examples of the calcium phosphate type compound of the present invention include hydroxyapatite , fluoroapatite , chloroapatite and tricalcium phosphate . a commonly employed method for orienting grains in the direction of a certain crystallographic axis is by utilizing the electric or magnetic polarity of the grains . this method , however , is not highly effective in orienting the crystals of calcium phosphate type compounds because their polarity is rather low . if calcium phosphate type compounds are synthesized by a wet method under properly selected conditions , it is possible to prepare rod - like crystal grains that are elongate in the direction of the c - axis as described in j . am . chem . soc ., oct . 25 , 1967 , pp . 5535 - 5541 . for instance , such grains can be synthesized by mixing a calcium compound with a phosphoric acid compound , the ph of the reaction solution being adjusted to either neutral or alkaline with the temperature held in the range of from 10 ° to 100 ° c . the ratio of the long axis to the short axis of the rod - like grains is preferably 1 . 1 or more , more preferably 1 . 5 or more , and particularly preferably 2 . 0 or more . the present inventors found that when the thus prepared rod - like grains were placed in a flowing fluid , they could be oriented in the direction in which the fluid was flowing . these rod - shaped crystal grains ( primary particles ) can form secondary particles when the slurry is dried . the secondary particles as such will not be oriented if they are subjected to shaping , so it is necessary to pulverize the secondary particles into primary particles by a suitable method such as a jet - mill attrition process and a supersonic mill attrition process . the secondary particles of a calcium phosphate type compound can be pulverized into primary particles when they are mixed with an organic binder and water or an organic solvent . if the slurry is not dried but is subjected to filtration , no secondary particles will form and the synthesized crystal grains may be directly used without being pulverized . therefore , in accordance with the present invention , the rod - shaped crystal grains of a calcium phosphate type compound are used as a starting powder , which is mixed with suitable amounts of a heat - decomposable organic binder and water or an organic solvent , as well as optionally with other additives such as a dispersant and a plasticizer . the ingredients are kneaded and the resulting clay - like composition is extruded to shape the same . various kinds of heat - decomposable organic binders and dispersants may be used without any particular limitation and illustrative examples include organic high - molecular weight compounds and sublimable solid substances . specific examples of the binders include methyl cellulose , butyl methacrylate , ethylene - vinyl acetate copolymers and pullulan . illustrative organic solvents include various kinds of alcohols , hydrocarbons , halogenated hydrocarbons , ethers , ketones and esters . any shaping method can be used as long as it permits the composition to be given the desired shape as it is extruded in a specified direction . illustrative examples are extrusion molding , extrusion spinning and doctor blading as well as injection molding . if injection molding is used , the desired orientation can be achieved by shaping the composition with a mold having a structure which permits the molding composition which is being fed to flow in a specified direction . in the doctor blading , the grains being shaped are subjected to a smaller force than in other shaping methods . it is therefore necessary that the dispersability of the ceramic particles in the composition be increased to make it less viscous . as the spinning method , those described in jp - a - 61 - 106166 can be employed . in the case where extrusion molding is employed , the molding pressure is preferably from 0 . 2 to 500 kg / cm 2 . in the case where injection molding is employed , the molding pressure is preferably from 500 to 3 , 000 kg / cm 2 . the shaped article , in which the crystallographic axes are oriented only in the surface of the article , can be prepared , e . g ., by extrusion molding in which the composition is extruded at a relatively low pressure through a die having a large diameter . by using such molding conditions , the inside part of the shaped article becomes less oriented because of a small stress during the molding . the sinter , in which the crystallographic axes are oriented only in the surface of the sinter , can be prepared by sintering such a shaped article . the resulting shaped article is heated to decompose and thereby remove the organic binder and other organic substances by a conventional method . subsequently , the article is fired by a conventional method , e . g ., at a temperature of from 900 ° to 1 , 400 ° c . to make a sinter that is dense and that has satisfactory strength . the following examples are provided for the purpose of further illustrating the present invention but are in no way to be taken as limiting . in the examples , all &# 34 ; parts &# 34 ; and &# 34 ; percentages &# 34 ; are on a weight basis unless otherwise noted . a slurry of hydroxyapatite synthesized by a conventional wet method ( as described in j . am . chem . soc ., oct . 25 , 1967 , pp . 