 |
|
|
||||
|
The literal translation of the term "piezoelectricity" comes for the Greek word piezin, which means "to press". In a more specific sense, the term refers to the ability to create electricity by pressure, and conversely, the development of mechanical strain by electricity. Credit or the discovery of this phenomena is credited to Jacque Currie and his brother Pierre (who along with his wife, Marie, and Henri Becquerel shared the 1903 Nobel Prize in Physics). In 1880, the two brothers demonstrated that when a stress in the form of a weight was applied to quartz and other crystals, positive and negative charges on certain portions of their surfaced developed. These charges were proportional to the pressure exerted and disappeared when the pressure was withdrawn. Later they also confirmed that the change in crystal's dimension occurred when a voltage was applied. It was during World War I that French scientist, Paul Langevin, used piezoelectric. Subsequent work on ceramics in the late 1940's and 1950's led to their widespread application. The first commercial device to be made from piezoelectric barium titanate was a phonograph pickup and was produced in 1947. A number of other ceramics were found to have piezoelectric properties and in 1952, piezoelectric Lead Niobate was discovered in the United States and in 1955, Lead Zirconate Titanate (PZT) compositions were found, thus leading to the mass production of PZT products in the late 1950's. There are several piezoelectric ceramic compositions in common use today: barium titanate, lead zirconate titanate (and modified iterations such as PLZT), lead metaniobate and lead magnesium niobate, PMN, (including electrostrictive formulations). The lead zirconate titanate, PZT, compositions are the most widely usage in applications involving light shutters, micro-positioning devices, speakers and medical array transducers. Barium titanate is considered an outmoded piezoelectric material and is the least used material. The lead metaniobate compositions have excellent electrical characteristics for use in transducers for nondestructive testing and high temperature applications as well as high resolutions medical applications. The lead magnesium particular the cofired actuators. The major applications for piezoelectric ceramics are:
The manufacturing process for piezoelectric ceramics involves a solid state reaction process dependent on the compaction and densification of powders made from oxide materials of very high purity. Preparation of these powders involves a high level of knowledge and experience. Composition, homogeneity, purity, and temperature are all crucial and deviations in any variable will have a catastrophic effect on the final product. For this reason, the raw materials are evaluated on the basis of purity and particle size and then, assuming these raw materials meet the specifications, are blended together to form a compound. In order to assure a completely homogenous composition, the material is subjected to a high temperature (calcine process). This process removes impurities, carbon dioxide and water, it also effects a thermochemical reaction among the constituent oxides and minimizes subsequent volume shrinkage. After the calcine process, the compound is ground to further homogenize any compositional variations which may exist. Further manufacturing is dependent on the part being produced. Usually binders are added and mixed during the ball milling cycle to form a slip. This slip is then spray dried to make a powder suitable for forming. The forming operation is accomplished by pressing the powder into a desired shape using either a hydraulic mechanical, isostatic mechanical, hot press, or vacuum hot press process. High purity, fine-grain materials are made possible through the use of hot and vacuum hot processes. Flat and some tubular configurations are pressed mechanically by hydraulic pressure. Following the forming operation, the binders are removed by a bisque firing. This operation is necessary for the removal of the organic binders to prevent volume reduction of the shape when it is sintered (high fired). The high-fire process required the shape product be placed in a magnesium oxide (MgO) crucible. The reason for placing the shaped product in these crucibles is to maintain the lead balance and stoichiometry during the firing process. Typically, the temperature in the kiln is exceeds 1200 degrees Centigrade. In the high fire process the final material properties and eventual electrical properties are determined. Following high fire of the product, it can be machined by grinding, slicing, and lapping to meet specified dimensions. The specified dimensions will determine the electrical properties such as frequency and capacitance. At this point in the process the material in still inactive and will not have piezoelectric properties. In order to create a dipole, electrodes are applied to specific surfaces of the shaped product. The most common method is applications of the thick film silver by a silk screening. After the silver has air dried the product is sent through a tunnel kiln at a temperature of approximately 500 degrees C and the silver film paste is fused to the surfaces of the ceramic. The final process step is poling. The product is subjected to a high field voltage (55 volts per mil of thickness), temperature and time. At this point the electrical properties are achieved. The product is now ready for final inspections and testing to determine compliance in accordance with a customer's specification.
|