A composite material and a method for producing the same

Abstract: A composite material is provided which includes a discrete phase including grains made of a first substance; and a continuous phase including a thin coating film made of a second substance and formed on the surface of each of the grains. The thin coating film has a mean thickness smaller than the mean particle size of the grains. The grains are separated substantially from each other by the thin coating film. The porosity of the composite material is 5% or less.

Description: The present invention relates to an inorganic composite material which is suitable for many applications such as electronic and structural materials, and to a method for producing the composite material. Description of the prior art: Various materials with properties appropriate for their particular applications are required in the manufacture of electronic components, structural parts and other products, and composite materials combining the characteristics of two or more substances are often used in order to obtained the required properties. For example, those such as resin-ceramic and metal-ceramic composite materials are widely used. However, the aforesaid resin-ceramic composite materials have the disadvantage of low rigidity. On the other hand, metal-ceramic composite materials have, for example, the form of metal grains 101 dispersed in a ceramic matrix 102, as shown in Figure 7. The proportion of ceramic constituents in such materials is large, and the metal grains only occupy at most about 50% by volume. The ceramic properties and metallic properties of such composite materials are only manifested to an extent corresponding to the proportions of ceramic material and metal, respectively, which are contained in the mixture. Furthermore, although the rigidity of such composite materials is relatively high, the distances between the grains dispersed in the ceramic matrix is large, and consequently the mechanical strength of such composites is extremely low. Moreover, triangular regions 103 (i.e., regions in the cross-section bounded by at least three grains; triple point of grain boundaries) occupy a relatively large area in articles formed from such composite materials, and numerous pores are present in these triangular regions. Corrosion may begin from these triangular regions, and consequently cracks may occur. Hence, such composite materials tend to exhibit poor weather resistance and low mechanical strength. In addition to the above-described dispersed type of composite material, for example, multilayer laminated structures composed of thin metal films and thin ceramic films are well known as composite materials applicable to the fabrication of magnetic cores. However, if the relative density of 95% is provided to raise the mechanical strength of such a composite material, then plastic deformation will destroy the laminated structure, and consequently, as in the case of the above-mentioned dispersed type of composite material, both the ceramic properties and metallic properties are only manifested to an extent corresponding with the proportions of ceramics and metal, respectively, contained in the material. For example, no material developed so far has the sufficient magnetic properties of magnetic metals, while at the same time adequately exhibiting the electrical insulation properties associated with ceramics. US- A -4.101.348 discloses a process for preparing an aluminum-silicon-iron alloy compact exhibiting an improved combination of electrical and physical properties which process comprises: (1) providing an aluminum-silicon-iron alloy powder, (2) nitriding the powder, (3) sintering the nitrided powder, and (4) hot-pressing the powder simultaneously with or after the sintering until the powder is formed into a compact having a resistivity of at least 100 micro-Ohm·cms. The composite material of this invention, which overcomes the above-discussed and numerous other disadvantages and deficiencies of the prior art, comprises a discrete phase including grains made of a first substance comprising an iron-based metal alloy; and a continuous phase including a thin coating film made of a second dielectric substance and formed on the surface of the grains, the thin coating film having a mean thickness smaller than the mean particle size of the grains, wherein the grains are separated substantially from each other by the thin coating film and the porosity of the composite material is 5% or less. In a more preferred embodiment, the first substance contains an aluminum metal and the second substance contains an aluminum oxide or aluminum nitride. In a more preferred embodiment, the above-mentioned second substance contains at least one selected from the group consisting of boron, lead, vanadium, and bismuth compounds. According to the present invention, the mean thickness of the above-mentioned thin coating film is from 5 to 50 nanometers. In a preferred embodiment, the above-mentioned first substance is a metal and the above-mentioned second substance is an insulating or highly electric-resistive material comprising super-plastic ceramics. In a more preferred embodiment, the super-plastic ceramics contain at least one of the group consisting of apatite and zirconium oxide. In a preferred embodiment, the above-mentioned first substance is a magnetic metal such as an iron-based magnetic metal and the above-mentioned second substance is a highly electric-resistive soft magnetic material such as manganese-zinc ferrite or nickel-zinc ferrite. In a preferred embodiment, the above-mentioned first substance is an iron-based magnetic metal containing aluminum and the above-mentioned second substance is at least one selected from the group consisting of aluminum oxide, aluminum nitride, and aluminum oxide nitride. In a preferred embodiment, the above-mentioned grains have a platelet shape. The method for producing a composite material of this invention, which overcomes the above-discussed and numerous other disadvantages and deficiencies of the prior art, comprises the steps of providing grains made of a first substance comprising an iron-based metal alloy; forming a thin coating film on the surface of the grains to prepare coated grains, the thin coating film being made of a second dielectric substance and having a mean thickness smaller than the mean particle size of the grains; compacting the coated grains into a green body; and densifying the green body in an active gas atmosphere, while forming an additional thin coating film on the