5535 - 5541 ) was spray - dried to make a powder having an average particle size of about 7 μm . observation under a scanning electron microscope showed that the powder was composed of spherical secondary particles which were dense agglomerations of elongate primary grains having a width of from 200 to 500 å and length of about 1 , 000 å . these particles were calcined at 700 ° c . for 4 hours and pulverized with a supersonic mill ( product of nippon pneumatic co ., ltd .) to obtain a fine powder having an average particle size of about 1 μm . this fine powder ( feed powder ) was used as a starting material for the next shaping step . the fine powder prepared above ( 100 parts ), water ( 45 parts ), butyl alcohol ( 5 parts ), methyl cellulose ( 7 parts ), a wax ( paraffin ) emulsion ( 40 % solid content ; 1 . 5 parts ) and a stearic acid emulsion ( 20 % solid content ; 1 . 5 parts ) were mixed and kneaded to form a homogeneous clay - like composition . by mixing and kneading , the fine powder was completely pulverized into primary particles . the composition was extrusion - molded to form a cylinder , a hollow cylinder and a honeycomb . the molding pressure depended on the shape of the articles , and was about 30 kg / cm 2 for the cylinder , about 50 kg / cm 2 for the hollow cylinder , or about 75 kg / cm 2 for the honeycomb . the shaped articles were dried at room temperature , heated in a degreasing furnace at 700 ° c . to decompose and remove any residual organic substance in the article , and sintered at 1 , 200 ° c . in a box - shaped electric furnace . the result of x - ray diffractiometry for the surface of each shaped article is shown in fig1 and the result of x - ray diffractiometry for each article after it was pulverized in a mortar is shown in fig2 . in each figure , the open circle represents the diffraction peak for the ( 300 ) face and the solid circle represents the diffraction peak for the ( 002 ) face . x - ray diffractiometry was performed at 40 kv and 100 ma using cukα . when the surfaces of the shaped articles were directly measured , the diffraction intensity from the ( 300 ) face was higher than when the pulverized samples were measured . on the other hand , the diffraction intensity from the ( 002 ) face was lower in the intact shaped samples than in the pulverized samples . this shows the presence of orientation in the shaped articles . the &# 34 ; degree of orientation &# 34 ; of hydroxyapatite as defined below is shown in table 1 for each sample . as one can see from table 1 , the sinters of the shaped articles prepared in example 1 had the c - axis of the primary particles oriented in the direction of extrusion . however , the pulverized products of these sinters showed no appreciable orientation and the relative intensities of individual diffraction lines were in accord with the data of astm 24 - 33 . the &# 34 ; degree of orientation &# 34 ; of hydroxyapatite as used herein is defined by ## equ1 ## wherein i . sub . ( 300 ) is the diffraction intensity from the ( 300 ) face of a sample , and i . sub . ( 002 ) is the diffraction intensity from the ( 002 ) face of the sample . the astm value for i . sub . ( 300 ) / i . sub . ( 002 ) is 1 . 5 . using the powder feed prepared in example 1 , a composition of the following recipe was produced : ______________________________________ amountcomposition ( parts ) ______________________________________fine powder prepared in 100example 1butyl methacrylate 12 . 5ethylene - vinyl acetate 9copolymerwax emulsion 9 . 5dibutyl phthalate 3stearic acid 1 . 5______________________________________ the composition thus produced was kneaded as in example 1 and was injection molded at an injection pressure of 1 , 500 kg / cm 2 in a plate mold such that the composition flowed in the direction parallel to the plate mold to produce an article in a plate form . the shaped article was placed in a degreasing furnace and heated while increasing the temperature at a rate of 1 . 5 ° c ./ hour , followed by holding at 700 ° c . for 10 hours . after degreasing , the article was sintered at 1 , 200 ° c . the result of x - ray diffractometry on the shaped article is shown in fig3 and the result of x - ray diffractometry for the pulverized product of the shaped article is shown in fig4 . as is clear from these results and from the data shown in table 1 , the shaped article produced in example 2 had grain orientation as in example 1 . a slurry containing 4 % hydroxyapatite particles ( based on total slurry weight ) with a width of from 200 to 500 and a length of about 1 , 000 å was obtained by a conventional wet method of synthesis ( as described in j . am . chem . soc ., oct . 25 , 1967 , pp . 5535 - 5541 ). the slurry was concentrated by filtration , and the following composition was obtained . the composition containing the thus synthesized rod - shaped crystals of hydroxyapatite , a binder and water was spun by extrusion through an air nozzle ( diameter : 0 . 3 mm ) at an air pressure of 900 mmh 2 o according to jp - a - 61 - 106166 , and dried with hot air at 300 ° c . to form a gauze - like shaped article . the dried article was sintered at 1 , 100 ° c . for 4 hours and subjected to x - ray diffractiometry to check for the occurrence of orientation . when an unpulverized sample of the gauze - like article was loaded in a glass sample holder , grain orientation was observed in the same manner as in examples 1 and 2 . the result of x - ray diffractometry on the shaped article is shown in fig5 and the result of an x - ray diffractometry for the pulverized product of the article is shown in fig6 . as is clear from these scans and from the data shown in table 1 , the shaped article produced in example 3 had grain orientation as in example 1 . table 1______________________________________sample degree of orientation______________________________________example 1 shaped article 3 . 02example 1 pulverized product 0 . 05example 2 shaped article 1 . 94example 2 pulverized product 0 . 15example 3 shaped article 10 . 