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Regarding advanced 5G mobile communication, microwave dielectric ceramics are considered as the most potential materials to develop new-generation base station resonators. Herein, ZnNb 2 O 6 ceramics with ε r of approximately 24 have been prepared using the solid-state reaction method, with tailored extra ZnO of x mol% (x = 1, 2 and 3). We have for the first time applied the P-V-L chemical bond theory to investigate ZnNb 2 O 6 ceramics with ZnO doping, by exploring the relationship of dielectric properties and chemical bond characteristics. Particularly, the Raman spectra demonstrates that the full width at half maximum of υ 1 (A g) vibration mode can exhibit significant correlation with the quality factor (Q × f). To further support the experimental study, we have also conducted the first-principle calculation of electron density difference via CASTEP package, which further confirms the change of temperature coefficient of resonance frequency (τ f). Our newly designed ZnNb 2 O 6 ceramics doped with 1 mol% ZnO exhibit excellent dielectric properties, i.e., ε r = 23.74, Q × f = 102,824 GHz and τ f = −55.38 ppm/°C, which demonstrates great potential to construct miniaturized 5G base station with advanced ceramic dielectrics. [ABSTRACT FROM AUTHOR]

Ceramics joining has been widely applied in MEMS sensors, microwave and imaging devices as well as other ceramic packaging applications to fabricate multifunctional ceramic components. In this study, a glass frit with gradient thermal expansion coefficient (TEC) has been applied as the bonding material to join the ceramic of Al 2 O 3 and AlN. This glass frit consists of two types of glasses with different TEC. The Al 2 O 3 and AlN ceramics have been successfully joined with this glass frit in muffle furnace at 850 °C for 1 h. The bonding strength of Al 2 O 3 /AlN ceramic joined via this glass frit exceeds 40 MPa without any cracks in the joining interface, which is much superior compared with that of conventional glass frit bonding. Our work demonstrates that the gradient-TEC glass frit bonding is an effective method to join different types of ceramics. [ABSTRACT FROM AUTHOR]

PbO–ZnO–B 2 O 3 (PZB) glass with low-melting temperature has been widely applied in microelectromechanical systems (MEMS) packaging. However, the high thermal expansion coefficient (TEC) of PZB glass usually leads to strict packaging requirements. Here, in order to achieve ideal TEC, we have introduced β -eucryptite into PZB/ β -eucryptite composite glass and optimized the composite fabrication process. Firstly, PbO:ZnO:B 2 O 3 in mol ratio of 65:15:20 was selected as glass matrix owing to its low-melting temperature and good wettability. Then, β -eucryptite ceramics with ultra-low TEC were synthesized via a facile solid-state reaction method to optimize subsequent composite TEC value. The combination of corresponding material and device characterizations suggest that the pure phase and regular columnar grains of β -eucryptite ceramic can be obtained by sintering at 1450 °C for three hours, which can result in the lowest negative TEC of α 30-300°C = −10.74 × 10−6 K−1. Furthermore, regarding the PZB/ β -eucryptite composite glasses, systematic experiments demonstrated that the β -eucryptite introduction can greatly impact the TEC of matrix glass with only negligible effect on the softening point (T s). Importantly, composite glasses with TEC of α 30-300°C = 3.26 × 10−6 K−1, T s = 380 °C and wetting angle < 30° are ideal for MEMS packaging. This work implements optimized fabrication process to obtain low-melting PZB/ β -eucryptite composite glasses with ideal TEC values, which can be envisaged to be applied in advanced MEMS packaging applications. [ABSTRACT FROM AUTHOR]

The influence of BaO content (up to 15 mol%) on the crystallization behaviour, structure, thermal properties and microwave dielectric properties of the BaO-CaO-B 2 O 3 -SiO 2 glasses and glass-ceramics system was investigated. The glasses were produced by melting at 1400 °C and quenching into water, and the glass-ceramics were produced via heat treatment at temperatures between 750 and 800 °C. The results of X-ray diffraction analysis showed that increasing the BaO content raised the resistance of the glass against crystallization and favoured the transformation of β-CaSiO 3 and α-CaSiO 3 phases, which crystallized in the Ba-free and in low BaO content compositions, into SiO 2 and Ba 4 Si 6 O 16 , which crystallized in compositions with higher concentrations of BaO. The BaO content had little influence on the glass transition temperature ( T g ) and the linear coefficient of thermal expansion (CTE), but strongly reduced the softening point ( T s ). Even the addition of BaO as minor additives resulted in a dramatic reduction of the T s ; for example, the T s decreased from 902 °C for the Ba-free composition to 682 °C for the BaO-containing one (5%). Low values of the dielectric constant (5.9 ≤  ε r ≤ 6.63) and dielectric loss (1.12 × 10 −3 ≤ tan δ  ≤ 3.15 × 10 −3 ) were measured. [ABSTRACT FROM AUTHOR]

