Electroceramic heating devices

#This invention relates to metallized electroceramic heating devices and particularly to such devices having radially extending lead wires, the inner ends of which have been ultrasonically welded to the devices.

#Metallized electroceramic heating devices are generally made from doped barium titinate ceramics which have a sharp positive temperature coefficient of resistance. The ceramics are designed such that below a critical temperature, the resistance of the ceramic remains at a low value and is essentially constant. When a particular temperature is reached, a crystalline phase change takes place in the ceramic and this change in crystalline structure is accompanied by a sharp increase in the resistance at the crystalline grain boundaries. The result of this crystalline change is an increase in the heater resistance of several orders of magnitude over a small temperature change. For example, barium titinate heaters with a room temperature resistance of 3.0 ohms can increase to 1,000 ohms or more during the crystalline phase change. The temperature at which the cyrstalline phase change takes place can be adjusted in the manufacturing process through the use of appropriate chemical additives and can attain temperatures as high as 300.degree. C., and even higher.

#Commonly, current is carried to the device by means of wires that are attached to the metallized coatings which form the electrodes on the electroceramic body. If attachment is by way of soldering, when making devices that can operate temperatures as high as 300.degree. C., special solders or solder pastes frequently have to be used, together with special fluxes. These special fluxes may seriously harm the devices since they can impregnate the grain boundaries of the ceramic when the device is operated and cause detrimental changes to the resistivity of the heater.

#FIG. 1 is a perspective view of the heater of the present invention showing a metallized surface, the ceramic that forms the body of the heater, and the ultrasonically-attached lead wires.

#FIG. 2 is a side elevational view of the heater shown in FIG. 1.

#FIG. 3 is a perspective view of an ultrasonic welding device that can be used to weld the lead wires to the metallized surfaces of the heater of the present invention.

#Referring now to FIG. 1, the electroceramic device has a thermally sprayed. metallized surface that forms the electrodes 3 which are disposed upon the ceramic substrate 1. The electrodes 3 are separated from each other by the substrate 1 and shoulders 5 which are disposed around the perimeter of the substrate 1. The substrate 1 is made in a flat shape and is formed, generally, of doped barium titinate ceramics which have a sharp positive temperature coefficient of resistance. The device is designed such that below a critical temperature, the resistance of the ceramic remains at a low value and is essentially constant. When the cereamic's anomaly temperature is reached, a crystalline phase change takes place in the ceramic and this change in crystal structure is accompanied by the sharp increase in the resistance at the crystalline grain boundaries. The result of this crystalline change is an increase in the heater resistance of several orders of magnitude over a very small shift in temperature. The temperature at which the crystalline phase change takes place can be adjusted in the manufacturing process through the use of appropriate chemical dopents and can be varied between 50.degree. C. and 300.degree. C. and even higher. When energized with a suitable voltage by applying current to the opposite sides of the device, the ceramic rapidly heats up to its predetermined operating or anomaly temperature and then "locks in" at this temperature.

#This rapid heating is due to the initial low resistance of the ceramic which results in an internal high power of the heater. The "lock in" is due to the abrupt increase in resistance which causes generated power to be reduced until it equals dissipated power. At this point, thermal equilibrium is achieved and the substrate 1 self-regulates itself at the temperature.

#The lead wire 7 is attached to each of the metallized electrodes 3. The lead wire 7 is divided into three sections for the purposes of explaining my invention. At its inner end 7a, the lead wire is swaged and ultrasonically welded to the metallized surface 3. Intermediate section 7b is disposed between outer and 7c and inner end 7a. The intermediate section 7b of the lead 7 commences where the weld between the metallized surface 3 and the inner end 7a terminates. The location of the other end of the intermediate section 7b is less easy to define, but for purposes of explaining this embodiment, it can be considered as being located on a plane that is normal to the edge of the ceramic body 1.