Low temperature firing of Co2Y–NiCuZn ferrite composites

Hexagonal structure magnetoplumbite ferrites have revealed a higher dispersion frequency than that of nickel ferrites because of the magnetoplumbite's magnetic anisotropy. The magnetoplumbite ferrite densification temperature always exceeds 1000 °C and the initial low temperature firing permeability of magnetoplumbite ferrites with added glass is too low (μ_i = 2–4). Therefore, it is desirable to develop a material that has a higher permeability at above 300 MHz and can be densified at temperatures below 900 °C. The Bi₂O₃–B₂O₃–ZnO–SiO₂ (BBSZ) glass addition effects on the densification and magnetic properties of Co₂Y–NiCuZn ferrite composites with various Co₂Y/NiCuZn ferrite ratios were investigated. The densification of Co₂Y–NiCuZn ferrite composites was enhanced by the addition of glass at low sintering temperatures (<900 °C) due to the liquid phase sintering. Co₂Y–NiCuZn ferrite composites with 4 wt% BBSZ glass sintered at 900 °C show a relative density above 90%, a high-initial-permeability of 5–6, a quality factor of above 30 in the 200–300 MHz frequency and a resonance frequency above 1 GHz, which can be used in high frequency multilayer chip inductors.     

High-Q microwave dielectric properties of Li(Zn0.95Co0.05)1.5SiO4 ceramics for LTCC applications

This study investigated the effects of Li₂O-MgO-ZnO₂-B₂O₃-SiO₂ (LMZBS) glass on the microstructure, sintering behaviour and microwave dielectric properties of Li(Zn_0.95Co_0.05)_1.5SiO₄ ceramics. Li(Zn_0.95Co_0.05)_1.5SiO₄ powders were synthesised by a traditional solid-state route and added with different amounts of LMZBS glass (0–4 wt%) to decrease the sintering temperature of the ceramics to approximately 900 °C. The XRD patterns showed that no chemical reactions occurred between the Li(Zn_0.95Co_0.05)_1.5SiO₄ ceramics and the LMZBS glass within the doping range. The SEM images indicated that the sample added with 1.5 wt% glass and sintered at 900 °C exhibited a compact and uniform microstructure. In particular, the microwave dielectric properties of the products were related to LMZBS glass content and sintering temperature. The sample with 1.5 wt% LMZBS glass exhibited excellent microwave dielectric properties, namely, ${\epsilon }_r$=6.12, $Q\times f$=83,600 GHz and ${\tau }_f=-39.1\mathrm{ppm}/\mathrm{℃}.$     

Influence of Li2O–B2O3–SiO2 glass on the sintering behavior and microwave dielectric properties of BaO–0.15ZnO–4TiO2 ceramics

This paper reports the investigation of the performance of Li₂O–B₂O₃–SiO₂ (LBS) glass as a sintering aid to lower the sintering temperature of BaO–0.15ZnO–4TiO₂ (BZT) ceramics, as well as the detailed study on the sintering behavior, phase evolution, microstructure and microwave dielectric properties of the resulting BZT ceramics. The addition of LBS glass significantly lowers the sintering temperature of the BZT ceramics from 1150 °C to 875–925 °C. Small amount of LBS glass promotes the densification of BZT ceramic and improves the dielectric properties. However, excessive LBS addition leads to the precipitation of glass phase and growth of abnormal grain, deteriorating the dielectric properties of the BZT ceramic. The BZT ceramic with 5 wt% LBS addition sintered at 900 °C shows excellent microwave dielectric properties: ε_r=27.88, Q×f=14,795 GHz.     

