramics

MgSiO₃ ceramics were synthesized and their microwave dielectric properties were investigated. The Mg₂SiO₄ phase was formed at temperatures lower than 1200°C, while the orthorhombic MgSiO₃ phase started to form by the reaction of SiO₂ and Mg₂SiO₄ in the specimen fired at 1200°C. The structure of the MgSiO₃ ceramics was transformed from orthorhombic to monoclinic when the sintering temperature exceeded 1400°C. A dense microstructure was developed for the specimens sintered at above 1350°C. The excellent microwave dielectric properties of ɛ_r=6.7, Q × f =121 200 GHz, and τ_f=−17 ppm/°C were obtained from the MgSiO₃ ceramics sintered at 1380°C for 13 h.     

Spark Plasma Sintering of Nano-Sized TiN Prepared from TiO2 by Controlled Hydrolysis of TiCl4 and Ti(O-i-C3H7)4 Solution

Nano-sized TiO₂ powders were prepared by controlled hydrolysis of TiCl₄ and Ti(O-i-C₃H₇)₄ solutions and nitrided in flowing NH₃ gas at 700°–1000°C to form TiN. Nano-sized TiN was densified by spark plasma sintering at 1300°–1600°C to produce TiN ceramics with a relative density of 98% at 1600°C. The microstructure of the etched ceramic surface was observed by SEM, which revealed the formation of uniformly sized 1–2 μm grains in the TiCl₄-derived product and 10–20 μm in the Ti(O-i-C₃H₇)₄-derived TiN. The electric resisitivity and Vickers micro-hardness of the TiN ceramics was also measured.     

Processing and Electromechanical Properties of (Bi0.5Na0.5)(1−1.5x)LaxTiO3 Ceramics

Bismuth sodium titanate (Bi_0.5Na_0.5TiO₃, BNT) with 0–6 at.% lanthanum was prepared by the conventional mixed oxide method. Each composition was calcined at 800–900°C for 2–5 h to form a pure perovskite phase. Green pellets were sintered at 1050–1150°C for 1–4 h to obtain dense ceramics with at least 95% of theoretical density. X–ray diffraction (XRD) showed phase distortion as lanthanum was added to this system. Meanwhile, a small amount of La was found to affect the grain size and had an influence on the poling conditions and electrical properties. The BNT–based composition with 1 at.% La doping provided a dielectric constant ( K ) of 560, a piezoelectric charge constant ( d ₃₃) of 92 pC/N, and a hydrostatic piezoelectric coefficient ( d _h) of 72 pC/N.     

Influence of SiO2 Addition on the Dielectric Properties and Microstructure of (Ba0.96Ca0.04)(Ti0.85Zr0.15)O3 Ceramics

In this study, the effect of SiO₂ doping on the sintering behavior, microstructure, and dielectric properties of BaTiO₃-based ceramics was investigated. Silica was added to (Ba_0.96Ca_0.04)(Ti_0.85Zr_0.15)O₃ (BCTZ) powder prepared using the solid-state method. SiO₂-doped BCTZ ceramics with a high density and a uniform grain size were obtained and sintered at 1220°C in a reducing atmosphere. A second phase (BaTiSiO₅) existed in samples when SiO₂ was added in excess of 1%. The amount of the second phases was observed to increase as the number of SiO₂ additives increased. It was found that BCTZ ceramics sintered with SiO₂ are helpful in reducing the sintering temperature for a typical thick film and MLCC applications. However, there were disadvantageous effects on the dielectric properties with mere addition of SiO₂ addition (3% and 5%) due to higher formation of BaTiSiO₅. Doping with a small amount of silica can improve the sintering and dielectric properties of BCTZ ceramics. In addition, to understand the effect of the BaTiSiO₅ phase on the dielectric properties of BCTZ ceramics, the BaTiSiO₅ composition was synthesized from individual BaCO₃, TiO₂, and SiO₂ powders using conventional solid-state methods. X-ray diffraction results show the presence of mainly the crystalline phase, BaTiSiO₅, in the sintered ceramics.     

Sintering and Properties of Monazite-Type CePO4

Monazite-type CePO₄ powder (average grain size 0.3 μm) was dry-pressed to disks or bars. The green compacts began to sinter above 950°C. Relative density ≧ 99% and apparent porosity <1% were achieved when the specimens were sintered at 1500°C for 1 h in air. The linear thermal expansion coefficient and thermal conductivity of the CePO₄ ceramics were 9 × 10⁻⁶/°C to 11 × 10⁻⁶/°C (200° to 1300°C) and 1.81 W/(m · K) (500°C), respectively. Bending strength of the ceramics (average grain size 4 μm) was 174 ± 28 MPa (room temperature). The CePO₄ ceramics were cracked or decomposed by acidic or alkaline aqueous solutions at high temperatures.     

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.     

Preparation of Ba(Mg1/3Ta2/3)O3 Using Oxine

A solution containing Mg²⁺ and Ta⁵⁺ was added to an aqueous ammonia solution of oxine, resulting in a precipitate. After the precipitate was thermally decomposed and fired, it was mixed with BaCO₃ powder and fired again at high temperatures to obtain Ba(Mg_1/3Ta_2/3)O₃ (BMT). This method resulted in BMT formation at temperatures lower than those used in the conventional mixed-oxide method, and single-phase BMT formed directly at 1300°C without intermediates.     