uncoated surface of the grains, the additional thin coating film being made of the second substance or a third substance and having a mean thickness smaller than the mean particle size of the grains the combined thickness of said first and second thin coating films is from 5 to 50 nm. In a preferred embodiment, the above-mentioned thin coating film is farmed by heat treatment in an active gas atmosphere. In a more preferred embodiment, the above-mentioned treatment is nitridation in an atmosphere which contains nitrogen. In a more preferred embodiment, the above-mentioned treatment is oxidation in an atmosphere which contains oxygen. In a preferred embodiment, the above-mentioned thin coating film is formed by heat treatment in air. In a preferred embodiment, the above-mentioned thin coating film is formed by means of a sputtering treatment. In a preferred embodiment, the above-mentioned thin coating film is formed by a mechanical alloying treatment. In a preferred embodiment, the above-mentioned green body is densified at a temperature of 300°C or higher under a pressure of 100 kg/cm 2 or more. In a more preferred embodiment, the pressure is applied to the green body in one direction to deform the grains into a platelet shape. In a preferred embodiment, the coated grains mentioned above are mixed with a sintering aid and compacted into a green body which is then densified by a sintering treatment. In a more preferred embodiment, the sintering aid is at least one selected from the group consisting of boron, lead, vanadium, and bismuth compounds. In a preferred embodiment, the above-mentioned grains are heat-treated in an atmosphere containing an active gas to form a thin coating film on the surface of the grains until an increase in the weight of the coated grains falls in the range of 0.01 to 2.5%, and the coated grains are encapsulated into an airtight vessel containing an active gas and then heat-treated under hydrostatic pressure to form a composite material. Thus, the invention described herein makes possible the objectives of (1) providing an inorganic composite material, composed of two or more substances, which can exhibit the desired properties of the constituent substances in a synergistic rather than merely an additive manner; (2) providing a composite material which is of high density, rigid, and has excellent mechanical strength, thermal resistance, and weather resistance; (3) providing a composite material having certain desirable combinations of particular characteristics, for example, composite material which consists of 99.9% metal but is also an electrical insulator, and (4) providing a method for producing a composite material having the desirable characteristics mentioned above. BRIEF DESCRIPTION OF THE DRAWINGS This invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows: Figure 1 is an enlarged partial sectional view showing the structure of a composite material of this invention. Figure 2 is an enlarged partial sectional view showing the structure of another composite material of this invention. Figure 3 is an enlarged partial sectional view showing the structure of still another composite material of this invention. Figure 4 is a graph showing the relationship between the magnetic permeability and the film thickness of a composite material of this invention. Figure 5 is a graph showing the relationship between the electrical resistivity of the composite material and the volume percent occupied by the thin coating films in the composite material. Figure 6 is a graph showing the relationships between the magnetic permeability and the frequency with respect to the composite materials obtained in Examples 2 and 3 of this invention and Example I (Comparative). Figure 7 is an enlarged partial sectional view showing a convention composite material. DETAILED DESCRIPTION OF THE INVENTION The composite material of the present invention comprises a discrete phase including grains made of a first substance comprising an iron-based metal alloy, and a continuous phase including a thin coating film made of a second dielectric substance. The thin coating film is formed on the surface of each or the grains and has a mean thickness smaller than the mean particle size of the grains. The grains constituting the discrete phase are separated substantially from each other by the thin coating film. The aforesaid first and second substances are appropriately selected in accordance with the properties required in the final composite material. For example, the first and second substances can be selected from the group consisting of inorganic substances such as metals, metal oxides, metal nitrides, and ceramics. For example, as the first and second substances, a metal comprising an iron-based metal alloy and a dielectric (i.e., aluminum oxide or aluminum nitride) are selected, respectively, thereby obtaining a composite material combining the properties of these two substances. Excellent magnetic materials can be obtained by selecting a magnetic metal as the first substance. For example, as the first substance, materials with high magnetic permeability such as Si-Al-Fe, Fe-Al, Fe-Ni, or Mo-Ni-Fe alloys, may be used which can provide high saturated magnetic flux density. Soft magnetic materials with electrical insulating properties or high electrical resistivity, such as Mn-Zn ferrite or Ni-Zn ferrite, are suitable for use as the second substance. Compounds such as aluminum oxide, aluminum nitride, or aluminum oxide nitride are also applicable for this purpose. By appropriately selecting the aforesaid types of materials, there can be obtained composite materials capable of attaining high saturated magnetic flux density and also having electrically insulating surfaces. The production of the composite materials comprises preparing coated grains by forming a thin coating film of the second substance on the surfaces of grains made of the first substance; compacting these coated grains into a green body; and densifying the green body. Any of the well-known methods can be used to form the thin coating film made of the second substance. For example, there may be employed a method of allowing grains made of the first substance to come into contact with an active gas, whereby the active gas reacts with the first substance on the surfaces of the grains, thus forming a layer of a substance different from the original first substance on the grain surfaces; or a method using a sputtering procedure to form a layer of the second substance on the surfaces of grains made of the first