A thermistor is one of semiconductor electronic parts and is known as a semiconductor electronic parts, which can determine temperature by measuring electric resistance utilizing a property (having a negative or positive temperature coefficient) that electric resistance of the semiconductor changes depending on increase in temperature.
Particularly, a thermistor 10 called a bead thermistor or a glass thermistor includes, as shown in FIG. 1, a semiconductor (thermistor chip) 1, a wire lead 2, and glass for semiconductor encapsulation 3 and the thermistor chip 1 and part of the wire lead 2 are covered and encapsulated by the glass for semiconductor encapsulation (glass for thermistor chip encapsulation) 3, the thermistor can be used at a high temperature and under an oxidative atmosphere. In this connection, as the thermistor chip 1, there are oxide-based materials and at least one non-oxide-based material selected from the group consisting of nitride, carbide, boride and silicide, but oxide-based materials have been widely used mainly in view of properties and price. Moreover, as the wire lead 2, Dumet wire (Ni—Fe alloy covered with Cu), platinum wire, and the like have been widely used.
For such glass for thermistor chip encapsulation, the following are required: (1) to have a sufficiently high volume resistivity in the operating temperature range so as not to influence electric resistance property of the thermistor chip; (2) to match a thermal expansion coefficient of the glass with those of the wire lead and the thermistor chip so that cracks do not generate at the time when the wire lead and the thermistor chip are encapsulated; (3) to be able to achieve encapsulation at a temperature lower than heat-resistant temperature of the wire lead and the thermistor chip.
Hitherto, as glass for thermistor chip encapsulation which satisfies such requirements, there have been proposed PbO—SiO2—B2O3—K2O-based high-lead-content glass (e.g., see Patent Document 1) and alkali borosilicate glass (e.g., see Patent Document 2).
Patent Document 1: JP-A-8-67534
Patent Document 2: JP-A-2002-37641