Compositions – Electrically conductive or emissive compositions – Metal compound containing
Patent
1980-02-12
1983-05-31
Barr, J. L.
Compositions
Electrically conductive or emissive compositions
Metal compound containing
252512, 252513, 252518, 252519, 338 20, 338 21, 501152, H01B 106
Patent
active
043860222
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to a voltage non-linear resistance ceramic which has superior protection ability against the overvoltage and is prepared by firing an essential component of zinc oxide added with one or more elements as accessary components in the form of elements or compounds.
BACKGROUND OF THE INVENTION
It is conventionally known that the ceramics prepared by firing an essential component of zinc oxide added with one or more elements as accessary components in the elemental or compound form have a large voltage non-linear coefficient with less leakage current and is suitable to protect against overvoltage in electronic devices such as a semiconductor device which has a less overcurrent capacity so that they may be used for various purposes instead of the varister of SiC and the like.
It is particularly known that the ceramics prepared by firing zinc oxide added with preseodymium, cobalt, potassium and chromium as accessary components in the form of elements or compounds has a superior voltage non-linearity in the region of larger currents with an advantageous property in protection against the overvoltage. However, conventional ceramics have a relatively larger leakage current and are inconvenient for use at high working voltages with poor protection ability against the overvoltage.
DISCLOSURE OF THE INVENTION
A general object of the invention is to further reduce the leakage current of the conventional voltage non-linear resistance ceramics which consists essentially of zinc oxide and is characterized by addition of accessory components of praseodymium, cobalt, potassium, chromium and at least one of magnesium and calcium in the form of elements or compounds. The voltage non-linear resistance ceramics according to the invention comprises (a) zinc oxide as an essential component, as well as (b) praseodymium, cobalt, potassium and chromium either in their elemental or compound form as accessory components, said ceramic being calcined at 1200.degree. to 1400.degree. C. in an oxidizing atmosphere, the improvement in which the respective amounts of the accessory components are, calculated on the elemental basis, in 0.2 to 5.0 atom % for the prasedymium, 0.5 to 5.0 atom % for the cobalt, 0.1 to 0.5 atom % for the potassium and 0.05 to 0.5 atom % (but not exceeding the amount of potassium to be added) for the chromium, and in which the accessory components further comprises at least one of magnesium and calcium either in their elemental or compound form, each in an amount of 0.01 to 2.0 atom %, calculated on the elemental basis. Each accessory component may be added either in its elemental or compound form. If the accessory component is used in the form of compound, it may usually be an oxide. However, any compound, such as carbonate or fluoride may be used, provided that the compound may be converted to its oxide during calcination in an oxidizing atmosphere. Of course, the accessory component, may be added in their elemental form, if desired, and converted to their oxides during calcination in the oxidizing atmosphere. Preferably, the accessory components, namely prasedymium, cobalt, potassium, chromium, magnesium and calcium may be added as PrPhd 6O.sub.11, Co.sub.3 O.sub.4, K.sub.2 CO.sub.3, Cr.sub.2 O.sub.3, MgO and CaO, respectively. In any case, the amount of each accessory component to be added is calculated on its elemental basis. The optimum firing temperature is varied in accordance with the additives. Firing below 1,200.degree. C. is not advantageous since the density of the sintered body goes down with degradation of the electrical characteristics, whereas firing over 1,400.degree. C. invonveniently reduces the voltage non-linearity. Thus, the preferable firing temperature is selected in the range of from 1,200.degree. C. to 1,400.degree. C.
THE MOST PREFERABLE EMBODIMENTS OF THE INVENTION
The invention is described more fully with reference to various embodiments.
At first, ZnO is added with the predetermined amounts of Pr.sub.6 O.sub.11, Co.sub.3 O.sub.4, K.sub.2 C
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Ishii Takashi
Mukae Kazuo
Nagasawa Ikuo
Sakaguchi Toyoshige
Tsuda Koichi
Barr J. L.
Fuji Electric & Co., Ltd.
Schaffer Murray
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