Abstract: Porous sialon ceramics have been cost-effectively prepared by pressureless sintering from a mixture of elongated SHS β-sialon powders with an α-sialon precursor composition composed of α-Si3N4, AlN, Y2O3 and Al2O3. The obtained porous sialon ceramics exhibited low shrinkage and homogeneous pore size distribution. The results of X-ray diffraction and scanning electron microscopy showed that a uniform microstructure with elongated and intermingled β-SiAlON grains, which were strongly connected by α-SiAlON phase, has been obtained. [Copyright &y& Elsevier]

• B-site precursor method can effectively inhibit the appearance of second phase. • B-site precursor method can obtain high reactivity Mg 4 Nb 2 O 9 powders. • B-site precursor method can promote integrality of the crystal structure. Mg 4 Nb 2 O 9 ceramics have been prepared by B-site precursor method in this work. Experimental results demonstrate that the uniform particles-size distribution of Mg 4 Nb 2 O 9 powders can be obtained, which exhibit high reactivity to promote the grain growth of Mg 4 Nb 2 O 9 ceramics. Moreover, the Mg 4 Nb 2 O 9 ceramics via B-site precursor method can be sintered at a wide range of sintering temperature from 1275 to 1375 °C with single phase Mg 4 Nb 2 O 9 composition and high crystallinity. Compared to the conventional solid-state method, the Mg 4 Nb 2 O 9 ceramics prepared by B-site precursor method exhibit excellent Q × f value by above 168,756 GHz. [ABSTRACT FROM AUTHOR]

Abstract: The oxygen content is usually difficult to control in direct bonding copper process. In this study a new method for preparation of direct bonding copper on alumina ceramic substrates was realized. 96% Al2O3 ceramic substrates were first oxidized by pasting a thin layer of Cu2O and firing at 1150 °C in air. Then copper foil was bonded to the substrate by heating to 1070 °C in pure N2 atmosphere. Microstructure and composition of the interface between the copper and Al2O3 ceramic were analyzed. The XRD and EDS results show that an interphase of CuAlO2 was formed and a eutectic transformation between oxygen and copper took place at the interface of the copper and the ceramic substrate. The interface was much thicker than the traditionally bonded substrates, which resulted in a better bonding strength. The directly bonded copper alumina substrate samples showed no evidence of de-bonding after 50 thermal cycles comprising quenching from 220 °C to room temperature. [Copyright &y& Elsevier]

BaO-Sm 2 O 3 –5TiO 2 (BST) added by Al 2 O 3 was prepared by solid-state reaction method. The main phase is Ba 3.99 Sm 9.34 Ti 18 O 54 of orthorhombic tungsten bronze structure, and the secondary phase is Ba 1.18 Al 1.90 Ti 6.09 O 16 of tetragonal structure, which has two types of grains: one is grey and cubic, and the other is white and quasi-rectangular. Microstructure was characterized by XRD, SEM, and Raman spectra. Micromechanical properties of different phases were assessed by nanoindentation and micro-scratch tests with Berkovich indenter. Volume fractions of different phases obtained by statistical analysis of grid indentation are in good agreement with those obtained by XRD analysis. Vickers hardness method is more appropriate for the determination of the fracture toughness of dielectric ceramics. BST ceramic without addition of Al 2 O 3 has better mechanical properties than BST-A with addition of Al 2 O 3 , indicating that addition of Al 2 O 3 impairs the micromechanical properties of dielectric ceramics. [ABSTRACT FROM AUTHOR]

Dense AlN ceramics with a thermal conductivity of 180W/m·K were obtained at the sintering temperature of 1750 °C using CaF2 and YF3 as additives. At temperatures below 1650 °C, the shrinkage of AlN ceramics is promoted by liquid (Ca,Y)F2 and Ca12Al14O32F2. Liquid CaYAlO4 mainly improves the densification of the sample when the sintering temperature increases to 1750 °C. The formation of liquid (Ca,Y)F2 at a relatively low temperature results in homogeneous YF3 distribution around the AlN particles, which benefits the removal of oxygen impurity in the AlN lattice, and thus a higher thermal conductivity. [Copyright &y& Elsevier]

Effects of Li2O on the densification and the thermal conductivity of AlN ceramics sintered at a low temperature were studied by using Li2CO3, CaF2 and Y2O3 as additives. DTA and XRD were employed to identify the formation of the liquid and the changes of the phase compositions during the sintering. SEM and TEM were used to observe the microstructures of AlN specimens sintered at 1650 °C. The results show that Li2O is an effective sintering aid, leading to high densification and high thermal conductivity at low sintering temperatures. AlN specimens with full densification and thermal conductivity were achieved with Li2O addition. The high densification comes from the promotion of the liquid containing Li and CaYAlO4. The high thermal conductivity is attributed to the purification of the AlN lattice, the enhancement of the density and the elimination of some grain-boundary phases. [Copyright &y& Elsevier]