1. Field of the Invention
The present invention relates to a voltage nonlinear resistor.
2. Description of the Related Art
Voltage nonlinear resistor elements (varistor elements) are elements having a structure in which a voltage nonlinear resistor is interposed between a pair of electrodes and are widely used in various sensors such as humidity sensors and temperature sensors, arresters for electric power systems, and the like, as elements for protecting electronic circuits or the like from abnormal voltage. FIG. 2 shows the current-voltage characteristic of a voltage nonlinear resistor. The voltage nonlinear resistor has an excellent protective function and low voltage VH in a high-current region surrounded by a dotted line in FIG. 2. It is known that in a voltage nonlinear resistor containing zinc oxide as a major component, the voltage VH decreases with the decrease in resistivity of inner portions of zinc oxide grains and the resistivity of the inner portions of the zinc oxide grains can be lowered in such a way that a trivalent element such as Al, Ga, or In is added to zinc oxide as a dopant so as to form a solid solution. Furthermore, zinc oxide is known to have a hexagonal crystal structure and anisotropic characteristics depending on a crystal structure.
Examples of the voltage nonlinear resistor containing zinc oxide as a major component include those disclosed in Patent Literatures 1 and 2. Patent Literature 1 discloses a voltage nonlinear resistor which is formed in such a way that acicular zinc oxide particles crystallographically grown in the c-axis direction are oriented in the c-axis direction, are molded, and are sintered. This voltage nonlinear resistor has been measured for varistor voltage in the a- and c-axis directions thereof. Patent Literature 2 discloses a (110) plane-oriented zinc oxide sintered body in view of the fact that the conductivity in the a-axis direction is high.