Bo Su

Dielectric and microwave properties of barium strontium titanate (BST) thick films on alumina substrates

Abstract The dielectric and microwave properties of barium strontium titanate (BST) thick films on alumina substrates have been investigated. The BST films were screen printed and sintered at temperatures below 1300xa0°C. At temperatures below the Curie point the BST films exhibit tunability in the range 15–35% under a DC bias field of 2 kV/mm. The dielectric loss is critically dependent on film thickness with lower losses ( 100 μm). A relaxation process appears to take place for the BST films in the MHz to GHz frequency regime. The variation of permittivity with bias field exhibits hysteretic behaviour in both the ferroelectric and paraelectric regions. This is believed to arise due to the non-uniform composition and existence of micro/nano-polar phases in the films.

The processing and properties of barium strontium titanate thick films for use in frequency agile microwave circuit applications

Abstract Barium strontium titanate (BST) thick films for use in frequency agile microwave circuit applications have been investigated. A potentially cost-effective processing route has been used to make BST thick films from mixed-oxide powders. BST thick films have been press-formed in a single step onto alumina substrates from a viscous polymer tape. Films with thicknesses ranging from 10 to 140 μm have been obtained. The microstructure and dielectric properties of the BST thick films have been characterised. The results show that Ba 0.7 Sr 0.3 TiO 3 thick films exhibit ∼20% tunability at room temperature under a DC bias field of 5.5 kV/cm. Possibilities for further improvements are discussed in terms of reducing the interactions between the BST films and alumina substrates.

Interactions between barium strontium titanate (BST) thick films and alumina substrates

Abstract Barium strontium titanate (BST) thick films have been prepared on alumina substrates using a novel press forming route. The effect of the sintering temperature on the microstructure and dielectric properties of the films has been investigated. Severe interactions between the BST films and alumina substrates has been observed for sintering temperatures >1250xa0°C, with Ba and Sr ions diffusing into the substrates. The counter-diffusion of Al ions into the films resulted in the formation of strontium aluminate and the destabilisation of the BST solid solution. The use of B2O3 as a sintering aid and MgO as a buffer layer is also reported and discussed. MgO is shown to be effective in preventing inter-diffusion between the films and substrates and, although B2O3 is effective in reducing the sintering temperature and improving the dielectric properties of the films, it is not compatible with the current thick film processing route.

Processing effects on the microstructure and dielectric properties of Barium Strontium Titanate (BST) ceramics

Barium Strontium Titanate (BST) ferroelectric thick films have been investigated as potential candidates for use in frequency agile microwave circuit devices. Powder processing techniques such as screen-printing have been used to make BST thick films. However, due to the interactions between the BST and substrates such as alumina, the sintering temperatures for the BST thick films are limited and the resultant films are difficult to achieve full densification. In this paper, the effects of different powder processing conditions (calcination, sintering temperature and time) on the sintering behaviour and dielectric properties of the BST ceramics have been investigated. The dielectric behaviour of the ceramics has been correlated with composition and microstructural features such as chemical homogeneity, grain size and domain wall movements.

Hydrothermal and electrophoretic deposition of lead zirconate titanate (PZT) films

Abstract Hydrothermal synthesis in conjunction with colloidal processing has been used to make PZT films in an aqueous solution. Fine PZT powders with a particle size of 200 nm and a narrow particle size distribution were synthesised hydrothermally at 300°C. After washing, a dispersant (di-ammonium citrate) was added to the slurry to make a stable suspension. Electrophoretic deposition (EPD) was then employed to deposit PZT films directly from the hydrothermal PZT suspensions. The effects of the synthesis conditions on the particle size and size distribution of the PZT powders are briefly discussed. Various stabilisation mechanisms for the hydrothermal slurries have been investigated. The effects of the slurry properties and deposition parameters on the microstructure of deposited films are discussed. It is shown that PZT films with a thickness 1000°C) in comparison with powder compacts. In addition, interaction with the substrate was observed when PZT films were deposited and sintered on Pt-coated alumina substrates.

