Novel microwave dielectric LTCCs based uponV2O5 doped M2+Cu2Nb2O8 compounds (M2+ = Zn,Co, Ni,Mg and Ca)

The copper-niobates, M²⁺Cu₂Nb₂O₈ (M²⁺ = Zn, Co, Ni, Mg or Ca) have good microwave dielectric properties when sintered between 985–1010 °C and 1110 °C for CaCu₂Nb₂O₈. Therefore, they would be potential dielectric LTCC materials if they could be made to sinter below 960 °C (melting point of silver). To this end, additions of 3 wt.% V₂O₅ were made to ZnCu₂Nb₂O₈, CoCu₂Nb₂O₈, NiCu₂Nb₂O₈, MgCu₂Nb₂O₈ and CaCu₂Nb₂O₈, and their sintering and dielectric behaviour was investigated for samples fired between 800 and 950 °C. Doping lowered sintering temperatures to below the 960 °C limit in all cases. Doping had the general effect of reducing ɛ_r, density, Qf and τ_f, although doped CaCu₂Nb₂O₈ had a Qf value of 9300 GHz, nearly four times that of the best undoped sample. Doped ZnCu₂Nb₂O₈ fired to 935 °C had Qf = 10,200 GHz, and for doped CoCu₂Nb₂O₈ fired to 885 °C Qf = 7500 GHz. When doped and undoped samples all fired to 935 °C were compared, all doped samples had greater ɛ_r and density, and all except ZnCu₂Nb₂O₈ had a smaller τ_f. All doped samples had a more linear relationship between frequency and temperature in the range 250–300 K.     

Testing media influences on the dielectric properties of lead sodium niobate glass-ceramics

The influences of testing media on the breakdown strength (BDS) and dielectric properties of glass-ceramics in the Na₂O–PbO–Nb₂O₅–SiO₂ system were investigated. This work was brought out by consideration of the electric conductance, dielectric constant and breakdown strength of different testing media, which are the main reasons for the different dielectric properties and BDS values of the identical dielectric sample. Leakage current (LC), P–E hysteresis loops, C–V curves and breakdown tests show that the BDS and the dielectric properties of the glass-ceramics could be optimized through using appropriate testing medium. It turns out that three favorable characteristics of the dielectric composites could be optimized in silicon/castor oil mixture: the lowest LC (LC = 6.72 × 10^?6 A, at E = 25 kV/mm), thin P–E hysteresis loops and low hysteresis. Furthermore, the highest BDS of the glass-ceramic was obtained in glycerin (BDS = 105.6 kV/mm with sample thickness of 0.108 mm) compared to other media.     

Formation and micro-Raman spectroscopic study of Aurivilius and fluorite-type SrBi2Nb2O9 nanocrystallites obtained using an ‘amorphous citrate’ route

The crystallization of a SrBi₂Nb₂O₉ gel-glass obtained using the amorphous citrate method was studied by micro-Raman scattering, X-ray diffraction, and electron microscopy techniques. A citric acid–ethanolamine gel with the stoichiometric proportion of the metallic cations was prepared as polymeric precursor and calcined to obtain the amorphous complex. Nanocrystallites with a metastable fluorite-type structure nucleate from the amorphous complex below 500 °C, as shown by X-ray scattering and confirmed by electron microscopy. The morphological study by scanning electron microscopy revealed the nucleation of nanocrystals in the glass-like amorphous powder after thermal treatment at 500 °C. Raman features characteristic of the stable Aurivillius nanocrystals can be detected after thermal treatment at 550 °C, while using X-ray diffraction the crystallization of the Bi-layered perovskite phase is observed only after treatment at 625 °C or higher temperatures. Both X-ray and Raman scattering detected single phase nanocrystallites with Aurivillius structure above 650 °C. The distinctive Raman features of the different SrBi₂Nb₂O₉ nanocrystallites and its evolution with thermal treatment is presented.     

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.     

