Technology

Tape Casting and Templated Grain Growth. 

    Tape casting is a method to create uniform sheets of material. It has been in use since the 1950s to create dielectric tape for the capacitor industry. A slurry of the raw ceramic and special additives is placed in a chamber and extruded through a small   gap onto a conveyor belt  (Figure 1). The gap and resulting tape thickness are controlled using a “doctor blade.” The conveyor belt can be covered with a non-stick polymer or sacrificial paper that is burned in the sintering process. Tape casting has been used to produce conventional thick-film piezoceramics in the past. It has recently been combined with the process called templated grain growth (TGG) to produce textured PMN-PT ceramics. This breakthrough, known as “textured material” (TM).

Figure 1

    The key to achieving improved properties is to mechanically align (texture) the jumbled crystal domains rather than align them by crystal growth. Texturing polycrystalline ceramic causes its properties to become more anisotropic, yielding performance gains in certain directions. A small fraction of template particles or grains is added to a slurry containing a much finer equiaxed matrix powder. The slurry is then shear-formed using the tape casting method described above to align the anisotropic particles in the direction of least resistance to flow. An electron microscope photograph of the textured result is shown in Figure 2.

Figure 2

    TGG is a cost-effective alternative to Bridgman-grown single crystals. Textured PMN-PT obtained through this process not only shows better piezoelectric properties than random ceramic PMN-PT, but it can approach Bridgman single-crystal performance. The tape casting process, combined with TGG, can be used to make flat plates and other shapes. Wrapping the tape around a mandrel produces a ring-shaped ceramic with domains aligned primarily in the radial direction. 

     However, early indications are that combining the well known commercial tape casting process with TGG enables the production of high-performance flat plates and cylindrical shapes at a fraction of the cost of a single crystal. Textured material promises to be a significant part of the unfolding piezoelectric ceramic technology revolution. 

 CRYSTAL GROWTH TECHNOLOGY

CZOCHRALSKI, WITH DIAMETER CONTROL

    The Czochralski (CZ) furnace equipment consist of a compact set-up for growing a great variety of single crystals, such as Lithium Niobate, pure and doped, bulk Periodic Poled Lithium Niobate, oxides, alkaly halides, fluorides (with control atmosphere) PMN-PT and others materials. All the equipment is controlled by a personal computer, which monitors the entire growth process. The set-up contains a typical PID program and an innovative fuzzy-logic control program developed in our lab, using our experience in the materials field for more than 20 years. All the phases of the growth process can be controlled such as the shoulder formation, the diameter of the crystal, the growth parameters, etc. The furnace works up to 1600ºC. With some modifications it is possible to use the equipment like a Liquid Phase Epitaxy furnace.