Abstract: Sintered aluminous abrasive grains having a density of at least 95% of theoretical density and having an alpha alumina crystal size of smaller than one micron. Which are obtained by drying and crushing a lithium containing alumina dispersion and sintering the crushed objects at 1100.degree.-1500.degree. C., said abrasive grains have the strength and hardness equal to or higher than conventional abrasive grains, superior properties in grinding resistance and wear resistance and superior self-sharpening property in course of grinding.

Abstract: Pourable molding compounds containing sinterable powders contain, based on the total compound,a) from 60 to 95% by weight of sinterable powders,b) from 0.1 to 10% by weight of a surfactant, obtainable by alkoxylation of an aliphatic alcohol, fatty acid, fatty acid amide, fatty acid ester or aliphatic amine,c) from 2 to 35% by weight of an organic solvent having a melting point <10.degree. C. and a boiling point of from 40.degree. to 180.degree. C., selected from the group comprising the alkanes, ethers, esters and ketones.

Abstract: A mix and a high alumina refractory shape with high resistance to aluminum penetration comprising aluminum oxide and at least about 1% by weight of boron phosphate.

Abstract: A beta alumina sintered body having a wide optimum sintering temperature range and a high sodium ion conductivity includes 8.0-10.0 wt % of sodium oxide, 3.0-5.5 wt % of magnesium oxide, 0.01-1.0 wt % of tin oxide, and the remainder of aluminum oxide. This sintered body is manufactured by mixing raw materials including 8.0-10.0 wt % of sodium oxide, 3.0-5.5 wt % of magnesium oxide, 0.01-1.0 wt % of tin oxide, and the remainder of aluminum oxide to obtain raw material powder; forming the raw material powder to obtain a formed body; and sintering the formed body to obtain a beta alumina sintered body.

Abstract: A lightweight propping agent made from kaolin clay or mixtures of kaolin clay and lightweight aggregate and having an alumina content between 25% and 40%, a specific gravity of from 2.20 to 2.60 and better conductivity than sand at pressures up to 8,000 psi.

Abstract: There is disclosed an Al.sub.2 O.sub.3 based ceramic which includes 5% to 50% by weight of a hard dispersed phase of at least one compound selected from metal carbide, metal nitride, metal carbonitride, metal oxy-carbide, metal oxy-nitride and metal carbo-oxy-nitride. The compound is represented by M(CNO), wherein M is at least one metal selected from the group consisting of Ti, Zr and Hf. The ceramic contains no greater than 25% by weight of ZrO.sub.2, and balance Al.sub.2 O.sub.3 matrix. which contains an additive dissolved in the Al.sub.2 O.sub.3 grains so as to form solid solution therewith. The additive is contained in an amount of 0.01% to 12% by weight with respect to the Al.sub.2 O.sub.3 matrix and is at least one oxide of an element selected from the group consisting of Y, Mg, Cr, Ni, Co, and rare earth elements. The Al.sub.2 O.sub.3 grains have an average grain size no larger than 1.0 .mu.m while the hard phase constituent has an average grain size no larger than 0.6 .mu.m.

Abstract: Self supporting ceramic composite structures having filler embedded therein produced by a method which includes infiltrating a permeable mass of filler with polycrystalline material comprising an oxidation reaction product obtained by oxidation of a parent metal such as aluminum and optionally containing therein non-oxidized constituents of the parent metal. The structure is formed by placing a parent metal adjacent to a permeable filler and heating the assembly to melt the parent metal and provide a molten body of parent metal which is contacted with a suitable vapor-phase oxidant. Within a certain temperature region and optionally aided by one or more dopants in or on the parent metal, molten parent metal will migrate through previously formed oxidation reaction product into contact with the oxidant, causing the oxidation reaction product to grow so as to embed the adjacent filler and provide the composite structure.

Abstract: The invention concerns self-supporting ceramic structures, including ceramic composite structures embedding a filler, and methods of making them. The ceramic structures comprise a polycrystalline material made by oxidation of a body of molten parent metal with an oxidant. The polycrystalline material has a first region substrate surmounted by a terminal region stratum which is integral with the first region. The terminal region stratum, may be harder and of denser, finer crystalline structure than the first region substrate, is formed in a reaction stage subsequent to the reaction stage in which the first region of polycrystalline material is formed. Growth of the first stage is attained by attenuating or interrupting the transport of molten parent metal to the first region under conditions which nonetheless leave or maintain therein enough oxidizable molten parent metal to form the polycrystalline material of the terminal region.