Micropatterning of fine scale ceramic structures

We report a method which combines colloidal processing with polymer mould for the net shape fabrication of fine scale ceramic structures with feature sizes in the micrometer range and aspect ratios up to 10. Both soft and solid polymer moulds have been used, which can be made using lithography, laser micromachining or CNC machining. Fine scale ceramic structures are formed via embossing, moulding and casting. Polymer moulds are removed using either chemical or mechanical methods depending on the aspect ratio and feature size of the fine scale ceramic structures. Some fine scale ceramic structures have been demonstrated in examples such as PZT microrod arrays and alumina microgears.

Routes to net shape electroceramic devices and thick films

Abstract The net shape fabrication of a range of electroceramic devices and thick films is described. The fabrication routes involve producing homogeneous and formable ceramic dough using a viscous polymer processing technique, with various subsequent shape-forming operations to produce devices with sizes ranging from tens of millimetres to tens of microns. The advantages of these processing routes are discussed and examples of large (>1 mm) planar and 3-D components (hemispheres, tubes and helices) and structures with small (

Dielectric and mechanical losses in (Ba,Sr) TiO3 systems

In the application of tuneable microwave devices of ferroelectric (BaSr)TiO3 systems the two critical parameters needed for optimal device performance are high tunability and low dielectric loss. The dielectric loss of the materials is strongly dependent on microstructure. This paper is concerned with an investigation of the variation in the dielectric and mechanical losses in BaxSr1 − xTiO3 systems (x = 0.5, 0.6, 0.7 and 1.0) with microstructure (grain sizes from 1 μm to 50 μm). The magnitude of the loss peak and sharpness of the anomaly in the dielectric constant/elastic modulus observed for the phase transitions in BaxSr1 − xTiO3, depend not only on the composition and but also on the grain size. A relaxation peak has been observed in large grain material, which is indication of interactions between different configurations of domain walls and the diffusion of oxygen vacancies in the domains.

Investigation of Ferroelectric Thick-Film Varactors for Microwave Phase Shifters

This paper presents a novel cost-effective microwave phase-shifter technology using thick-film ferroelectric varactors as the tuning component. The devices are fabricated by conventional screen-printing techniques with silver/platinum as the conducting layers and Ba0.7 Sr0.3TiO3 as the tunable dielectric layer. The microwave performance of the ferroelectric varactors is optimized in terms of both material process parameters and the varactor gate area dimensions. S-parameter measurements on a tunable reflective circuit are used to evaluate the performance of the varactor at around 2 GHz. The varactor parameters such as RF capacitance, tunability, and the Q factor of the varactor are studied as a function of the gate dimension. At 100-V dc-biasing voltage, the BSTO varactor loaded tunable reflective circuit exhibits a differential phase shift of 70deg with a return loss of -1.2 dB. This is equal to a figure-of-merit value of 58deg/dB. Full analog reflection-type and all-pass network phase shifters are also implemented in order to explore the monolithic integration of phase control devices into the RF system based on low-cost ceramic thick-film technology

Preparation of concentrated aqueous alumina suspensions for soft-molding microfabrication

Abstract This paper reports the preparation of highly concentrated aqueous alumina suspensions for demonstrating the microfabrication of ceramics by using soft molds of Poly(dimethyl siloxane) (PDMS) instead of the traditional solid molds. Zeta potential measurements showed very good dispersing effects of the dispersant NH 4 PAA. The rheological properties of concentrated aqueous alumina suspensions have been characterized with varying pH, NH 4 PAA concentration, solids loading and ultrasonic processing time. The intrinsic pH of the suspension was found suitable for molding. The optimum dispersant concentration is 0.14 wt.% and viscosities increase with more NH 4 PAA. A stable and easily processable suspension of 84.0 wt.% was achieved and suitable for soft-molding with viscosity of 0.30 Pa s at 100 s −1 . Embossing and microtransfer molding have been used for soft-molding microfabrication and finally, crack-free and dense microstructures have been fabricated successfully with feature sizes of ∼30 μm.