Low-dielectric-constant LiAlO2 ceramics combined with LBSCA glass for LTCC applications

This study used Li₂O–B₂O₃–SiO₂–CaO–Al₂O₃ (LBSCA) glass to reduce the sintering temperature of LiAlO₂ ceramics and to realise the low dielectric constants (ɛ_r<5) of low-temperature co-fired ceramic (LTCC) materials. LBSCA glass remarkably enhanced the densification of LiAlO₂ ceramics. X-ray diffraction patterns indicated that only the γ-LiAlO₂ phase occurred within the doping range of 1 wt% to 3.5 wt%. Scanning electron microscopy images showed dense and uniform grains in samples with 3.0 wt% LBSCA glass. These samples also exhibited low dielectric constants and low dielectric loss when sintered at 900 °C and 950 °C (i.e., ɛ_r=4.48, Qf=35,540 GHz and τ_f=−53 ppm/°C at 900 °C; ɛ_r=4.50, Qf=38,979 GHz and τ_f=−55 ppm/°C at 950 °C, respectively). The material prepared was chemically compatible with silver and showed potential in applications of high-frequency LTCC microwave substrates.     

Low-temperature sintering and microwave dielectric properties of Nd(Co1/2Ti1/2)O3 ceramics using glass addition of oxides

The microstructures and microwave dielectric characteristics of complex perovskite Nd(Co_1/2Ti_1/2)O₃ ceramics with 60P₂O₅–15ZnO–5La₂O₃–5Al₂O₃–5Na₂O–5MgO–5Yb₂O₃ (PZLANMY) additions (1–4 wt%) prepared through the conventional solid-state route were investigated. It was found that Nd(Co_1/2Ti_1/2)O₃ ceramics can be sintered at 1210 °C owing to the sintering aid of PZLANMY-glass addition. At 1300 °C, Nd(Co_1/2Ti_1/2)O₃ ceramics with 1 wt% of PZLANMY-glass addition possess a dielectric constant (ε_r) of 27, a Q×f value of 64,000 GHz and a temperature coefficient of resonant frequency (τ_f) of ?29 ppm/°C. The PZLANMY-glass doped Nd(Co_1/2Ti_1/2)O₃ ceramics can find applications in microwave devices that require low sintering temperature.     

Ba0.7Sr0.3TiO3–glass–silver percolative composite

Ba_0.7Sr_0.3TiO₃–glass–silver (BST–glass–Ag) composites were prepared by the solid state ceramic route. Their percolation behavior and dielectric properties were examined. The pure BST had a percolation limit of 24 vol% of silver whereas an addition of 8 wt% of 50PbO–30B₂O₃–20SiO₂ (PBS) glass lowered the percolation limit to 14 vol% of Ag. Glass addition lowered the sintering temperature of BST from 1300 to 975 °C and addition of Ag further lowered the sintering temperature to 925 °C, minimizing the Ag loss during sintering. The relative permittivity increased from 2700 for pure BST to about five orders of magnitude in the BST–glass–Ag composites near the percolation threshold.     

Silver cofirable (Ca0.9, Mg0.1)SiO3 microwave dielectric ceramics with Li2CO3–Bi2O3 additive

The effects of Li₂CO₃–Bi₂O₃ (LB) additive on the microstructure, phase formation, microwave dielectric properties and applicability for low-temperature co-fired ceramics (LTCC) technology of (Ca_0.9Mg_0.1)SiO₃ (CMS) ceramics were investigated. The sintering temperature of the CMS ceramics was reduced from 1290 °C to 890 °C by the addition of LB. Secondary phases SiO₂ and Bi₄(SiO₄)₃ were detected when LB content was less than 9 wt%. Low melting point liquid phases were formed when LB content was 11–14 wt%. The Q_f value initially increased with the addition of LB and attained the maximum value for the 9 wt% LB-doped CMS ceramic. When the LB content exceeded 9 wt%, the Q_f value decreased because of the presence of liquid phase and abnormal growth of grains. ?_r of 6.92, Q_f of 27,600 GHz and τ_f of ?43.6 ppm/°C were obtained for 9 wt% LB-doped CMS ceramics sintered at 890 °C for 2 h. Also the ceramics can be well co-fired with Ag electrode.     