Abstract: A self-supporting ceramic body produced by oxidation of a molten precursor metal with a vapor-phase oxidant to form an oxidation reaction product and inducing a molten flux comprising said molten precursor metal through said oxidation reaction product. A second metal is incorporated into said molten flux during the oxidation reaction. The resulting ceramic body includes sufficient second metal such that one or more properties of said ceramic body are at least partially affected by the presence and properties of said second metal in the metallic constituent.

Abstract: The present invention relates to a method of forming ceramic matrix composite bodies comprising a parent metal reacting with an oxidant to produce an oxidation reaction product which infiltrates a loose unbounded mass or a preform comprising a filler material mixture. Specifically, the filler material mixture used in the method of the present invention comprises a material having varying compositions, sizes and/or shapes of filler material within the mass or preform. By utilizing a filler possessing varying compositions, sizes and/or shapes, enhanced packing of the filler material is achieved which may result in improved properties, such as erosion resistance, corrosion resistance, etc. Further, the use of filler material containing varying sizes and/or shapes may also enhance the growth rate of oxidation reaction product, thereby reducing processing times for formation of the resultant ceramic composite body.

Abstract: Microcrystalline alpha-alumina based ceramic articles comprising at least 60% alumina by weight, and 0.1-7.0 percent by weight iron equivalence in which substantially all the alumina is in the alpha phase having a uniform grain structure comprising alpha alumina crystallites with an average crystallite size less than 0.5 micrometer and a density greater than 90% theoretical are disclosed. Ceramic articles particularly useful as high modulus refractory fibers, among other shaped bodies such as beads, flakes, coatings, and shaped or randomly-shaped abrasive particles, are produced from a unique sol-gel process wherein hydroxy iron polymers are utilized to nucleate the alpha alumina transformation. An inherent advantage of the unique process disclosed is the resulting fine grained microstructure, and a surface roughness height of less than 100.0 nm, which is essential if a ceramic article such as an alpha alumina fiber is to have reasonable strength.

Abstract: Ceramic foams in which the open cells are connected by a three-dimensional, substantially continuous ceramic matrix formed of interconnected hollow ligaments, are made from an open-cell, reticulated precursor metal, i.e. a metal foam. The precursor metal first is treated so as to allow a support coating to form thereon, and thereafter the coated precursor is heated above the melting point of the metal in the presence of an oxidant to form an oxidation reaction product.

Abstract: The present invention provides an aluminum borate ceramic having a high porosity and a tightly controlled pore size distribution, while maintaining good mechanical strength. The ceramic body can be formed by decomposing boric acid-stabilized aluminum acetate to form an aluminum borate powder, and sintering the powder to form a ceramic body.

Abstract: A method for producing a ceramic is disclosed. The method involves comminuting a batch composed of from 6 to 95 percent of particulate ZrO.sub.2 and from 5 to 94 percent of particulate Al.sub.2 O.sub.3. A part of the particulate ZrO.sub.2 is in a metastable tetragonal crystal structure, for example, as a consequence of 3.sup.m/o of Y.sub.2 O.sub.3 dissolved therein and a part has a monoclinic crystal structure. The comminuted batch is compacted into a shape, and the shape is fired. The degree of comminution of the batch, the firing conditions and the proportions of stabilized and monoclinic ZrO.sub.2 in the batch are controlled to produce a dense, gas impervious ceramic which contains both metastable tatragonal and monoclinic ZrO.sub.2, and in such proportions that the ceramic has improved strength by comparison with an otherwise identical ceramic wherein all of the ZrO.sub.2 is in either a metastable tetragonal or a monoclinic crystal structure.

Abstract: Process for the production of a composite of metal oxides, according to which a metal alcoholate is hydrolysed in the presence of metal (hydr)oxide particles in order to convert the metal alcoholate into a metal (hydr)oxide gel enveloping the metal (hydr)oxide particles. Powders of a composite of metal oxides obtained by this process and ceramic materials obtained by sintering a powder of this type.