Thermal Expansion of Homogeneous and Gradient Solid Solutions in the System KZr2(PO4)3─KTi2(PO4)3

Low-thermal-expansion ceramics having arbitrary thermal expansion coefficients were synthesized from homogeneous solid solutions in the system KZr₂(PO₄)₃─KTi₂(PO₄)₃ (KZP–KTP). Dense and strong ceramics were fabricated by sintering at 1100° to 1200°C with 2 wt% MgO. The thermal expansion coefficient increased from 0 to +3 × 10⁻⁶/°C with increasing x in KZr_{2 − x}Ti_x (PO₄)₃ (KZTP). In addition, a functionally gradient material with respect to thermal expansion was prepared by forming a series of KZTP solid solutions in a single ceramic body. By heating a pile of KZP and KTP ceramics in contact with each other, KZP and KTP bonded together to form a KZTP gradient solid solution near the interface.     

Preparation and Sintering Behavior of Fine-Grained A12O3-SiO2 Composite

A fine, uniform A1₂O₃-SiO₂ powder was prepared by heterocoagulation of narrow Al₂O₃ and SiO₂ powders. This composite powder was dispersed, compacted, and fired in air at 900° to 1580°C for 1 to 13 h. Full density was achieved at 1550°C with the formation of a mullite phase. Relative densities of 83% and 98% (0.3 μm grain size) were measured for samples sintered at 1200°C for 13 h and at 1400°C for 1 h, respectively.     

Effects of La on Dielectric and Piezoelectric Properties of Pb1−xLa2x/3(Nb0.95Ti0.0625)2 O6 Ceramics

Ceramics with the chemical compositions of Pb_{1− x} La_{2 x /3}(Nb_0.95Ti_0.0625)₂O₆ (0≤ x ≤0.060) (PLTN) were prepared by the conventional solid-state reaction method. X-ray diffraction analysis indicated that Ti and La doping not only decreased the rhombohedral–tetragonal phase transformation temperature, but also stabilized the orthorhombic phase of PLTN ceramics. All ceramics sintered at 1190°–1250°C had shown the pure orthorhombic ferroelectric phase. La doping suppresses grain growth and inhibits the formation of pores and cracks, resulting in an increase in relative density up to 97%. The amount of La doping to PLTN ceramics obviously affect ceramics' piezoelectric constant ( d ₃₃) and dielectric loss (tanδ). The sample with x =0.015 possesses high Curie temperature ( T _c=560°C), low dielectric loss (tanδ=0.0054), and excellent piezoelectric constant ( d ₃₃=92 pC/N), presenting a high potential to be used in high-temperature applications as piezoelectric transducers.     

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.     

High-Temperature Reactions of Clay Mineral Mixtures and Their Ceramic Properties: II, Reactions of Kaolinite-Mica-Quartz Mixtures Compared with the K2O— Al2O3—SiO2 Equilibrium Diagram

Two series of mixtures containing respectively 15 and 35% quartz, together with kaolinite and muscovite in varying proportions, were fired at 1000°, llOO°, 1200°, and 1300°C. for 2-hour periods. The resulting crystalline phases were measured quantitatively by X-ray diffraction and the amount of noncrystalline material was estimated by difference. The results for samples fired to 1300°C. are compared with data calculated from the K₂O-Al₂O₃-SiO₂ equilibrium diagram, and a general agreement is obtained with the calculated equilibrium relations for this temperature. In particular, the observed and calculated amounts of mullite agree within 5%. The main departure from equilibrium is the persistence of unreacted quartz, and consequently the estimated amount of noncrystalline residue tends to be less than the calculated liquid (viz. glassy) component. At 1100° and 1200°C., the fired materials approach less closely to the equilibrium relations. The decomposition products of muscovite are discussed in an appendix.     

High-Temperature Chemistry and Oxidation of ZrB2 Ceramics Containing SiC, Si3N4, Ta5Si3, and TaSi2

The effect of Si₃N₄, Ta₅Si₃, and TaSi₂ additions on the oxidation behavior of ZrB₂ was characterized at 1200°–1500°C and compared with both ZrB₂ and ZrB₂/SiC. Significantly improved oxidation resistance of all Si-containing compositions relative to ZrB₂ was a result of the formation of a protective layer of borosilicate glass during exposure to the oxidizing environment. Oxidation resistance of the Si₃N₄-modified ceramics increased with increasing Si₃N₄ content and was further improved by the addition of Cr and Ta diborides. Chromium and tantalum oxides induced phase separation in the borosilicate glass, which lead to an increase in liquidus temperature and viscosity and to a decrease in oxygen diffusivity and of boria evaporation from the glass. All tantalum silicide-containing compositions demonstrated phase separation in the borosilicate glass and higher oxidation resistance than pure ZrB₂, with the effect increasing with temperature. The most oxidation-resistant ceramics contained 15 vol% Ta₅Si₃, 30 vol% TaSi₂, 35 vol% Si₃N₄, or 20 vol% Si₃N₄ with 10 mol% CrB₂. These materials exceeded the oxidation resistance of the ZrB₂/SiC ceramics below 1300°–1400°C. However, the ZrB₂/SiC ceramics showed slightly superior oxidation resistance at 1500°C.     

Pressure and Temperature Dependencies of the Direct-Current Resistance of Semiconducting (Ba, Sr)TiO3 and (Ba, Pb)TiO3

The direct-current resistance of semiconducting compositions in the (Ba, Sr)TiO₃ and (Ba, Pb)TiO₃ systems has been measured under hydrostatic pressures from 0 to 35 MPa, and temperatures from 20° to 135°C. The temperature dependence of the hydrostatic piezoresistance coefficients is presented. Results are discussed in terms of barrier layer sensitivity to mechanical stress.     

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.