The effect of sintering temperature and time on microwave properties of Sm(Zn1/2Ti1/2)O3 ceramics for resonators

The microwave dielectric properties of Sm(Zn_1/2Ti_1/2)O₃ ceramics have been investigated. Sm(Zn_1/2Ti_1/2)O₃ ceramics were prepared by conventional solid-state route with various sintering temperatures and times. The prepared Sm(Zn_1/2Ti_1/2)O₃ exhibited a mixture of Zn and Ti showing 1:1 order in the B-site. Higher sintered density of 7.01 g/cm³ can be produced at 1310 °C for 2 h. The dielectric constant values (ɛ_r) of 22–31 and the Q × f values of 4700–37,000 (at 8 GHz) can be obtained when the sintering temperatures are in the range of 1250–1370 °C for 2 h. The temperature coefficient of resonant frequency τ_f was a function of sintering temperature. The ɛ_r value of 31, Q  ×  f value of 37,000 (at 8 GHz) and τ_f value of −19 ppm/°C were obtained for Sm(Zn_1/2Ti_1/2)O₃ ceramics sintered at 1310 °C for 2 h. For applications of high selective microwave ceramic resonator, filter and antenna, Sm(Zn_1/2Ti_1/2)O₃ is proposed as a suitable material candidate.     

High-Q microwave dielectrics in low-temperature sintered (Zn1−xNix)3Nb2O8 ceramics

The effects of Ni substitution for Zn on microwave dielectric properties of (Zn_1−xNi_x)₃Nb₂O₈ (x = 0.02–0.08) ceramics were investigated in this study. The XRD patterns of the sintered samples reveal single-phase formation with a monoclinic structure. The tremendous improvement of Q × f value can be achieved by a small level of Ni substitution (x = 0.05). The τ_f value was found to decrease with a decreasing A-site bond valence. In addition, B₂O₃ and CuO were used as a sintering aid to lower the sintering temperature from 1180 to 900 °C. Excellent microwave dielectric properties (ɛ_r ∼ 20.7, Q × f ∼ 98,000 GHz and τ_f ∼ −85.2 ppm/°C) and a chemical compatibility with Ag electrodes can be obtained for 4 wt% B₂O₃–CuO doped (Zn_0.95Ni_0.05)₃Nb₂O₈ ceramics sintered at 930 °C for 2 h. This constitutes a very promising material for LTCC applications.     

Synthesis and characterization of LTCC compositions with middle permittivity based on CaO-B2O3-SiO2 glass/CaTiO3 system

CaTiO₃ ceramics with the addition of CaO-B₂O₃-SiO₂ (CBS) glass (45–55 wt%) composites were sintered at 830 °C, 850 °C, 875 °C and 900 °C. To illustrate influence mechanism of the different glass contents and sintering temperatures on the properties of the composites, we focused on the multiple performances of the composites by employing different qualitative and quantitative instruments. Composites with 50 wt% glass sintered at 875 °C presented fairly ideal performance: the bulk density was 3.20 g/cm³, the dielectric constant was 25.7 and the dielectric loss was 0.0009 at 7 GHz. Micro-Structure analysis of the composites showed a dense and pore-less microstructure except for few pores with size around 1 μm. In addition, the composite could meet the shrinkage requirement of Ag electrodes and could not possibly react with Ag electrodes any more. This makes them suitable for various dielectric applications at low sintering temperature.     

Densification and biocompatibility of sintering 3.0 mol% yttria-tetragonal ZrO2 polycrystal ceramics with x wt% Fe2O3 and 5.0 wt% mica powders additive