Abstract: A calcined microsphere of ultrafine bauxite particles having a mean particle size in the range 0.01 to 0.3 micrometers, said microsphere having a particle size less than 30 micrometers and being characterized by a substantially solid form having a pycnometric density substantially falling in the range 3.2-3.9 g/cm.sup.3, a BET surface area substantially falling in the range 0.05 to 0.5 m.sup.2 /g and a crystal grain size less than 4 micrometers, the surface chemistry of the microsphere being modified to enhance the wettability of the microsphere and to improve its ability to bond strongly with matrix materials in use.

Abstract: Hard crystalline sintered ceramic products, and a method for making them at substantially reduced sintering temperatures, are disclosed, the products being manufactured by the sintering of mixtures comprising one or more ceramic powders selected from among zirconia, stabilized zirconia, partially stabilized zirconia, alumina, mullite, cordierite, and MgAl.sub.2 O.sub.4 spinal together with a combination sintering aide including TiO.sub.2 and at least one transition metal oxide selected from the group consisting of copper oxide, manganese oxide, cobalt oxide and zinc oxide, the mixture comprising at least about 3.5 mole percent TiO.sub.2, at least about 1.3 percent of transition metal oxides, about 4.8-50 mole percent total of TiO.sub.2 +transition metal oxides, and the remainder ceramic powder. Sintering of these mixtures at temperatures typically 200.degree.-350.degree. C.

Abstract: A ceramic green sheet which is fired to give a porous ceramic structure, and which is formed of a composition which principally consists of a ceramic powder, a binder, and a multiplicity of planar or elongate flakes which disappear upon application of heat, and which are oriented in the ceramic green sheet in a direction substantially parallel to opposite major surfaces of the green sheet. Also disclosed is an electrochemical element having a protective layer covering at least one electrode, which layer is prepared from the ceramic green sheet indicated above, and has a multiplicity of planar or elongate pores corresponding to the planar or elongate flakes included in the green sheet. A method of producing the electrochemical element is also disclosed.

Abstract: A magnesia-alumina type spinel clinker contains Fe.sub.2 O.sub.3 and TiO.sub.2 at boundaries between crystal grains, and a refractory produced by using the clinker has excellent corrosion resistance and cement coating properties. The refractory can thus be applied to the fields of cement rotary kiln refractories, refractories for iron making, steel making and nonferrous refining and the like.

Abstract: A boron compound, capable of forming boron oxide on hot pressing in an oxygen-containing atmosphere, is used as a densification aid for hot pressing of alumina-containing ceramics in oxygen-containing atmospheres. The boron compound may be incorporated into the material to be hot pressed or may be applied to the surface of that material. Air may be used as the hot pressing atmosphere. The alumina-containing ceramic to be hot pressed may be formed by reaction processing techniques. Boron nitride is a preferred boron compound.

Abstract: A method of producing a self-supporting ceramic composite body having therein at least one cavity which inversely replicates the geometry of a positive mold of parent metal. The method includes embedding the mold of parent metal within a conformable bed of filler to provide therein a cavity shaped and filled by the mold. The assembly is heated to melt the parent metal mold, e.g., an aluminum parent metal mold, and contacted with an oxidant to oxidize the molten parent metal to form a polycrystalline material which grows through the surrounding bed of filler, the molten metal being drawn through the growing polycrystalline material to be oxidized at the interface between the oxidant and previously formed oxidation reaction product whereby the cavity formerly filled by the mold of parent metal is eventually evacuated of the metal. There remains behind a cavity whose shape inversely replicates the original shape of the mold.

Abstract: This invention relates generally to a novel method of preparing self-supporting bodies, and to novel products made thereby. In its more specific aspects, this invention relates to a method of producing self-supporting bodies comprising one or more boron-containing compounds, e.g., a boride or a boride and a carbide, by reactive infiltration of a molten parent metal actinide into (1) a bed or mass containing boron carbide and, optionally, (2) at least one of a boron donor material (i.e., a boron-containing material) and a carbon donor material (i.e., a carbon-containing material), (3) a bed or mass comprising a mixture of a boron donor material and a carbon donor material and, optionally, (4) one or more inert fillers in any of the above masses, to form the body.