Effects of Sr substitution for Ba on dielectric and energy-storage properties of SrO-BaO-K2O-Nb2O5-SiO2 glass-ceramics

In this work, [xSrO, (1 − x)BaO]-K₂O -Nb₂O₅-SiO₂ (SBKNS, x = 0.2, 0.4, 0.6, 0.8) glass-ceramics were synthesized through the controlled crystallization method. The phase structure, dielectric and energy-storage properties were systematically studied through the Sr substitution for Ba. It was found that the dielectric properties were improved due to the formation of solid liquid phase Sr_0.5Ba_0.5Nb₂O₆. Breakdown strength firstly increases and then decreases, which strongly depends on the variation in interfacial polarization. The highest value of breakdown strength reaches 1828 ± 88 kV/cm for x = 0.4, which is attributed to more uniform and dense microstructure and lower interfacial polarization. Correspondingly, the optimized theoretical energy-storage density reaches up to 17.45 ± 0.74 J/cm³. The maximum of discharged energy-storage density of 1.45 J/cm³ from P-E loop was acquired under electric field of 500 kV/cm. Moreover, discharged power density of the capacitor was evaluated and reached a high value of 1.76 MV/cm³ in pulsed charged-discharged circuit.     

Ni4Nb2O9 ceramics prepared by the reaction-sintering process

Fabrication of Ni₄Nb₂O₉ ceramics via a reaction-sintering process was investigated. A mixture of raw materials was sintered into ceramics by bypassing calcination and subsequent pulverization stages. Ni₄Nb₂O₉ phase appeared at 1300 °C and increased with increasing soak time. Ni₄Nb₂O₉ content was found >96% in 1350 °C/2 h sintering pellets. A density of 5.71 g/cm³ was obtained for pellets sintered at 1350 °C for 2 h. This reaches 96.5% of the theoretical density. As the sintering temperature increased to 1350 °C, an abnormal grain growth occurred and grains >100 μm could be found. ?_r of 15.4–16.9 are found in pellets sintered at 1200–1300 °C. Q × f increased from 9380 GHz in pellets sintered at 1200 °C to 14,650 GHz in pellets sintered at 1250 °C.     

Synthesis of glass–ceramics in the CaO–MgO–SiO2 system with B2O3, P2O5, Na2O and CaF2 additives

Glass–ceramics based on the CaO–MgO–SiO₂ system with limited amount of additives (B₂O₃, P₂O₅, Na₂O and CaF₂) were prepared. All the investigated compositions were melted at 1400 °C for 1 h and quenched in air or water to obtain transparent bulk or frit glass, respectively. Raman spectroscopy revealed that the main constituents of the glass network are the silicates Q¹ and Q² units. Scanning electron microscopy (SEM) analysis confirmed liquid–liquid phase separation and that the glasses are prone to surface crystallization. Glass–ceramics were produced via sintering and crystallization of glass-powder compacts made of milled glass-frit (mean particle size 11–15 μm). Densification started at 620–625 °C and was almost complete at 700 °C. Crystallization occurred at temperatures >700 °C. Highly dense and crystalline materials, predominantly composed of diopisde and wollastonite together with small amounts of akermanite and residual glassy phase, were obtained after heat treatment at 750 °C and 800 °C. The glass–ceramics prepared at 800 °C exhibited bending strength of 116–141 MPa, Vickers microhardness of 4.53–4.65 GPa and thermal expansion coefficient (100–500 °C) of 9.4–10.8 × 10⁻⁶ K⁻¹.     

Hot corrosion of V2O5-coated NdMgAl11O19 ceramic in air at 950 °C

NdMgAl₁₁O₁₉ ceramic was prepared by solid-state reaction at 1700 °C for 10 h in air, and exhibited a single phase of magnetoplumbite structure. Reaction between molten V₂O₅ and NdMgAl₁₁O₁₉ was investigated at 950 °C using an X-ray diffractometer, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Molten V₂O₅ reacts with NdMgAl₁₁O₁₉ to form α-Al₂O₃, NdVO₄ and MgAl₂O₄ at 950 °C in air. After hot corrosion at 950 °C for 50 h, α-Al₂O₃ is the main corrosion product. The thickness of the corrosion layer gradually increases with increasing corrosion time from 10 to 50 h.     