substance; or a method using a mechanical alloying process to deposit a layer of the second substance on the grain surfaces. The layer of the second substance should have a mean thickness smaller than the mean particle size of the grains made of the first substance; the appropriate sizes of the grains and thin coating film will vary according to the particular purpose and type of composite material, but ordinarily the range of mean particle sizes is 0.1-100 µm while the mean thickness of the thin coating film is 5-50 nm. Specific examples of the formation of the thin coating film, include the formation of metal oxide films on the surfaces of metal grains by oxidation treatment. Next, the coated grains prepared by using any of the above methods are compacted into a green body which is then subjected to a densification process, thereby obtaining the composite material of the present invention. In general, densification can be achieved by high-temperature and/or high-pressure treatment. Ordinarily, densification is accomplished by treatment at a temperature of 300°C or higher under a pressure of 100 kg/cm 2 or higher. When sintering is performed, sintering aids may be added, if necessary. The sintering aids which can be used include compounds of boron, lead, vanadium, or bismuth, such as baron oxide (B 2 O 3 ), lead oxide (PbO), vanadium pentoxide (V 2 O 5 ), and bismuth trioxide (Bi 2 O 3 ). The first substance being a metal the workability and thermal conductivity of the composite material are improved. The reaction products formed from the sintering aids during the sintering treatment remain intermixed with the thin coating film made of the second substance in the composite material obtained. In the step of densification, a portion of the thin coating films on the grains may in some cases be broken by the operations of compression, etc., thereby exposing the grain surfaces made of the first substance. Therefore, densification is ordinarily performed in such a manner that a thin coating film is again formed by the second substance or some third substance. For example, if the densification is performed in an atmosphere of an active gas, the first substance on the exposed surfaces reacts with the active gas, thereby forming an additional thin coating film. As the active gas, oxygen or nitrogen, for example, may be used. A mixture of these active gases may also be used. By selecting a metal as the first substance and an electrical insulator as the second substance, a composite material combining metallic properties and highly effective electrical insulation characteristics can be obtained. In such cases, the following method can also be employed. Firstly, a covering phase made of the second substance is formed on the surfaces of grains made of the first substance, and using these coated grains, green bodies with a relative density of 80-95% are prepared. The sintered bodies are prepared so as to have open pores. Next, these sintered bodies are brought into contact with an electrically insulating substance, or a substance which can generate an electrical insulator, or a substance which can react with the metal and thereby form some electrically insulating substance, and are then once again densified under high pressure to form a composite material with a relative density of 95% or greater. For example, Fe-Si-Al, Fe-Ni, Fe-Si, Fe-Al, or Mo-Ni-Fe are used as the first substance, and oxide films are formed on the surfaces of grains made of this first substance. Next, these grains are brought into contact with a substance, such as oxygen or titanium ethoxide, which can react with the first substance, thereby, forming an electrically insulating substance, while simultaneously performing compression molding, and thus obtaining a composite material which has metallic properties as well as effective electrical insulation characteristics. Moreover, the following method of manufacturing magnetic core materials may be mentioned as an application of the aforesaid method. First, grains made of a magnetic alloy are subjected to oxidising treatment to form an oxide coating on the grain surfaces. This is done in such a manner that the increase in weight amounts to 0.01-2.5%. These coated grains are then encapsulated into an airtight vessel containing an oxygen gas, and heat-treated under hydrostatic pressure until the porosity is reduced to 3% or less. By this procedure, magnetic core materials having magnetic properties of the same order as those of conventional laminated magnetic core materials can be obtained. If a metal is used as the first substance, and if an electrically insulating or highly electric-resistive material containing inorganic ceramics exhibiting super-plasticity is used as the second material, then the aforesaid super-plastic inorganic ceramics conform well with the deformations of the metal grains during the molding process. Consequently, high densification is attained, and moreover, breakdown of the continuous phase composed of the second substance (in this case, an electrically insulating layer) does not occur. Super-plastic inorganic ceramics applicable for the present purpose include apatite, zirconium dioxide (ZrO 2 ), bismuth trioxide (Bi 2 O 3 ), magnesium oxide (MgO), and uranium dioxide (UO 2 ). The apatite is a mineral of the generic formula X 5 (YO 4 ) 3 Z, where X is Ca 2+ , Mg 2+ or Pb 2+ , and Y is P 5+ or As 5+ , and Z is F - , Cl - , OH - . In particular, if a material containing at least one of the apatite and zirconium dioxide is used, then, since these substances exhibit super-plasticity even at relatively low temperatures, the aforesaid advantage can be fully attained. The composite materials formed in this manner have, for example, the structure shown in Figure 1, where the grains 1 made of the first substance are dispersed as a discrete phase in a continuous phase 2 consisting of a thin coating film formed from the second substance. Likewise, composite materials with the structure shown in Figure 2 can also be obtained by compacting and sintering grains made of a substance either identical with or entirely different from the first or second substance together with the aforesaid coated grains. In Figure 2, the face 10 composed of the first substance exhibits complete electrical insulation characteristics, while the opposite face 100 where the first and second substances are intermingled with each other is electrically conductive. A composite material with the structure shown in Figure 3, containing a discrete phase consisting of platelet grains 21 made