Abstract: Ceramic abrasive grits are formed by an impregnation process that involves preparing an alumina hydrate sol, drying the sol to form a porous solid, crushing the solid to produce particles, calcining the dried particles, preparing a mixture of an additive metal oxide or its precursor in a liquid vehicle, impregnating the mixture into the calcined particles, drying the impregnated particles, calcining the dried impregnated particles, and firing the particles to produce ceramic abrasive grits. The concentration of the modifying additive metal is greater at or near the surface of the grit than at the interior of the grit.

Abstract: In a method for producing ceramic body by oxidation of a parent metal having a graded microstructure characterized by a plurality of zones differing from each other in one or more properties by altering the process conditions during formation of said ceramic body such that a zone of the oxidation reaction product formed posterior to said altering has one or more properties different from a zone of the oxidation reaction product formed anterior to said altering.

Abstract: The processes of the invention involve the formation of an initial reactant mixture by combining sources of SiO.sub.2, B.sub.2 O.sub.3, aluminum, and carbon. The mixture may then optionally be shaped or compacted. The reactant mixture is then heated to start a reaction which is described by the formula:4 C+3 SiO.sub.2 +2 B.sub.2 O.sub.3 +8 Al.fwdarw.4 Al.sub.2 O.sub.3 +B.sub.4 C+3 SiC.Depending on the actual reactant mixture compositions and reaction conditions, other reactions may also occur.

Abstract: Ceramic composites, and in particular structural ceramic composites, have enhanced strength, toughness and abrasion resistance due to the presence of up to about 40 volume percent of relatively large boron carbide whiskers. Although whiskers of various sizes were studied, those of particular interest are whiskers having an average diameter greater than about 3 micrometers, and particularly about 5 to about 8 micrometers, with an average aspect ratio of about 50. Due to the larger size, which had been predicted to be detrimental, there is less respiratory health risk when using these whiskers in the manufacturing of thse improved ceramic composites. Since boron carbide whiskers have many properties superior to silicon carbide whiskers, the most common strengthening agent, the findings should lead to several improved ceramic composites.

Abstract: Self-supporting ceramic composite bodies of desired shape are produced by infiltrating a permeable self-supporting preform with polycrystalline matrix material comprising an oxidation reaction product obtained by oxidation of a parent metal precursor, such as aluminum, and optionally containing therein metallic constituents. The composite body is formed by contacting a zone of a permeable self-supporting preform, having at least one defined surface boundary spaced from said contacting zone, with a body of molten metal which is reacted with a suitable vapor-phase oxidant to form an oxidation reaction product.

Abstract: Preformed silica shapes such as pins and spouts useful in the casting of molten aluminum are provided by which a high strength chemical bond is developed by adding 2 to 10 wt. % of an alkali phosphate-modified alumino-silicate binder to a fused silica mix. To this silica and binder mix, sufficient water is added to obtain a castable consistency. Then, a shape is cast, dried, and shipped for service.

Abstract: A grinding tool is described which comprises self-bonded particles of a ceramic abrasive and has a voids volume of from 5 to 65%.

Abstract: The instant invention is directed to the production of ceramic bodies of at least one disperse inorganic component embedded in a phase of aluminum oxide by producing a mixture of aluminum powder and optionally an alloying element, and a disperse inorganic component which contains at least 40 vol. % of aluminum oxide, attriting the mixture, forming a body, and sintering the body in an oxidizing atmosphere.

Abstract: A method of producing shaped, self-supporting ceramic bodies includes preparing a mold by applying a permeable, conformable material to a shape-defining surface of an expendable pattern. The permeable, conformable material, when set or stable, provides a mold with a shaped surface which is defined by, and is therefore substantially congruent to, the shape-defining surface. Upon heating, the material of the expendable pattern combusts or volatilizes and thus establishes the shaped cavity mold. A molten parent metal is then vaporized with a vapor-phase oxidant in such a manner as to form a ceramic body which grows into the mold cavity, and is shaped by it. The ceramic body is recovered from the mold having a shaped surface replicating the shape-defining surface of the expandable pattern.

Abstract: Sintered ceramic material which comprises doped aluminum titanate and mullite having the composition:50-61.5% by weight Al.sub.2 O.sub.3,36-47.5% by weight titanium oxide, expressed as TiO.sub.2,2.5-5% by weight SiO.sub.2,with the sum total of the three components adding up to 100%, and which further comprises 0.3 to 1% by weight MgO and up to 1% by weight impurities, useful as, inter alia, filters, catalyst supports and internal combustion engine parts, are prepared by mixing and shaping materials of suitable composition and then sintering the shaped composition at temperatures of 1250.degree. to 1600.degree. C. over a holding time of 0.5 to 100 hours.