Superionic conducting Na5SmSi4O12-type glass-ceramics: Crystallization condition and ionic conductivity

Glass-ceramics of the phosphorus-containing Na₅RSi₄O₁₂ (N5)-type (R = rare earth element; Sm) Na⁺-superionic conductors (NaRPSi) were prepared by crystallization of glasses with the composition Na_3.9Sm_0.6P_0.3Si_2.7O₉. The optimum conditions for crystallization were discussed with respect to the conduction properties and the preparation of uncracked N5-type glass-ceramics. Most of the N5-type NaSmPSi compounds were obtained as uncracked glass-ceramics when the heating time for nucleation was more than 6 h. Also studied were the microstructural effects on the conduction properties, which were dependent upon the heating conditions of crystallization. Large enhancement of electrical conductivity was observed in the glass-ceramics as the grain growth was promoted with the increase of the heating time for crystallization. The ionic conductivity of the glass-ceramic Na_3.9Sm_0.6P_0.3Si_2.7O₉ heated at 900 °C for 42 h was 9.07 × 10⁻² S/cm at 300 °C.     

Structure and dielectric properties of perovskite ceramics in the system Ba(Ni1/3Nb2/3)O3–Ba(Zn1/3Nb2/3)O3

Ceramics in the system Ba(Ni_1/3Nb_2/3)O₃–Ba(Zn_1/3Nb_2/3)O₃ (BNN–BZN) were prepared by the mixed oxide route. Powders were mixed and milled, calcined at 1100–1200 °C then pressed and sintered at temperatures in the range 1400–1500 °C for 4 h. Selected samples were annealed or slowly cooled after sintering. Most products were in excess of 96% theoretical density. X-ray diffraction confirmed that all specimens were ordered to some degree and could be indexed to hexagonal geometry. Microstructural analysis confirmed the presence of phases related to Ba₅Nb₄O₁₅ and Ba₈Zn₁Nb₆O₂₄ at the surfaces of the samples. The end members BNN and BZN exhibited good dielectric properties with quality factor (Qf) values in excess of 25,000 and 50,000 GHz, respectively, after rapid cooling at 240 °C h⁻¹. In contrast, mid-range compositions had poor Qf values, less than 10,000 GHz. However, after sintering at 1450 °C for 4 h and annealing at 1300 °C for 72 h, specimens of 0.35(Ba(Ni_1/3Nb_2/3)O₃)–0.65(Ba(Zn_1/3Nb_2/3)O₃) exhibit good dielectric properties: τ_f of +0.6 ppm °C⁻¹, relative permittivity of 35 and quality factor in excess of 25,000 GHz. The improvement in properties after annealing is primarily due to an increase in homogeneity.     

Wetting,densification and phase transformation of La2O3/A2O3/B2O3-based glass–ceramics

A lead-free, non-alkali La₂O₃–Al₂O₃–B₂O₃ (LAB) glass with Al₂O₃ filler had been investigated for low temperature co-firing ceramic (LTCC) application. The glass forming window and several physical properties of the LAB systems were investigated by ICP, TMA, XRD, DSC, and SEM/EDS. The results show that the densification and crystallization temperatures of LAB/Al₂O₃ were between 700 °C and 950 °C and depended greatly on the formulation. Crystalline phase LaBO₃ (LB) and LaAl₂B₃O₉ (L2A3B) crystallized starting at 825 °C and 925 °C, respectively. High degree of densification and crystallization of one glass–Al₂O₃ composition (L30A) was observed with the microstructure composed of tabular L2A3B grains interlocking with submicron Al₂O₃ and LB grains.     

Oxidation behaviour of a (Mo,W)Si2-based composite in dry and wet oxygen atmospheres in the temperature range 350–950 °C

The oxidation of a (Mo, W)Si₂-based composite was investigated in the temperature range (350–950 °C). The influence of temperature and water vapour on the oxidation was examined. The kinetics was studied using a thermobalance whereas the morphology and composition of the oxides were examined using X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive X-ray (EDX). Focused ion beam (FIB) milling was performed on some of the oxide scales which allowed us to look at a non-mechanically disturbed scale/oxide in cross-section. Rapid oxidation was found to occur in the 550–750 °C temperature range. The mass gains were significantly larger in O₂ than in O₂ + 10%H₂O. The different mass changes in the two exposure atmospheres were attributed to the higher vapour pressure of the volatile MoO₂(OH)₂ and WO₂(OH)₂ species in O₂ + 10%H₂O than that of (MoO₃)₃ and (WO₃)₃ in dry O₂. The peak mass gain was found to occur at a temperature of about 750 °C in O₂ and 650 °C in O₂ + 10%H₂O. At temperatures above 850 °C, especially when water vapour is present, the removal of Mo and W from the oxide scales is rapid enough to allow partial healing of the silica, causing the oxidation rate to drop. At 950 °C in O₂ + 10%H₂O, a protective SiO₂ scale could be re-established quickly and maintained, causing the oxidation to essentially cease.     