The densification and biocompatibility of sintered 3.0 mol% yttria-tetragonal zirconia polycrystal (3Y-TZP) ceramics, with X wt% Fe₂O₃ and 5.0 wt% mica powders (denoted by 3Y-TZP: X-5.0 wt% mica) have been studied. When the pellets of 3Y-TZP: X-5.0 wt% mica were sintered at 1300 °C for 1 h, the relative shrinkage increases from 19.20–19.43% with the X increased from 0.3 to 1.0. The relative shrinkage of pellets containing 1.0 wt% Fe₂O₃ (X=1.0) increased from 19.43–19.59% when sintering temperatures were raised from 1300 °C to 1450 °C. X-ray diffraction results show that the pellets of 3Y-TZP: X-5.0 wt% mica sintered at 1400 °C for 1 h only contained single phase of tetragonal ZrO₂ (t-ZrO₂). When the sintering temperature was higher than 1400 °C, the Vickers microhardness was greatest in the pellets with X=0.5. Within pellets with the same Fe₂O₃ content, the dominant wavelength (λ_d) was only slightly different for pellets sintered at 1300 °C and those sintered at 1450 °C. The results of the materials were evaluated in vitro cytotoxicity tests reveals that the powders and sintered pellets are safe materials. The oral mucosa irritation tests did not find erythema or histopathological change including normal epithelium, and was free from leucocyte infiltration, vascular congestion and oedema.     

Temperature stability and high-Qf of low temperature firing Mg2SiO4–Li2TiO3 microwave dielectric ceramics

In this work, a series of low-temperature-firing (1−x)Mg₂SiO₄–xLi₂TiO₃–8 wt% LiF (x = 35–85 wt%) microwave dielectric ceramics was prepared through conventional solid state reaction. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses showed that the Li₂TiO₃ phase was transformed into cubic phase LiTiO₂ phase and secondary phase Li₂TiSiO₅. Partial substitution of Mg²⁺ ions for Ti³⁺ ions or Li⁺Ti³⁺ ions increased the cell volume of the LiTiO₂ phase. The dense microstructures were obtained in low Li₂TiO₃ content (x ≤ 65 wt%) samples sintered at 900 °C, whereas the small quantity of pores presented in high Li₂TiO₃ content (x ≥ 75 wt%) samples sintered at 900 °C and low Li₂TiO₃ content (x = 45 wt%) sintered at 850 and 950 °C. Samples at x = 45 wt% under sintering at 900 °C for 4 h showed excellent microwave dielectric properties of ε_r = 10.7, high Q × f = 237,400 GHz and near-zero τ_f = − 3.0 ppm/°C. The ceramic also exhibited excellent chemical compatibility with Ag. Thus, the fabricated material could be a possible candidate for low temperature co-fired ceramic (LTCC) applications.     

Near net-shape fabrication of alumina glass composites

The purpose of the present study is to fabricate alumina glass composites by melt infiltration with better dimensional control through reducing both the presintering and infiltration temperature. Main efforts were put to develop glasses that are chemically compatible with alumina. After extensive investigations, a glass of 21SiO₂–24B₂O₃–35Al₂O₃–15Li₂O–5CaO wt.% was successfully developed. The glass shows good chemical compatibility with alumina at elevated temperatures and low viscosity above 900 °C. Dense alumina glass composites can be fabricated by the melt infiltration process at 950 °C, which is 150 °C lower than the current state-of-art. Investigations showed improved net-shape capability for the newly developed composites, where the total linear shrinkage for the sintering and infiltration at 950 °C is less than 0.1%, as compared with the shrinkage of 0.5% induced by the presintering and infiltration at 1100 °C. Preliminary mechanical tests showed that the fracture strength and toughness of the composites are 303 MPa and 3.4 MPa m^1/2, respectively. The lower processing temperature and the better dimensional control are the major advantages for the newly developed alumina glass composites.     