Abstract: A porous filter for metal hot melt comprises ceramic aggregate particles bound by an inorganic binder. The aggregate particles contain not less than 50 wt % of particles with a shape factor in the range of 100 to 130 and the binder has needle-shaped crystals deposited on the surface thereof. By employing this filter, debris catching ability and initial impregnation of metal hot melt are highly improved.

Abstract: Substantially dense, void-free ceramic-metal composites are prepared from components characterized by chemical incompatibility and non-wetting behavior. The composites have a final chemistry similar to the starting chemistry and microstructures characterized by ceamic grains similar in size to the starting powder and the presence of metal phase. A method for producing the composites requires forming a homogeneous mixture of ceramic-metal, heating the mixture to a temperature that approximates but is below the temperature at which the metal begins to flow and pressing the mixture at such pressure that compaction and densification of the mixture occurs and an induced temperature spike occurs that exceeds the flowing temperature of the metal such that the mixture is further compacted and densified. The temperature spike and duration thereof remains below that at which significant reaction between metal and ceramic occurs. The method requires pressures of 60-250 kpsi employed at a rate of 5-250 kpsi/second.

Abstract: A self-supporting ceramic composite body produced by a method which comprises preparing a polycrystalline material as the oxidation reaction product of a parent metal with a vapor-phase oxidant, comminuting the resulting material to a particulate, forming a permeable mass of said particulate as filler, and infiltrating said particulate with an oxidation reaction product of a parent metal with a vapor-phase oxidant, thereby forming said ceramic composite body.

Abstract: The method of the present invention is useful in preparing an improved alpha alumina-based ceramic. The improvement consists of the addition of a precursor of an alpha phase nucleating to transition alumina. The method is particularly useful in preparing alpha alumina-based abrasive grain.

Abstract: The present invention relates, generally, to a self-supporting porous ceramic composite body and to methods for producing such articles to net or near net shape, which bodies exhibit superior thermal and mechanical properties. The invention also relates to the manufacture of ceramic-metal composite bodies which have ceramic articles incorporated therein.

Abstract: There is disclosed a method for making a self-supporting ceramic composite article having a porous core bearing a dense surface layer formed integrally with said core. A preform comprises a filler material and parent metal distributed therethrough, wherein the volume percent of parent metal is sufficient to form a volume of oxidation reaction product exceeding the total volume available within said preform. The parent metal is melted and reacted with an oxidant to form an oxidation reaction product filling the spatial volume and leaving voids. The reaction is continued to further transport molten parent metal through the oxidation reaction product to at least one surface of the preform to form oxidation reaction product on said surface substantially free of voids thereby forming a relatively dense surface layer.

Abstract: A beta alumina sintered body having no tantalum component in an intergranular phase is manufactured by mixing raw materials consisting essentially of 8.0-10.0 wt % of sodium oxide, 3.0-5.5 wt % of magnesium oxide and/or 0.1-2.0 wt % of lithium oxide, 0.01-0.5 wt % of tantalum oxide, and the remainder of aluminum oxide to obtain raw material powders; forming the raw material powders to obtain a formed body; and sintering the formed body.

Abstract: A high alumina ceramic composition has controlled small amounts of MgO and CaO. When refractories made from the composition are fired, the alumina and MgO react to form spinel. The CaO promotes such reaction while minimizing undesirable expansion, and results in a refractory having significantly improved hot strength and other desirable physical properties.

Abstract: The process of the invention consists in co-sintering a fine-grain alumina (8,9,10) with a metallizable alumina (11,12). The component obtained has dielectric properties which are homogeneous in its useful part (8,9,10) and its metallizable parts (11,12) permitting the assembly thereof very solidly with metallic components (1).

Abstract: A ceramic raw material mixture with improved processing properties comprising at least two chemically different raw material components, wherein at least two of the raw material components have the same type of cation on the surface thereof, at least one of the raw material components being coated with said type of cation and the coating, calculated as cation oxide, amounting to 0.01 to 6% by weight, based on the coated component, and all the raw material components having a surface with an isoelectric point of .ltoreq.pH 7.