Preparation of infrared transparent YAG-based glass-ceramics with nano-sized crystallites

Transparent YAG-based glass-ceramics were prepared by a novel method called amorphous sintering followed by controlled crystallization (ASCC) from the compositions of 62.5Al₂O₃–(37.5 ? x)Y₂O₃–xLa₂O₃ (in molar ratio, x = 5, 7, 10 and 20). The stability of the YAG glass was improved by the incorporation of La₂O₃, which increased the activation energy for crystallization. With 10 mol% La₂O₃, bulk YAG glass was prepared by hot-pressing and showed an infrared transmittance of 66%. The YAG glass was converted into glass-ceramics by post annealing at 875 °C for 5 h for controlled crystallization. The obtained glass-ceramic sample showed a crystallite size of 20–50 nm and an infrared transmittance of 60%. With increasing annealing time, the crystallites grew up quickly, resulting in a significant decrease in transparency. In the hot-pressed glass, nano-sized YAG nuclei (～5 nm) were found, which were probably responsible for the crystallization behavior observed at temperatures (e.g. 875 °C) below the onset crystallization temperature.     

Li2O excess (Na0.51K0.47Li0.02)(Nb0.8Ta0.2)O3 lead free piezoelectric ceramics

1 mol% Li₂O excess (Na_0.51K_0.47Li_0.02)(Nb_0.8Ta_0.2)O₃ ceramics were prepared by the conventional mixed oxide method and sintered from 950 to 1200 °C. Also, Li₂O was employed as a sintering aid for high densification and low temperature sintering process. X-ray diffraction results of 1 mol% Li₂O excess (Na_0.51K_0.47Li_0.02)(Nb_0.8Ta_0.2)O₃ lead free piezoelectric ceramics indicated that the specimens were well crystallized and have tetragonal structure. The specimens which sintered at 1050 °C showed the highest piezoelectric properties compared with others. The measured piezoelectric constant and electromechanical coupling coefficient were 231 pC/N and 38.9%, respectively. Curie temperature of (Na_0.51K_0.47Li_0.02)(Nb_0.8Ta_0.2)O₃ ceramics was 344.32, 344.4 and 344.5 °C at 1, 10 and 100 kHz, respectively.     

Oxidative dehydrogenation of ethane to ethylene over V2O5/Nb2O5 catalysts

V₂O₅/Nb₂O₅ catalysts with various V₂O₅ contents were prepared by impregnation and characterized by various techniques in detail. Oxidative dehydrogenation of ethane was carried out in a fixed bed quartz reactor at 500–600 °C. XPS analysis indicated a clear enrichment of vanadium on the near-surface-region and UV–vis diffuse reflectance spectroscopy revealed the nature of VO_x structures formed. 10 wt.% V₂O₅/Nb₂O₅ catalyst has displayed the best performance (X = 28%, S = 38% at 600 °C) due to enrichment of vanadium in the near-surface-region and formation of optimum amount of monomeric/oligomeric VO_x species.     

Effect of Nb2O5 content and sintering temperature on the microstructure and bending strength of porous Ni-YSZ cermets

Porous Ni-YSZ cermets are prepared by reducing NiO-YSZ composites upon exposure to (Ar+6% H₂) gas. The porous cermets are prepared by the addition of carbon black (0.123 mol) to mixed NiO-YSZ powders and the conversion of NiO to Ni in the NiO-YSZ composites. The microstructure and bending strength of porous Ni-YSZ cermets as functions of sintering temperature and Nb₂O₅ content are discussed. The Ni-YSZ cermets consist of uniformly distributed Ni and YSZ grains as well as pores. Both higher sintering temperature and higher Nb₂O₅ content yield lower porosity, thus increasing the bending strength. The bending strength of 0.00470 mol% Nb₂O₅–containing Ni-YSZ cermets sintered at 1400 °C (111 MPa) is about two times higher than that of Nb₂O₅–free Ni-YSZ cermets sintered at 1400 °C (59 MPa).     