of the first substance and a continuous phase 22 consisting of thin coating films made of the second substance, can be obtained by applying pressure in only one direction during the densification process employed in the preparation of the composite material. Also, platelet grains can be used from the start for the preparation of the composite material. The composite materials obtained exhibit high hardness along the direction of the longitudinal axes of the grains, and superior wear resistance. In all of the aforesaid composite materials, the cross-sectional area occupied by triangular regions (i.e., regions bounded by three or more particles, indicated in the Figures 1-3 by reference numerals 3, 13, and 23, respectively) is small as compared with conventional types of composite materials, and therefore the composite materials of the present invention exhibit superior mechanical strength. The porosity of the composite materials of the present invention should ordinarily be 5% or less (i.e., the density thereof being 95% or more), and preferably 3% or less. If the porosity is 5% or less, then the existing pores will be closed and isolated from each other in the composite material, and the interior of the composite material will not communicate with the exterior. Therefore, the composite material will exhibit high mechanical strength as well as superior weather and chemical resistances. The composite materials obtained in this manner have a synergistic combination of the properties of the first and second constituent substances. Far example, if the first substance is a metal and the second substance is a metal oxide, then the composite material will have both metallic properties and dielectric characteristics. For example, using the method of the present invention, it is possible to produce composite materials composed of 99.9% metal, while also having electrically insulating characteristics. DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention will be further explained by reference to the following examples. Example 1 A powder consisting of grains made of Fe-5% Al alloy (first substance), with a mean particle size of 30 µm, was oxidized by heating in air at 800°C for one hour, thereby forming a uniform thin coating film of aluminum oxide (second substance) on the surfaces of the alloy grains. Aluminum oxide is a dielectric, and accordingly the coated grains exhibited complete electrically insulating characteristics. This powder consisting of coated grains was then mixed with 0.05 wt% of a wax volatile at low temperatures as a binder, molded, and, after removal of the binder, sintered in air at a temperature of 1350°C under a pressure of 50 kg/cm 2 for one hour. The sintered body (composite material) so obtained was sufficiently dense, with a porosity of about 4.2%. When this sample was cut and polished, the surface thereof is a mirror surface, displaying a metallic luster which manifested the characteristics of the Fe-Al alloy constituting the first substance in the composite material. The areal proportion of triangular regions was found to be approximately 3%. Owing to the compression of the grains, the mean size of these triangular regions was extremely small (i.e., approximately 1.2-2 µm), or less than about 1/15 of the mean particle size of the grains made of the first substance. The electrical resistance of this polished surface was measured by applying a tester, and found to be approximately 15 megohm. Thus, whereas the electrical resistance of a sintered body prepared from a Fe-Al alloy containing 5% aluminum without forming an oxide film exhibited an electrical resistance nearly equal to zero ohm, this composite material, prepared by the method of the present invention, exhibited a metallic luster combined with extremely high electrical resistance, properties ordinarily regarded as incompatible in conventional materials. The hardness of the composite material obtained in this way was also greatly improved, being approximately 30% higher than that of samples formed from the Fe-5% Al alloy in the conventional manner. Moreover, the temperature limit indicating the thermal resistance of the composite material was not merely of the order of about 100°C, as in the case of conventional resin-based composite materials, and in fact no problems of thermal resistance arose even at a temperature of 1000°C. The composite material obtained in the aforesaid manner was loaded into a thermal-resistance vessel, and the vessel was charged with an active gas (oxygen or nitrogen) at ordinary pressure. This sample was then heated at 800°C for one hour, and then subjected to hot pressing under a pressure of 2000 kg/cm 2 for two hours, thereby obtaining a high-density sintered body With a porosity of 0.1% or less. Analysis revealed that the continuous phase of the final sintered body had been made entirely of oxides or nitrides. The areal proportion of triangular regions in the cross-section was 0.1% or less. The electrical resistance of this sample at the cross-section thereof was 20 megohm or more, above the limit of measurement of the tester, while the hardness was approximately 25% higher than that prior to the hot pressing. In another experiment, the composite material obtained in the aforesaid manner was loaded into a thermal-resistance vessel, and the vessel was charged with an inactive gas (e.g., argon) at ordinary pressure. This sample was then heated at 800°C for one hour, and then subjected to hot pressing under a pressure of 2000 atm for two hours, thereby obtaining a high-density sintered body with a porosity of 1.5% or less. However, the electrical resistance of this sample at the cross-section thereof was 1 ohm or less, suggesting that the films at the grain boundaries had been broken. This sort of destruction was also observed by electron microscopy. Example 2 A spherically granular powder (mesh #250, mean particle size approximately 30 µm) composed of an Si-Al-Fe allay containing 10 wt% of silicon, 6 wt% of aluminum, and 84 wt% of iron was immersed in an aqueous solution of nickelous chloride, zinc chloride, and ferrous chloride at 70°C, then the mixture was adjusted to pH 7-8 and allowed to react. Thus, metal was precipitated onto the surfaces of powder grains immersed in this solution, and next this precipitated film was oxidized in air to effect ferritization. By repeating the aforesaid two steps, soft magnetic Ni-Zn ferrite films of thickness 5-50 nm were formed. In separate experiments, the magnetic and electrical characteristics of Ni-Zn ferrite films of