52example 3 pulverized product 0 . 51______________________________________ a sinter was produced in the same manner as in example 1 except that the powder synthesized , dried and calcined as in example 1 was immediately used without being pulverized by jet milling . x - ray diffraction analysis of the surface of the sinter showed no grain orientation . this would be because the diameter of the secondary particles in the powder was so large that precluded the orientation of primary particles during shaping . a sinter was produced in the same manner as in example 2 except that shaping was performed with a cubic mold in place of the plate mold . x - ray diffraction analysis of the surface of the sinter showed no grain orientation . this would be because a mold having a larger cross section than the length of the final shaped article caused the injected composition to diffuse without having the grains oriented . an hydroxyapatite cylinderical molded article , which was molded , dried and degreased in the same manner as in example 1 , was fired at 1 , 100 ° c . to obtain a sinter . the sinter was cut in the direction parallel to the molding direction or in the direction perpendicular to the molding direction . the cut surfaces were polished and measured for x - ray diffraction scans . the x - ray diffraction scan for the cut surface parallel to the molding direction is shown in fig7 . the x - ray diffraction scan for the cut surface perpendicular to the molding direction is shown in fig8 . separately , the sinter was pulverized in a mortar and measured for an x - ray diffraction scan which is shown in fig9 . from the diffraction intensity for the ( 300 ) face ( i . sub . ( 300 )) which is parallel to the c axis and the diffraction intensity for the ( 002 ) face ( i . sub . ( 002 )) which is perpendicular to the o axis , it was found that the ratios of i . sub . ( 300 ) / i . sub . ( 002 ) were 2 . 87 , 1 . 03 and 1 . 67 for fig7 and 9 , respectively . it is understood from the above results that the a axis of the primary particles of this sinter was oriented in the direction perpendicular to the molding direction , and the c axis was oriented in the direction parallel to the molding direction . an hydroxyapatite plate - like molded article , which was molded , dried and degreased in the same manner as in example 4 , was fired at 1 , 100 ° c . to obtain a sinter . from the measurement of x - ray diffraction scans , it was found that the sinter had the same orientation properties as in the sinter obtained in example 4 . the sinter was cut into a dimension of 3 × 4 × 40 mm , and the cut surfaces were mirror - polished followed by annealing at 1 , 050 ° c . for 2 hours in the air . the cut direction was determined such that the longitudinal direction of the cut specimen became parallel or perpendicular to the molding direction . the specimen in which the longitudinal direction is parallel to the molding direction was designated sample a , and the specimen in which the longitudinal direction is perpendicular to the molding direction was designated sample b . samples a and b were measured for the following mechanical properties . the hardness was measured by the vickers method at a load of 300 g . the penetrator was penetrated in the face parallel to the orientation direction in sample a , and in the face perpendicular to the orientation direction in sample b . the breaking toughness ( k 1c ) was measured by the im method and niihara &# 39 ; s equation using the dents formed upon measuring the hardness . the bending strength was measured by the three - points method according to jis r1601 . the modulus of elasticity was measured by a bending load method using a strain gauge . the results obtained are shown in table 2 below . the values in table 2 are the average values and 95 % confidence intervals for 10 specimens . table 2______________________________________ fracture bending modulus of hardness toughness strength elasticitysample ( gpa ) ( mpam . sup . 1 / 2 ) ( mpa ) ( gpa ) ______________________________________a 5 . 0 ± 0 . 4 0 . 74 ± 0 . 05 99 . 8 ± 21 . 9 114 . 3 ± 6 . 4b 4 . 6 ± 0 . 2 0 . 63 ± 0 . 05 85 . 2 ± 11 . 0 121 . 9 ± 8 . 1______________________________________ it was found from the variance analysis of the results in table 2 that significant differences between samples a and b was 95 % for hardness and 99 % for fracture toughness . therefore , the sinters of the present invention have anisotropy for mechanical properties . in accordance with the present invention , a certain crystallographic axis of the primary particles in at least the surface or the entire portion of the shaped article to be produced can be oriented in one direction by a simple method . the shaped article of a calcium phosphate calcium compound or the sinter thereof which is produced by the present invention has grain orientation so that their mechanical , electrical and chemical properties are sufficiently anisotropic to provide improved characteristics over conventional materials . further , the shaped article and sinter thereof are improved in fracture toughness . while the invention has been described in detail and with reference to specific embodiments thereof , it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof .
an oriented shaped article of a calcium phosphate type compound is disclosed , in which at least two crystallographic axes of the primary particles of which the shaped article is composed each is oriented in one direction , respectively , in at least the surface of said article . a sinter of the oriented shaped article and a processes for producing the article and the sinter are also disclosed .