Two specially designed Carbolite electric furnaces integrated with a pair of hydraulic presses are providing thermal processing facilities for Infrared Integrated Systems Ltd (Irisys), a global leader in people counting and queue management systems based in Northampton, UK. At the heart of the Irisys systems are small wafers of specially formulated lead zirconate titanate (PZT), a pyroelectric ceramic that develops a voltage difference across two of its faces when it experiences a temperature change. As a result, when bonded to a read-out circuit, it can be used as a sensor to identify and count people.
PZT is produced initially in powder form and then subjected by Irisys to a two-stage process to densify and compress it so that it will provide the required performance. The first stage involves cold pressing the powder and then sintering in one of six Carbolite chamber furnaces at the site at temperatures above 1,000°C for several hours to form discs 50 mm or 75 mm diameter.
After cooling, the discs are pressed at up to 40 metric tons in conjunction with precisely controlled temperatures in the Carbolite furnaces. This involves using the inbuilt process controller to ramp the furnace temperature between ambient and up to 1,200°C, dwell for a number of hours, and then a controlled cool to ambient.
The furnaces are in two sections, one fixed and the other removable, to provide a vertical cylindrical chamber that fully encloses the base and ram of the press while also allowing the PZT discs to be loaded easily. The front half of each furnace is handled using a wheeled lifting mechanism. Various guides and locking mechanisms are incorporated to ensure safe, secure attachment of the two halves before operation. Temperature controllers and associated power control equipment are housed in separate cabinets.
Processing throughout the cycle is provided by double-spiral silicon carbide heating elements suspended vertically in the chamber and controlled by a PID instrument suitable for the air pressure and hydraulic values. In addition, an over-temperature control system linked to an independent thermocouple automatically cuts power to the elements if an over-temperature condition occurs.