Abstract: An aqueous plastic composition of an inorganic powder which per se has substantially no plasticity which comprises: the inorganic powder and a polysaccharide of natural origin.The molding and sintering of the plastic composition provide a sintered body of high mechanical strength and dimensional accuracy.

Abstract: A cubic boron nitride-containing high-density inorganic composite is prepared by mixing cubic boron nitride with a powdered inorganic compound capable of forming a rigid sintered article having a density of at least 85% and a Vickers hardness of at least 800, which does not promote the conversion of cubic boron nitride into graphite-type phase boron nitride (hBN). The powder mixture is compressed into a mold, and fired at low pressure of between 1,000-2,000 MPa, at a temperature of not more than 1,500.degree. C., under which conditions the cubic boron nitride does not convert to hBN, but is sufficiently metastable to permit the formation of the high-density inorganic composite.

Abstract: This invention relates to a process for the preparation of a crystalline compound having an empirical formula LaCr.sub.x A.sub.1-x O.sub.4 .multidot.yH.sub.2 O where A is a metal selected from the group consisting of Mg, Sr, Ca and Ba, x ranges from 0.99 to about 0.7 and y ranges from 0 to 0.15. The process comprises adding a solution of the appropriate metal salts to a solution of ammonium hydroxide thereby precipitating a hydroxide gel intermediate, which is dried and calcined in air to yield the resultant crystalline compound which has a huttonite structure. Additionally, this invention relates to a process for the preparation of a ceramic powder having the empirical formula LaCr.sub.x A.sub.1-x O.sub.3, where A is a metal selected from the group consisting of Mg, Sr, Ca and Ba and x ranges from 0.99 to about 0.7. Finally, a ceramic composition having the empirical formula LaCr.sub.x S.sub.1-x O.sub.4 .multidot.yH.sub.2 O having a huttonite structure is also disclosed.

Abstract: A method of producing self-supporting ceramic bodies having a modified metal-containing component includes first providing a self-supporting ceramic body comprising (i) a polycrystalline oxidation reaction product formed upon oxidation of a molten parent metal precursor with an oxidant, and (ii) an interconnected metal-containing component at least partially accessible from one or more surfaces of said ceramic body. The surface or surfaces of the ceramic body is contacted with a quantity of foreign metal different from said interconnected metal-containing component at a temperature and for a time sufficient to allow for interdiffusion, whereby at least a portion of said metal-containing component is displaced by said foreign metal. The resulting ceramic body, having an altered metal-containing component, exhibits modified or improved properties.

Abstract: The invention encompasses unsintered and sintered compositions comprising boride and alumina. A sintered material containing boride and alumina may be produced by a method comprising:a) combining sources of borate glass frit and aluminum to form a mixture;b) compacting the mixture to form a shape;c) heating the shape, whereby the sources react to form boride and alumina; andd) densifying the reacted shape.Specific embodiments of the invention include the use of rare earth borate glasses such as La.sub.2 O.sub.3. 6 B.sub.2 O.sub.3 glass to form compositions containing rare earth boride.

Abstract: Novel self-supporting ceramic structures are produced by the oxidation reaction of a molten metal precursor with a vapor-phase oxidant to form an oxidation reaction product. Molten metal is drawn through the oxidation reaction product towards the oxidant to cause continued growth of the product at the interface between oxidant and previously formed product. This reaction or growth is continued to form a thick, self-supporting ceramic body. The resulting ceramic material of the polycrystalline growth product consists essentially of an oxidation reaction product and, optionally, one or more non-oxidized constituents of the metal precursor.

Abstract: By adding very-finely-divided or highly-dispersed titanium dioxide or aluminum titanate, optionally in a mixture with aluminum oxide, sintered products of great hardness, whose fracture toughness can be adjusted over a wide range, are obtained from aluminum hydroxide according to a sol-gel process depending on the selection of the sintering conditions. The materials are especially suitable as abrasives or for the production of ceramic powder or components.

Abstract: This invention relates to a cutting tool material with improved toughness behavior using alumina or silicon nitride as the matrix material. The improved toughness is achieved by the addition of whiskers and optionally zirconia in a monoclinic or tetragonal phase, to the matrix. The whiskers are disc-shaped with a thickness of 0.5-8 .mu.m and an equivalent diameter (hypothetical diameter of a circle with the same area as the disc) of about 5-50 .mu.m.