thickness 5-50 nm, as such, were evaluated using Ni-Zn ferrite films deposited on glass substrates. The results of these measurements showed that these films exhibited saturated magnetic flux densities B s of about 3000 gauss, magnetic permeabilities µ (CGS) of about 1000, and electrical resistivities β ranging from about 10 5 to 10 9 ohm-cm. The aforesaid Si-Al-Fe alloy powder coated with soft magnetic ferrite was molded by compression at 500 kg/cm 2 , and then hot pressed in nitrogen gas at a temperature of 1000°C under a pressure of 300 kg/cm 2 for two hours, thereby obtaining a high-density composite magnetic material. The magnetic and electrical characteristics of this composite magnetic material are shown in Figures 4 and 5, respectively. Figure 4 shows the relationship between the thickness of the Ni-Zn ferrite film and the magnetic permeability µ (CGS) at the frequency 100 kHz) of the corresponding composite material. Figure 5 shows the relationship between the volume percent occupied by Ni-Zn ferrite films in the composite material and the electrical resistivity of the composite material. From these results, one observes that the magnetic permeability µ (CGS) was 5000-6000, virtually independent of thickness in the range 5-50 nm, while the electrical resistivity β ranged from about 10 5 to 10 7 ohm-cm. The saturated magnetic flux density B s of this composite magnetic material was 9600 gauss, almost identical with that of the original powdered material. Similar results were obtained in the case where the same powder was coated with an Mn-Zn ferrite film. The magnetic and electrical characteristics of Mn-Zn ferrite films were evaluated using films deposited on glass substrates. The results of these measurements showed that these films displayed saturated magnetic flux densities B s of about 5000 gauss, magnetic permeabilities µ (CGS) of about 1000, and electrical resistivities β ranging from about 1 to 10 3 ohm-cm. Example 3 A spherically granular powder composed of the Si-Al-Fe alloy which was employed in the preparation of Example 2 was immersed in an aqueous solution of ethoxysilane and the mixture was thoroughly agitated. Then, this mixture was suction filtered and the residue was dried at 80°C, thereby forming a nonmagnetic silicon dioxide film of thickness 50 nm on the surfaces of the powder grains. This silica-coated Si-Al-Fe alloy powder was molded by compression at 500 kg/cm 2 , and the green body so formed was then subjected to hot pressing in an atmosphere of nitrogen gas under a pressure of 300 kg/cm 2 for a temperature of 1000°C for two hours, thereby obtaining a high-density composite magnetic material. This composite magnetic material exhibited a high resistivity of 10 9 - 10 10 ohm-cm. However, the magnetic permeability µ (CGS) at the the frequency of 100 Hz was low, i.e., 300. When the silicon dioxide film was prepared with a thickness of 5 nm, the electrical resistivity β of the composite magnetic material so obtained ranged from 10 6 to 10 8 ohm-cm, and the magnetic permeability µ (CGS) was 2500. Example I (Comparative) The spherically granular powder composed of the Si-Al-Fe alloy used in various examples described above was immersed in an aqueous solution containing nickelous chloride and ferrous chloride at a temperature of 70°C, then the mixture was adjusted to pH 7-8 and allowed to react, whereby metal was precipitated from the solution onto the powder grains, and an Fe-Ni (permalloy) film of thickness 50 nm was formed on the grain surfaces. This Si-Al-Fe alloy powder, coated with an Fe-Ni film, was then molded by compression at 500 kg/cm 2 , and the resulting green body was hot pressed in an atmosphere of nitrogen gas at a temperature of 1000°C under a pressure of 300 kg/cm 2 for two hours, thereby obtaining a high-density composite magnetic material. In separate experiments, the magnetic and electrical characteristics of Fe-Ni films with a thickness 50 nm, as such, were evaluated using Fe-Ni films deposited on glass substrates. The results of these measurements showed that these films exhibited saturated magnetic flux densities B s of about 8700 gauss, magnetic permeabilities µ (CGS) of about 20,000, and electrical resistivities β of about 60 microhm-cm. The electrical resistivity of the aforesaid composite magnetic material was 100 microhm-cm. Also, the magnetic permeability µ (CGS) of this composite magnetic material was 5500 at the low frequency of 100 Hz and 30 at the high frequency of 1 MHz. These results show that if the electrical resistivity of the composite magnetic material is low, then the magnetic permeability in the high frequency range drops abruptly because of eddy-current loss. Comparative Example II The spherically granular powder composed of the Si-Al-Fe alloy used in the preparation of Example 2 was immersed in an aqueous solution containing nickelous chloride and ferrous chloride at a temperature of 70°C, then the mixture was adjusted to pH 7-8 and allowed to react, whereby metal was precipitated from the solution onto the powder grains. This surface film was then ferritized by oxidation in air. By repeating the aforesaid step, a NiFe 2 O 4 ferrite film with a thickness of 100-1000 nm was formed on the grain surfaces. In separate experiments, the magnetic and electrical characteristics of these NiFe 2 O 4 films with a thickness of 100-1000 nm, as such, were evaluated using NiFe 2 O 4 films deposited on glass substrates. The results of these measurements showed that these films exhibited saturated magnetic flux densities B s of about 3400 gauss, magnetic permeabilities µ (CGS) of about 10, and electrical resistivities ρ of about 10 3 -10 4 ohm-cm. This Si-Al-Fe alloy powder, coated with the aforesaid ferrite film, was then molded by compression at 500 kg/cm 2 , and the resulting green body was hot pressed in an atmosphere of nitrogen gas at a temperature of 1000°C under a pressure 300 kg/cm 2 for two hours, thereby obtaining a high-density composite magnetic material. The magnetic permeability µ (CGS) of this composite magnetic material was 120 at the frequency of 1 MHz, while the electrical resistivity ρ was 10-10 2 ohm-cm. These measurements show that if the surfaces of the powder grains are covered with an insulating film of low magnetic permeability and a thickness of 100 nm or more, then the magnetic permeability of the resulting composite material decreases. As shown by the aforesaid and Examples 2-4, if a material with high electrical resistivity and soft magnetic properties is selected as the second substance