Lifu Chen

Preparation and properties of polymer matrix piezoelectric composites containing aligned BaTiO3 whiskers

A piezoelectric composite has been prepared with highly aligned BaTiO3 whiskers as the active phase and polyvinylidene fluoride (PVDF) as the matrix. Its dielectric and electromechanical properties are characterized. It is found that the dielectric constant (ε), piezoelectric constant (d33) and remnant polarization (Pr) are considerably higher in the whisker composite than in the corresponding composite containing BaTiO3 powders as the active phase, while the loss factors follow the opposite trend. For the whisker composite, ε, d33 and Pr along the direction of the whisker orientation are much higher than the values normal to the whisker orientation. The reasons for the observed differences are discussed.

Fabrication of polymer matrix composites reinforced with controllably oriented whiskers

Polyvinyl chloride (PVC) reinforced with controllably oriented potassium titanate whisker (PTW) has been prepared. The preparation includes wet-spinning a polymer solution that contains the whiskers, placing the resultant precursor fiber in a die and hot-pressing to form composite. The whiskers are highly aligned along the axis of the fiber as the result of extrusion and drawing in spinning. Therefore, the whisker orientation in the composite can be closely controlled through controlling the directions of the precursor. The degree of whisker alignment is found to depend strongly on drawing ratio, and a simplified mathematical relation is presented. The mechanical properties of the PTW reinforced PVC are reported. The applications of the composite processing technique described above in functional composites are also discussed.

Alignment of silicon carbide whiskers in polymer matrix

Polyacrylonitrile (PAN) loaded with up to 70% by weight of silicon carbide whiskers has been used as a model system to demonstrate the potential of whisker alignment by a technique similar to the conventional fibre spinning. Continuous fibres containing high percentages of whiskers can be readily produced using a laboratory scale conventional wet-spinning apparatus. Excellent whisker alignment along the fibre axes was found even for the as-extruded filaments without drawing. However, an improved alignment was obtained through optimizing whisker loading and by drawing the as-spun fibres. Drawing tends to improve the fibre mechanical properties considerably, rendering them easily handable or even useful as a “composite fibre” reinforcement in their own right. Although whisker alignment generally gets better with drawing, over-stretching leads to irregular fibre diameters. Preliminary experiments show that after some minor modifications, this technique may be applied to fabricate whisker reinforced ceramic composites.

Preparation of thin ceramic monofilaments for TEM observation with novel embedding processes

An applicable method to prepare transmission electron microscopy specimens from ceramic fibers for longitudinal and cross-sectional observations is investigated. The method includes novel embedding processes to fix fibers, a polishing process using a self-manufactured device to get uniformly low thickness (40 μm for L-fiber, 60 μm for C-fiber), a one-side dimpling process to grind the specimen to near electron transparency (about 5 μm in thickness for both L-fiber and C-fiber) and an efficient ion milling process using calculated parameters. These techniques are reliable to accomplish the preparation with high quality in a relatively short time. Many factors related to the preparation processes are discussed.

Multifiber ceramic capacitor

A new type of capacitor, multifiber capacitor (MFC) is proposed. It is made up of fiber capacitors (elements) connected in parallel and bonded together with a binder. Each element consists of fiber core (inner electrode), dielectric coating and outer electrode. Analysis indicates that MFC has the optimum capacitance in comparison with multilayer capacitor (MLC) when the diameter of fiber core is carefully matched by the thickness of dielectric coating. Since dielectric layer of a wide range of thickness can be produced more easily as fiber coating than as flat tape, MFC can cover a wider range of capacitance than MLC. Apart from as a possible substitute or supplement to MLC, MFC is potentially useful to bridge the gap between ceramic MLC and thin film capacitor used in integrated circuits. MFC also has better resistance to dielectric breakdown. The possible techniques for the preparation of MFC are also described.