Compositions – Electrically conductive or emissive compositions – Metal compound containing
Reexamination Certificate
2000-04-28
2003-09-16
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Metal compound containing
C264S617000
Reexamination Certificate
active
06620346
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a new method for manufacturing varistors using nanocrystalline powders obtained by intensive milling.
It also relates to the so manufactured varistors, which differ from similar products presently available in particular in that they have a very high break-down voltage.
BRIEF DESCRIPTION OF THE PRIOR ART
It has been known for a great numbers of years to use varistors containing zinc oxide to protect electrical equipments against over-voltages. Varistors are electrically active elements whose impedance varies in a non-linear manner as a function of the voltage applied to its terminals. These elements are usually in the form of pellets having a diameter of 3 to 100 mm and a thickness of 1 to 30 mm. They essentially consist of a material made of conducting grains of zinc oxide (ZnO) surrounded by insulating grain boundaries made of bismuth oxide (Bi
2
O
3
). After pressing, the pellets are subjected to sintering in a furnace at temperatures ranging from 1000 to 1500° C. for several hours.
At low voltages, the insulating barriers at grain boundaries prevent the current from flowing. Therefore, the material acts as an insulator. When the voltage exceeds a given value called “break down voltage”, the resistance of the boundaries decreases rapidly, thereby making the material a variable resistance or “varistor”. The material becomes then very conductive and the current can be diverted to the ground instead of damaging the electric equipment. Because of their structure, the varistors are mainly used in lightning-arresters like those of the electric energy transportation and distribution networks.
The lightning arresters presently available on the market usually comprise an insulating envelope in the form of a cylindrical tube. This envelope defines a cavity in which are mounted one or several columns of varistors packed one above the others. Each lighting arrester is connected in parallel to the electric equipment to be protected, in order to reduce the over-voltage that may be produced at the terminals of the same. From a practical standpoint, each lightning-arrester forms a normally open circuit which is “converted” into a closed circuit parallel to the equipment to be protected as soon as a significant over-voltage occurs at the terminals of the equipment. Such permits to reduce the insulation level of the electric equipment that is protected.
However, it is worth mentioning that there are numerous other potential applications for varistors, especially for the protection of secondary networks, domestic electric equipments, electronic or miniaturized equipments, etc.
Presently, there are numerous varistors available on the market, which are made of zinc oxide. By way of example of such varistors useful in lightning-arresters, reference can be made to those sold under the trademarks RAYCHEM and SEDIVER. These varistors are manufactured by sintering a mixture of powders of ZnO, Bi
2
O
3
and, optionally, other oxides such as Sb
2
O
3
and/or SiO
2
at temperatures of about 1,200° C. These varistors have an average grain size higher than 3 &mgr;m (about 10 &mgr;m for the RAYCHEM varistors and about 6 &mgr;m for the SEDIVER varistors). Their break-down voltage is proportional to the numbers of grain boundaries or insulating barriers of Bi
2
O
3
per unit length. Such break-down voltage is typically lower than 2.5 kV/cm (about 1.6 kV/cm for the RAYCHEM varistors and about 2 kV/cm for the SEDIVER varistors).
There are numerous scientific articles dealing with the structure and properties of ZnO-based varistors. Some of these articles suggest that the use of a pure or doped nanosize ZnO powder as a starting material would have numerous advantages, including, in particular, a substantial increase of their break-down voltage and of the coefficient of non-linearity of their current-voltage curve (hereinafter called “coefficient &agr;”). Indeed, the break-down voltage seems to be inversely proportional to the ZnO grain size and, accordingly, to the sintering temperature.
By way of example of such articles, reference can be made to the following:
S. HINGORANI et al, “Microemulsion mediated synthesis of zinc-oxide nanoparticles for varistor studies”, Mat. Res. Bull., 28 (1993), 1303
S. HINGORANI et al, “Effect of process variables on the grain growth and microstructure of ZnO—Bi
2
O
3
varistors and their nanosize ZnO precursors”, J. of Materials Research, 10 (1995), 461;
J. LEE et al, “Impedance spectroscopy of grain boundaries in nanophase ZnO”, J. of Materials Research, 10 (1995) 2295;
R. N. VISWANATH et al, “Preparation and characterization of nanocrystalline ZnO based materials for varistor applications”, Nanostructured materials, 6 (1995), 993.
In these articles, nanoparticles of ZnO are prepared by microemulsion (see the articles of S. HINGORANI et al), by gaseous phase condensation (see the article of J. LEE et al) or by colloid suspension and centrifugal separation (see the article of R. N. VISWANATH et al). In all the cases, the obtained powder is pressed to form a pellet or a disc which is then subjected to sintering at a temperature which can be as low as 600° C. to 750° C. to avoid undue increase of the crystallite size (see the articles of R. N. VISWANATH et al and J. LEE et al) or as high as 1,200° C. (see the articles of S. HINGORANI et al).
Recently, an article was published by the present inventors in the proceedings of ISMANAM-96. This article entitled “Ball milled ZnO for varistor applications”, reports the result of tests carried out on pellets prepared from a nanocrystalline powder of pure ZnO obtained by intensive mechanical grinding and subsequently subjected to pressing and sintering at 1,250° C. for 1 hour. These tests show that the so-obtained pellets have no varistor effects, contrary to those obtained from nanosize powder of ZnO obtained by gaseous phase condensation (see again the article of J. LEE et al).
In an article of Z. BRANKOVIC et al, <<Nanostructure constituents of ZnO-based varistors prepared by chemical attrition >>, Nanostructured Materials, 4 (1994), 149, there is disclosed a method for manufacturing a varistor comprising the following steps:
a) first preparing each of the main constituent phases of a ZnO-based varistor;
b) mixing together powders of the constituent phases;
c) intensively milling the powers after the mixing so that the obtained powders be nanocrystalline; and
d) submitting the so-milled mixture to a consolidation treatment comprising a pressing followed by a sintering at a temperature of 1,100° C. (1,373° K.) for 1 hour.
The final product that is so obtained, has the characteristics of a conventional varistor. The ZnO grain size ranges between 5.5 and 7.5 &mgr;m (see Table 2), that is in the typical range of conventional varistors. Moreover, the break down voltages have a value comprised between 4.1 and 6.6 KV/cm. The author mentions: “There is no significant difference in electrical properties between the milled samples and sample Z1 (the reference sample) sintered under the same conditions, but the milled samples have higher values for the sintered density . . . It is evident that varistor mixtures which were intensively milled before sintering are more active for sintering process. It is the consequence of increase of surface free energy and defects concentration, as well as uniform distribution of powder particles and a decrease of powder particles size”.
U.S. Pat. No. 4,681,717 discloses a chemical process for manufacturing varistors, comprising the coprecipitation of metals followed by an oxidation by calcination and a sintering at a temperature of 675 to 740° C. for periods exceeding 4 hours. The so-obtained varistors are disclosed as having a grain size lower than 1 &mgr;m, a break-down voltage of 10 to 100 KV, a coefficient &agr; of non-linearity higher than 30 and a density of about 65 to 99% of the theoretical density depending on the composition and the sintering temperature.
SUMMARY OF THE INVENTION
It has now been discovered that if:
on the one hand, use
Alamdari Houshang
Boily Sabin
Joly Alain
Schulz Robert
Van Neste Andre
Hydro-Quebec
Kopec Mark
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