Low temperature synthesis of anorthite based glass-ceramics via sintering and crystallization of glass-powder compacts

Anorthite based glass-ceramics were synthesized. The investigated glass compositions are located close to the anorthite-rich corner of the fluorapatite–anorthite–diopside ternary system. Glass powder compacts with mean particle size of 2 and 10 μm were prepared. Sintering behaviour, crystallization and the properties of glass-ceramics were investigated between 800 and 950 °C. In the case of specimens made from the finer particles, complete densification was achieved at a remarkably low temperature (825 °C) and the highest mechanical strength was obtained at 850 °C, but density significantly decreased at higher temperatures. The samples prepared from the larger particles exhibited higher values of density, shrinkage and bending strength within a wider temperature range (825–900 °C). Anorthite was predominantly crystallized between 850 and 950 °C, along with traces of fluorapatite. Diopside was detected only in the MgO richer compositions.     

Sintering,microstructure and electricity properties of ITO targets with Bi2O3–Nb2O5 addition

High density and low electrical resistivity ITO targets were prepared by normal pressure sintering in oxygen with Bi₂O₃–Nb₂O₅ addition. The relative density, microstructure and electrical properties of the ITO targets can be adjusted by changing the sintering temperature (1350 °C~1550 °C) and the content of Bi₂O₃–Nb₂O₅. The results show that the sintering temperature of ITO targets with Bi₂O₃–Nb₂O₅ decreased from 1550 °C to 1450 °C, and the maximum relative density (99.6%) and the lowest electrical resistivity (1.78×10⁻⁴ Ω cm) were reached when the sintering temperature was 1450 °C with 5 wt% Bi₂O₃–Nb₂O₅. The carrier concentration increased as the increase of the contents of Bi₂O₃–Nb₂O₅ and sintering temperature. The mobility first increased, and then decreased above 1450 °C as the sintering temperature increased.     

Hot corrosion behavior of V2O5-coated Gd2Zr2O7 ceramic in air at 700–850 °C

Gd₂Zr₂O₇ ceramic was prepared by solid state reaction at 1650 °C for 10 h in air, and exhibited a defect fluorite-type structure. Reaction between molten V₂O₅ and Gd₂Zr₂O₇ ceramic was investigated at temperatures ranging from 700 to 850 °C using an X-ray diffractometer (XRD) and scanning electron microscopy (SEM). Molten V₂O₅ reacted with Gd₂Zr₂O₇ to form ZrV₂O₇ and GdVO₄ at 700 °C; however, in a temperature range of 750–850 °C, molten V₂O₅ reacted with Gd₂Zr₂O₇ to form GdVO₄ and m-ZrO₂. Two different reactions observed at 700 °C and 750–850 °C could be explained based on the thermal instability of ZrV₂O₇.     

Excellent electrostrictive properties of low temperature sintered PZT ceramics with high concentration LiBiO2 sintering aid

The effect of LiBiO₂ (LBO) additive on the sintering of Pb_0.97La_0.03(Zr_0.53Ti_0.47)_0.9925O₃ (PLZT) ceramics was carefully investigated. 6.0 wt% LBO added PLZT powders could be fully densified to 98% relative density at a temperature as low as 950 °C. It is worthy to notice that there are distinct enhancements in piezoelectric and electrostrictive properties by increasing the soaking time from 2 h to 7 h, which could mainly originate from the improvement of crystallinity and grain size of PLZT ceramics. By controlling the soaking time and concentration of LBO addition, PLZT ceramics sintered at 950 °C could exhibit high curie temperature of 240 °C and very high S₁₁ of 0.22% under 3.0 kV/mm, which is even better than that of traditionally sintered PZT-5, PMN–PZT, and this is very promising for actuators designed in multilayer structure in high temperature environment.