Effects of glass additions on the microstructure and dielectric properties of barium strontium titanate (BST) ceramics

Commercial glass frits (lead borosilicate glasses) were employed as the sintering aids to reduce the sintering temperatures of BST ceramics. The effects of the glass content and the sintering temperature on the microstructures, dielectric properties and tunabilities of BST ceramics have been investigated. Densification of BST ceramics of 5 wt% glass content becomes significant from sintering temperature of 1000 °C. The glass content shows a strong influence on the Curie temperature T_c, permittivity and the diffuse transition. X-ray results show all BST ceramics exhibit a perovskite structure and also the formation of a secondary phase, Ba₂TiSi₂O₈. The shift of BST diffraction peaks towards higher angle with increasing the glass content indicates the substitution of Pb²⁺ in Ba²⁺ site, which mainly accounts for the diffuse transition observed in these BST ceramics. BST ceramics with 10 wt% glass additives possess the highest tunability at all four sintering temperatures. A tunability of 12.2% at a bias field of 1 kV/mm was achieved for BST ceramics with 10 wt% glass content sintered at 900 °C.     

Low sintering temperature of (Zn0.65Mg0.35)TiO3–xCaTiO3-based dielectric with controlled temperature coefficient

(Zn_0.65Mg_0.35)TiO₃–xCaTiO₃-based dielectric ceramics sintered at low temperature thanks to ZnO–B₂O₃ glass phase addition are investigated. The effects of such additions on the dilatometric curves, the microstructure, the phase composition and the dielectric properties have been carefully examined. It is shown that the sintering temperature is significantly lowered to 930 °C by the addition of 2 wt.% of ZnO–B₂O₃ glass phase. The temperature coefficient of permittivity (τ_?) could be controlled by varying the CaTiO₃ content and lead to near zero τ_? value. As an optimal composition, (Zn_0.65Mg_0.35)TiO₃ + 7%CaTiO₃, co-sinterable with silver electrodes at 930 °C, exhibits at 1 MHz, a relative permittivity of ?_r = 21, a temperature coefficient of the permittivity τ_? of ?4 ppm/°C and low dielectric losses (tan(δ) < 10^?3). These interesting properties make this system promising to manufacture Ag-based electrodes multilayer dielectric devices.     

Effects of Li2CO3 and Sm2O3 additives on low-temperature sintering and piezoelectric properties of PZN-PZT ceramics

To assist the development of applications for multilayer piezoelectric devices, the low-temperature sintering piezoelectric ceramics of 0.3Pb(Zn_1/3Nb_2/3)O₃-0.7Pb(Zr_0.49Ti_0.51)O₃ with Li₂CO₃ and Sm₂O₃ additives were fabricated by a conventional solid-state reaction, and their structural and piezoelectric properties were studied. With the addition of Li₂CO₃, the minimum sintering temperature of 0.3Pb(Zn_1/3Nb_2/3)O₃-0.7Pb(Zr_0.49Ti_0.51)O₃ piezoelectric ceramics was reduced from 1125 °C to 950 °C through the formation of a liquid phase and its piezoelectric properties showed almost no degradation. When the sintering temperature was below 950 °C, however, the piezoelectric properties degraded obviously. The additional Sm₂O₃ resulted in a significant improvement in the piezoelectric properties of 0.3Pb(Zn_1/3Nb_2/3)O₃-0.7Pb(Zr_0.49Ti_0.51)O₃ ceramic with added Li₂CO₃. When sintered at 900 °C, the optimized properties of the 0.3Pb(Zn_1/3Nb_2/3)O₃-0.7Pb(Zr_0.49Ti_0.51)O₃ piezoelectric ceramic with 0.3 wt% Li₂CO₃ and 0.3 wt% Sm₂O₃ were obtained as d₃₃ = 483 pC/N, k₃₁ = 0.376, Q_m = 73, ɛ_r = 2524, tan δ = 0.0178.     