referred to above, and if the surfaces of grains of a magnetic metal material are covered with this second substance, then the composite materials so obtained have high electrical resistivity as well as high saturated magnetic flux density and high magnetic permeability. If a magnetic material with high electrical resistivity is chosen as the second substance, then a composite material of high saturated magnetic flux density and high magnetic permeability can be obtained. As demonstrated by Example 2, if ferrite materials, which have high electrical resistivity and high magnetic permeability, and in particular, Mn-Zn ferrites or Ni-Zn ferrites, are employed as the second substance, then composite materials with high magnetic permeability in the high frequency range can be obtained. Example 4 This example will describe a composite material for use in magnetic cores. Substantially spherical grains composed of Sendust (an Fe-Si-Al alloy) with a mean particle size of 20 µm, prepared by gas atomization, were heat-treated at temperatures from 850°C to 950°C for periods ranging from one to ten hours in atmospheres with various concentrations of oxygen. The weight changes which occurred in this process were measured. Also, Auger electron spectroscopic analysis of the surfaces of these powder grains revealed that, in each case, aluminum oxide was the main compound formed during this treatment. Next, this powder was sealed in an airtight vessel together with oxygen, after which the material was sintered with a hot isostatic press apparatus for three hours at a temperature of 800°C under a pressure of 2000 kg/cm 2 . The surfaces of the sintered bodies so obtained were mirror polished, and the surface electrical resistance was measured with a tester over an arbitrarily selected interval of approximately 10 mm. Next, samples 0.5 mm in thickness were prepared, and the magnetic permeability of these samples was measured at the frequency of 1 MHz. The results of these measurements are shown in Table 2. The same tests were also applied to sintered bodies prepared from the same alloy powder which had not undergone the initial heat treatment described above; the results for these samples are also shown in Table 1. Weight Increase of Grains (%) Electrical Resistance (ohm) Magnetic Permeability (CGS) (at 1 MHz) Not heat treated ∼ 0 70 0.010 5 700 0.10 20 1500 0.5 50 1000 1.0 700k 800 1.8 > 20M 700 2.5 > 20M 100 3.0 > 20M 80 The data in Table 1 shows that greater weight increase of the powder is associated with greater electrical resistance. If the weight increase is 1.8% or greater, then the sintered body so obtained, although essentially a bulk metal in composition, exhibits electrical resistance close to that of an insulator. Measurements of the magnetic permeability of samples with a thickness of 0.5 mm at the frequency of 1 MHz revealed that the magnetic permeability (CGS) of samples prepared from untreated powder was low, i.e., about 70, due to eddy-current losses. The powder with a weight increase of 0.10-0.5% due to heat treatment exhibited nearly the maximum magnetic permeability, and weight increases beyond this range actually resulted in diminished magnetic permeability, which in fact, decreased to the low value of 80 for samples with 3% weight increase. This is attributed to the greater resistance to penetration of magnetic flux by thicker layers of nonmagnetic material (oxide layers) as well as loss of magnetic properties by the magnetic phase itself as a result of oxidation. As shown by the schematic illustration in Figure 1, the morphological structure of the sintered bodies (composite materials) obtained in this example comprises a discrete phase including grains 1 made of the first substance (a magnetic metal material) end a continuous phase including a thin coating film 2 made of the second substance (an insulating film, in the present case, aluminum oxide). In actual practice, the thickness of this insulating layer 2 is several tens of nanometers. The size of the grains used in such a composite material should be appropriately selected according to the frequency band in which the magnetic core is to be used, i.e., according to the degree of eddy-current loss. The powder is loaded in an airtight vessel containing oxygen when hot isostatic pressing is performed so that even if the insulating layer is broken by plastic deformation of the powder grains during the process of sintering, oxygen is replenished and an oxide film (insulating film) is reformed at the damaged portions. As a result, passage of current between the metal grains is suppressed. Although oxygen gas was used for this purpose in this example, any active gas which reacts with the first substance and thereby produces the second substance or a third insulating substance can also be used in place of oxygen. The process of sintering by means of heat and isostatic pressure which was used in the preparation of this example is advantageous in that the grains undergo isotropic deformation and therefore the insulating film is relatively insusceptible to breakage. The weight increase of the grains resulting from reaction with the active gas should desirably be in the range from 0.01% to 2.5%. If this weight increase is 0.01% or less, then the insulation effect is often inadequate, whereas if the weight increase amounts to 2.5% or more, then the magnetic characteristics deteriorate. The porosity of the composite material so obtained should desirably not be more than 3%, since composite materials with a porosity of 3% or less have sufficient mechanical strength. Even in cases where the composite material is to be used in magnetic heads, adequate performance is not secured if the porosity amounts to 3% or more. Although an Fe-Si-Al alloy was used in this example, similar results are obtainable if other alloys are used, for example, any appropriate alloy containing aluminum and silicon. Example III (Comparative) An Si-Al-Fe alloy composed of 10 wt% silicon, 6 wt% aluminum, and 84 wt% iron in the form of a spherically granular powder (mean particle size approximately 30 µm) was heat-treated in air at a temperature of 600°C for 5 minutes, thereby forming a coating film on the surfaces of the powder grains. The thickness of this coating film was estimated from the weight increase which occurred during heat treatment and the results of Auger electron spectroscopy as well as the argon sputtering depth profile, and was found to be approximately 9 nm. The powder was molded and hot pressed in an atmosphere of argon at a temperature