Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
1998-12-11
2001-01-16
Speer, Timothy M. (Department: 1775)
Stock material or miscellaneous articles
Composite
Of silicon containing
C428S467000, C428S469000, C428S701000, C428S702000, C428S900000, C428S928000, C148S302000
Reexamination Certificate
active
06174609
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a rare earth-based permanent magnet of high corrosion resistance or, more particularly, to a rare earth-based permanent magnet mainly consisting of a rare earth element, iron and boron and imparted with high corrosion resistance by providing a highly corrosion-resistant coating layer on the surface thereof as well as to a method for the preparation of such a rare earth-based permanent magnet of high corrosion resistance.
By virtue of the excellent magnetic properties and high economical merits for the high performance, the application fields of rare earth-based permanent magnets are rapidly expanding year by year mainly in the field of electric and electronic instruments so that an important issue in this field is to further upgrade the rare earth-based permanent magnets.
Among various types of rare earth-based permanent magnets currently under practical applications, the permanent magnets formed from a ternary alloy of a rare earth element, iron and boron, referred to as a R—Fe—B alloy or magnet hereinafter, in which R is a rare earth element including yttrium and the elements having an atomic number of 57 to 71, constitute the major current because, besides the very superior magnetic properties, the rare earth element R in the R—Fe—B alloy can be neodymium which is, as compared with the earlier developed rare earth-cobalt magnet, in which the rare earth element is mainly samarium, by far more abundant as the natural resources than samarium and hence less expensive and the relatively expensive metal of cobalt need not be employed as an alloying element. Accordingly, the application fields of the R—Fe—B permanent magnets are expanding not only as a substitute for the rare earth-cobalt magnets used heretofore in compact-size instruments constructed by using very small permanent magnets but also in the field where the magnet constructing the magnetic circuit was a large-size inexpensive permanent magnet of low magnetic performance, such as hard ferrite magnets, or an electromagnet.
As a counterbalancing disadvantage to the above mentioned great advantages, the R—Fe—B magnets in general have a serious problem of low corrosion resistance, due to the reactivity of the rare earth element and iron as the principal ingredients, readily to be oxidized in the air, in particular, containing moisture resulting in a decrease in the magnetic performance of the magnet and possible contamination of the ambience by the oxidized matter eventually falling off the magnets.
Therefore, various proposals and attempts were made heretofore for the improvement of the corrosion resistance of the R—Fe—B magnets by the surface treatment including coating of the surface with a resin-containing coating composition, dry-process metallic plating by the method of, for example, ion plating, wet-process metallic plating to form a plating layer of nickel and so on. These surface treatment methods in the prior art are in general very complicated and time-consuming unavoidably leading to a remarkable increase in the overall manufacturing costs of the R—Fe—B magnets.
SUMMARY OF THE INVENTION
The present invention accordingly has an object to provide a R—Fe—B magnet having high corrosion resistance which can be prepared by a convenient and very efficient surface treatment method undertaken at a low cost.
Thus, the R—Fe—B magnet of high corrosion resistance provided by the present invention comprises:
(a) a sintered block of a magnetic alloy mainly consisting of a rare earth element, iron and boron; and
(b) a coating layer on the surface of the sintered block of the magnetic alloy, the coating layer having a composition comprising, as a uniform blend, an alkali silicate and a thermosetting resin.
The above defined R—Fe—B magnet of high corrosion resistance is prepared by a method of the present invention which comprises the steps of:
(A) preparing an aqueous coating composition by admixing an aqueous solution of an alkali silicate with a water-soluble thermosetting resin or an aqueous emulsion of a thermosetting resin;
(B) coating the surface of a sintered block of a magnetic alloy mainly consisting of a rare earth element, iron and boron with the aqueous coating composition prepared in step (A) to form a coating layer;
(C) drying the coating layer; and
(D) subjecting the dried coating layer to a heat treatment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The base body, on which the corrosion-resistant coating layer of a unique composition is formed according to the invention, is a sintered block of a magnetic alloy mainly consisting of a rare earth element, iron and boron, i.e. a R—Fe—B alloy, of which the rare earth element denoted by R constitutes from 5 to 40% by weight of the alloy. The rare earth element R is selected from yttrium and the elements having an atomic number of 57 to 71 but it is preferable that the rare earth element is yttrium or selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, terbium, dysprosium, holmium, erbium, ytterbium and lutetium or, more preferably, the rare earth element R is selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, terbium and dysprosium. It is optional that the constituent R in the R—Fe—B alloy is a combination of two kinds or more of these rare earth elements.
The weight fraction of boron in the R—Fe—B alloy is in the range from 0.2 to 6% by weight. The weight fraction of iron, which is basically the balance to the rare earth element and boron, can be up to 90% by weight. It is optional that a part of the iron in the R—Fe—B alloy is replaced with a minor amount of cobalt in the range, for example, from 0.1 to 15% by weight as the weight fraction of cobalt in the alloy as a whole with an object to improve the temperature characteristic of the magnetic properties. This improvement cannot be accomplished if the weight fraction of cobalt is less than 0.1% by weight while the R—Fe—B magnet would suffer a decrease in the coercive force if the weight fraction of cobalt exceeds 15% by weight. It is further optional that the R—Fe—B alloy is admixed with a limited amount of an adjuvant element selected from the group consisting of nickel, niobium, aluminum, titanium, zirconium, chromium, vanadium, manganese, molybdenum, silicon, tin, copper, calcium, magnesium, lead, antimony, gallium and zinc with an object to improve the magnetic properties of the R—Fe—B magnet or to reduce the costs of the alloy.
The method for the preparation of a sintered block of the magnetic alloy is well known in the art and is not particularly limitative.
In step (A) of the inventive method for the preparation of the corrosion-resistant R—FE—B magnet, an aqueous coating composition is prepared by admixing an aqueous solution of an alkali silicate with a resinous ingredient. The alkali silicate can be selected from sodium silicate or so-called water glass, potassium silicate and lithium silicate either singly or as a combination of two kinds or more, of which sodium silicate is preferred in respect of the inexpensiveness and lithium silicate is preferred when improvement is desired in the water resistance of the coating layer formed according to the inventive method. The concentration of the alkali silicate in the aqueous coating composition is preferably in the range from 3 to 200 g per liter calculated as SiO
2
. When the concentration of the alkali silicate is too low, high corrosion resistance cannot be imparted to the permanent magnet block coated with the coating composition. When the concentration of the alkali silicate in the coating composition is too high, on the other hand, the aqueous solution of the alkali silicate has an unduly high viscosity and hence the coating composition with admixture of the alkali silicate solution with a resinous ingredient also has a high viscosity not to ensure good evenness of the coating layer on the permanent magnet block formed by coating with the coating composition followed by drying and a heat treatment.
The alk
Katsumi Kenichi
Minowa Takehisa
Dougherty & Troxell
McNeil Jennifer
Shin-Etsu Chemical Co. , Ltd.
Speer Timothy M.
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