Effect of boron oxide on the microstructure of mullite-based glass-ceramic glazes for floor-tiles in the CaO–MgO–Al2O3–SiO2 system

The effect of increasing replacement of Al₂O₃ by B₂O₃ in a parent glass on the sintering and further crystallization of mullite was investigated. The composition of the parent glass was chosen in the mullite primary phase field of the CaO–MgO–Al₂O₃–SiO₂ quaternary system. Glass powder pellets were heated under standard (10 °C/min and 2 h of hold time) and fast heatings (25 °C/min and 5 min of hold time) at different temperatures from 700 to 1190 °C. Sintering of B₂O₃-containing glasses took place in the range between 850 and 1050 °C. X-ray diffraction results showed that mullite formed as unique crystalline phase for glasses containing amounts of B₂O₃ larger than 6 wt%. For lower amounts of boron oxide cordierite was formed as secondary crystalline phase. Quantitative determination of mullite by Rietveld analysis indicated that the higher amount of mullite present in the glass-ceramic fast heated at 1160 °C was 19.5 wt% for the glass containing 9 wt% of B₂O₃. The final microstructure of the glass-ceramic glazes showed the presence of well shaped, long acicular mullite crystals dispersed within the residual glassy phase. Results of glass-ceramic glazes when applied as slurry and under industrial heating conditions pointed out promising mechanical properties.     

Effects of LiF addition on sintering behavior and microwave dielectric properties of (Mg0.95Zn0.05)2(Ti0.8Sn0.2)O4 ceramics

The effects of LiF addition on the sinterability and microwave dielectric properties of (Mg_0.95Zn_0.05)₂(Ti_0.8Sn_0.2)O₄ (MZTS) ceramics were investigated. A small amount of LiF addition can effectively lower the sintering temperature of MZTS from 1325 °C to 1150 °C due to the liquid phase effect and induce no apparent degradation of the microwave dielectric properties. With increasing LiF content, the apparent density and dielectric constant decreased gradually, the quality factor increased firstly and then decreased. In particular, MZTS–3.0 wt% LiF ceramics sintered at 1150 °C for 5 h exhibited good microwave dielectric properties of ?_r = 13.05, Q · f = 119,310 GHz (at 10 GHz) and τ_f = ?59.2 ppm/°C.     

Preparation and characterization of BaCe0.95Tb0.05O3−α hollow fibre membranes for hydrogen permeation

BaCe_0.95Tb_0.05O_3?α (BCTb) perovskite hollow fibre membranes were fabricated by spinning the slurry mixture containing 66.67 wt% BCTb powder, 6.67 wt% polyethersulphone (PESf) and 26.67 wt% N-methyl-2-pyrrolidone (NMP) followed by sintering at elevated temperatures. The influence of sintering temperature on the membrane properties was investigated in terms of crystal phase, morphology, porosity and mechanical strength. In order to obtain gas-tight hollow fibres with sufficient mechanical strength, the sintering temperature should be controlled between 1350 and 1450 °C. Hydrogen permeation through the BCTb hollow fibre membranes was carried out between 700 and 1000 °C using 50% H₂–He mixture as feed on the shell side and N₂ as sweep gas in the fibre lumen. The measured hydrogen permeation flux through the BCTb hollow fibre membranes reached up to 0.422 μmol cm^?2 s^?1 at 1000 °C when the flow rates of the H₂–He feed and the nitrogen sweep were 40 mL min^?1 and 30 mL min^?1, respectively.     

Effect of B2O3 addition on the sintering temperature and microwave dielectric properties of Zn2SiO4 ceramics

B₂O₃ (25.0 mol%) was added to Zn_2?xSiO_4?x ceramics (0.0 ≤ x ≤ 0.5) to decrease the sintering temperature. Specimens with 0.0 ≤ x ≤ 0.3 sintered at 900 °C were well sintered with a high density due to the formation of a B₂O₃ or B₂O₃–SiO₂ liquid phase. The Q × f value of the Zn₂SiO₄ ceramic was relatively low, 32,000 GHz, most likely due to the presence of a ZnO second phase. A maximum Q × f value of 70,000 GHz was obtained for the specimens with x = 0.2–0.3, and their ?_r and τ_f values were approximately 6.0 and ?21.9 ppm/°C, respectively. Ag metal did not interact with the 25.0 mol% B₂O₃-added Zn_1.8SiO_3.8 ceramic, indicating that Zn_2?xSiO_4?x ceramics containing B₂O₃ are a good candidate materials for low temperature co-fired ceramic devices.