of 800°C and a pressure of 1000 kg/cm 2 , thereby preparing the primary samples. Several of these primary samples were prepared by varying the hot pressing time between 5 minutes and 2 hours. Density measurements of these primary samples showed that the relative densities were 78% for those hot pressed for five minutes, 90% for those hot pressed for twenty minutes, and 98% for those hot pressed for two hours, with respect to the theoretical density of 6.89 g/cm 3 . These three varieties of samples wore designated as Nos. 1, 2, and 3, respectively. Next, the following three types of treatment were applied to each of these three varieties of molded samples, and the resulting samples were respectively designated as Nos. 4-6, Nos. 7-9 and Nos. 10-12. Samples Nos. 4-6 were prepared from samples interalia No. 3, by heat-treating in air at 700°C for approximately 10 minutes, followed by vacuum packing into airtight vessels. Samples Nos. 7-9 were prepared from samples interalia No. 3, by loading into airtight vessels together with approximately 10 cc of oxygen. Samples Nos. 10-12 were prepared as follows. Three 100 ml aliquots of a 0.01 M ethanol solution of titanium ethoxide were prepared, and interalia sample No. 3 was added to the three respective aliquots. Heat refluxing was performed at 70°C while a 1:3 water-ethanol mixture was added dropwise to each solution until the concentration of water reached 0.002 M after three hours. After completion of this process, the products were filtered, then the powders so obtained were dried at 150°C and heat-treated in air at 400°C for one hour. The powder samples obtained in this manner were then vacuum packed into airtight vessels. The first type of treatment described above was performed for the purpose of increasing the thickness of the insulating film around the pores of the primary samples. The second type of treatment described above was performed for the purpose of filling the pores of the primary samples with oxygen gas in order to induce a reaction between this gas and the metal at the pore sites during the high-pressure molding step (described below) and thereby increase the thickness of the insulating film. The third type of treatment described above was performed for the purpose of forming a titanium dioxide film on the surfaces of the pores of the primary samples. The nine varieties of samples obtained in this manner were subjected to isostatic pressing for one hour at 800°C and 2000 kg/cm 2 in an argon atmosphere, thereby obtaining the final secondary samples. Then, 2 x 1 x 12 mm prismatic test samples were cut from the high-density composite material so obtained, following which the specific gravity, electrical resistance (using a tester), and magnetic characteristics of these test samples were measured, with the results shown in Table 2. Physical properties of composite materials Sample No. Density (g/cm 3 ) Electrical Resistance (ohm) Saturated Magnetic Flux Density (kilogauss) Magnetic Permeability (CGS) Primary Sample Secondary Sample 1 kHz 1 MHz 3 6.75 - 1.0 11 1120 430 4 5.37 6.86 0.2 11 970 350 5 6.34 6.85 > 20M 11 1020 970 6 6.75 6.85 < 0.1 11 1100 120 7 5.37 6.84 0.4 11 850 230 8 6.34 6.84 > 20M 11 1060 990 9 6.75 6.83 < 0.1 11 1080 110 10 5.37 6.77 2k 11 690 470 11 6.34 6.79 > 20M 11 1000 930 12 6.75 6.82 < 0.1 11 1140 70 As can be seen from Table 2, sample No. 3, which was not subjected to a second pressurizing treatment, (density 98%) exhibited the lowest electrical resistance. Moreover, sample No. 3 exhibited a high magnetic permeability at 1 kHz, but the magnetic permeability at 1 MHz was less than half of the value at 1 kHz. Samples obtained by applying further high-pressure sintering after the above-described treatment had been applied to these primary samples, all exhibited low electrically insulating resistance, except for that with primary sinter density 90%, and consequently the frequency dependence of the magnetic permeability became very pronounced. On the other hand, the electrical resistance of the sample with primary density 90% was not greatly lowered by further densification, and therefore the resulting composite material exhibited high magnetic permeability even up to the high frequency range. For purposes of comparison, the aforesaid three varieties of primary samples were also vacuum packed into airtight vessels without any additional treatment, and composite materials were then obtained from these samples by applying isostatic pressure for one hour in an argon atmosphere under the same conditions. Evaluation of the properties of these composite materials revealed that all materials had low electrical resistance and that their magnetic permeability exhibited a pronounced frequency dependence. In general, even if the surface of a metal powder is completely covered by an electrical insulator, if this material is then compressed so as to form a molding of density nearly 100%, then the deformation of the metal powder grains causes a change in surface area, and a portion devoid of an insulating film is formed. As a result, the metal grains come into mutual contact, and this presumably causes a decrease in electrical resistance. As was demonstrated by the results concerning sample No. 3, destruction of the insulating film does not occur unless the density becomes quite high. However, by the method of this example, high density as well as high electrical insulation characteristics can be attained only with the formation of insulating substance at the sites where destruction of the insulating film is prone to occur and without increasing the overall quantity of the insulating substance. In addition to those described in this example, the inventors have also prepared samples with various primary sintered densities, and have ascertained that relative densities in the range from 80% to 95% are most advantageous for the present purpose. If the density of the primary sintered bodies in excessively high, then the pores are closed, which impedes filling with an insulating substance or a compound which can form an insulating substance by an appropriate chemical reaction. On the other hand, if the density of the primary sintered bodies is unduly low, and if a large quantity of an electrically insulating substance is added, then the fraction of the resulting composite material occupied by the insulating substance is unduly large, which results in low magnetic permeability, while conversely, if the quantity of electrically insulating substance added is small, then the insulation resistance is likely to be low.

Claims: A composite sintered material with a resistivity of at least 10 ohm-cm comprising: a discrete phase including grains made of a first substance, said first substance comprising an iron-based metal alloy, and a continuous phase including a first thin coating film made of a second dielectric substance comprising primarily highly electric-resistive compound and formed on the surface of said grains to prepare coated grains, said thin coating film having a mean thickness smaller than the mean particle size of said grains, wherein said grains are separated substantially from each other by said first thin coating film and the porosity of said composite sintered material is 5% or less, said coated grains being compacted into a green body and said green body being densified in an active gas atmosphere, while forming an additional second thin coating film on the surface of said grains coated with the first thin coating film, and said mean thickness of the combined thickness of said first and second thin coating film is from 5 to 50 nanometers. A composite sintered material according to claim 1, wherein said first substance contains an aluminum metal and said second substance contains an aluminum oxide or aluminum nitride. A composite sintered material according to claim 1, wherein said second dielectric substance contains at least one selected from the group consisting of boron, lead, vanadium, and bismuth compounds. A composite sintered material according to claim 1, wherein said mean thickness of the thin coating film is from 5 to 50 nanometers. A composite sintered material according to claim 1, wherein said first substance is an iron-based metal and said second dielectric substance is selected from manganese-zinc ferrite or nickel-zinc ferrite. A composite sintered material according to claim 1, wherein said first substance is an iron-based metal containing aluminum and said second sintered substance is at least one selected from the group consisting of aluminum oxide, aluminum nitride, and aluminum oxide nitride. A composite sintered material according to claim 1, wherein said grains have a platelet shape. A method for producing a composite sintered material with a resistivity of at least 10 ohm-cm, comprising the steps of: providing grains made of a first substance, said first substance comprising an iron-based metal alloy; forming a first thin coating film by heat treatment in an active gas atmosphere on the surface of said grains to prepare coated grains, said thin coating film being made of a second dielectric substance and having a mean thickness smaller than the mean particle size of said grains; compacting said coated grains into a green body; and densifying said green body in an active gas atmosphere, while forming an additional second thin coating film on the surface of said grains coated with the first thin coating film, said additional thin coating film being made of the second dielectric substance or a third dielectric substance and having a mean thickness smaller than the mean particle size of said grains, and said mean thickness of the combined thickness of said first and second thin coating films is from 5 to 50 nanometers. A method according to claim 8, wherein said treatment is nitridation in an atmosphere which contains nitrogen. A method according to claim 8, wherein said treatment is oxidation in an atmosphere which contains oxygen. A method according to claim 8, wherein said first thin coating film is formed by heat treatment in air. A method according to claim 8, wherein said green body is densified at a temperature of 300°C or higher under a pressure of 100 kg/cm 2 or more. A method according to claim 12, wherein said pressure is applied to said green body in one direction to deform said grains into a platelet shape. A method according to claim 8, wherein said coated grains are mixed with a sintering aid and compacted into a green body which is then densified by a sintering treatment. A method according to claim 14, wherein said sintering aid is at least one selected from the group consisting of boron, lead, vanadium, and bismuth compounds. A method according to claim 8, wherein said grains are heat-treated in an atmosphere containing an active gas to form a thin coating film on the surface of said grains until an increase in the weight of said coated grains falls in the range of 0.01 to 2.5 percent, and the coated grains are encapsulated into an airtight vessel containing an active gas and then heat-treated under isostatic pressure to form a composite material. A method according to claim 8, wherein said second dielectric substrate is selected from the group consisting of aluminum oxide, manganese-zinc ferrite, nickel-zinc ferrite and silicon dioxide. A method according to claim 8, wherein said second dielectric substance is aluminum nitride. A method for producing a composite sintered material with a resistivity of at least 10 ohm-cm comprising the steps of: providing grains made of a first substance, said first substance comprising an iron-based metal alloy; forming a first thin coating film by a sputtering treatment on the surface of said grains, said thin coating film being made of a second dielectric substance and having a mean thickness smaller than the particle size of said grains; compacting said coated grains into a green body; and densifying said green body in an active gas atmosphere while forming an additional second thin coating film on the surface of said grains coated with the first thin coating film, said additional thin coating film being made of the second dielectric substance or a third dielectric substance and having a mean thickness smaller than the mean particle size of said grains, and said mean thickness of the combined thickness of said first and second thin coating films is from 5 to 50 nanometers. A method for producing a composite sintered material with a resistivity of at least 10 ohm-cm comprising the steps of: providing grains made of a first substance, said first substance comprising an iron-based metal alloy; forming a first thin coating film by a mechanical alloying treatment on the surface of said grains to prepare coated grains, said thin coating film being made of a second dielectric substance and having a mean thickness smaller than the mean particle size of said grains; compacting said coated grains into a green body; and densifying said green body in an active gas atmosphere, while forming an additional second thin coating film on the surface of said grains coated with the first coating film, said additional thin coating film being made of the second dielectric substance or a third dielectric substance and having a mean thickness smaller than the mean particle size of said grains, and said mean thickness of the combined thickness of said first and second thin coating films is from 5 to 50 nanometers.

Inventors: HIROTA KEN, INOUE OSAMU, KUGIMIYA KOICHI, SATOMI MITSUO, SUGAYA YASUHIRO, HIROTA, KEN, INOUE, OSAMU, KUGIMIYA, KOICHI, SATOMI, MITSUO, SUGAYA, YASUHIRO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1990
Main classification: C04B35
IPC codes: C04B35, C22C33, H01F1, C22C38, C22C1, C22C32, B22F3, B22F1