Abstract:
A starting material composed of calcium carbonate is dissolved in a nitrate aqueous solution containing either Ca(NO 3 ) 2  or NH 4  NO 3  under application of heat and pressure. The starting material is hydrothermally synthesized within the nitrate aqueous solution to effect the rapid growth of calcium carbonate single crystal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of application Ser. No. 924,402, filed Oct. 29, 1986 and now U.S. Pat. No. 4,762,588. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method of manufacturing calcium carbonate single crystal (calcite) widely used as an optical polarizer, etc. 
     Single crystals of calcium carbonate (CaCO 3 ) are suitable for optical uses and, for this purpose, natural calcite crystals are currently used. Calcite single crystals exhibit double refraction of incident light. Calcite has a high refractive index and is used as a polarizing prism in optical apparatus. Because of recent advances in the design of optical apparatus, such as laser optics and an optical communication apparatus, there is an increased demand for a material with excellent optical characteristics. In this regard, calcite single crystal is an ideal material, and is expected to be more and more in demand. 
     Calcite single crystal has been obtained only from natural sources because it is not yet being industrially synthesized. For commercial use, natural calcite must be colorless and transparent, must have no bubbles or cracks and no twining and no internal strains, and must be greater than a certain size. However, calcite single crystal that will meet these requirements is found only in limited quantities in the Republic of South Africa, Mexico, etc., and reserves are running low. 
     There have been experiments to synthesize calcium carbonate single crystal. One method is crystallization from a solvent, another is the synthesizing from a gel, a third is crystallization from a flux or melt, a fourth is hydrothermal synthesis, and recently an FZ method under high pressure have been suggested. However, optical characteristics such as transparency of the resultant crystals have not been entirely satisfactory due to defects such as impurities, mixing, dislocations, inclusions, or internal strains. 
     Among the methods tried for the manufacture of calcium carbonate single crystal, hydrothermal synthesis is most similar to the process by which natural calcite is grown as a hydrothermal ore deposit. Therefore, hydrothermal synthesis can produce a desired calcium carbonate single crystal with characteristics similar to natural calcite. 
     The hydrothermal synthesis process for manufacturing calcium carbonate single crystal utilizes an aqueous solvent hold at a predetermined temperature and pressure in an autoclave. Alkaline aqueous solutions such as sodium hydroxide (NaOH) or alkali carbonate aqueous solutions such as sodium carbonate (Na 2  CO 3 ), potassium carbonate (K 2  CO 3 ), etc. are generally used as the aqueous solvent. This method for growth of calcium carbonate single crystal is essentially a modification of conventional growth technology for artificial crystals. Under the following conditions: 
     Solvent-6 mol K 2  CO 3  aqueous solution 
     Temperature-410° to 445° C. 
     Pressure-1720 atmospheres 
     Growth speed-50 μm/day 
     about 3 mm growth layer of a calcium carbonate single crystal has been obtained. 
     The above described hydrothermal synthesis is disclosed in D. R. Kinlock H, R. F. Belt, R. C. Puttbac H, Journal of Crystal Growth 24/25 (1974) 610-613. 
     A method of manufacturing calcium carbonate single crystal grown by hydrothermal synthesis using a chloride aqueous solution is described and claimed in the commonly assigned U.S. Pat. No. 4,685,995, of Shinichi Hirano and Seiko Instruments &amp; Electronics Ltd., issued Aug. 11, 1987. 
     In crystal growth method of calcium carbonate utilizing the conventional alkali carbonate aqueous solution, crystal can be grown, but there are problems to be solved as follows: 
     Firstly, due to the high concentration of solvent, inclusions frequently occur in the resultant crystals. These inclusions will result in inferior optical characteristics. Next, due to a high concentration of solvent, it is impossible to achieve sufficient pressure for quantitative production. In other words, the higher the solvent concentration, the lower the obtained pressure becomes even with the same filling-up rate. In the case of a 6 mol concentration of K 2  CO 3  aqueous solution at 445° C. and a filling-up rate of nearly 100%, it is impossible to obtain a pressure of 1720 atmospheres. Due to this, it is necessary to apply additional pressure from outside the autoclave thereby causing the apparatus and pressure system etc. to be too complicated. Using an alkali carbonate aqueous solution, the growth speed will be very slow, i.e., 50 μm/day, and therefore it will take more or less a year to grow crystals large enough to be used as optical elements. 
     It is an object of the present invention to provide a simple method to grow relatively quickly excellent calcium carbonate single crystal with good optical characteristics. 
     According to the present invention, there is provided a method for manufacturing a calcium carbonate single crystal by hydrothermal synthesis at a given temperature and pressure within a nitrate aqueous solution as a solvent. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a test tube used in Example 1 according to the present invention; and 
     FIG. 2 is a sectional view of a pressure vessel used in Example 2 according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The problems encountered in growing calcium carbonate single crystal by the prior art hydrothermal synthesis, namely, inferior optical quality, complicated apparatus, and long growth period, etc. result from having to use a high concentration solvent and high pressure. In other words, the problems stem from the choice of solvent and growth conditions. 
     A nitrate aqueous solution is now found best to avoid these problems, among various kinds of existing solvent such as alkaline, carbonate acid and chloride solutions. 
     In the present invention, hydrothermal synthesis initiating material is dissolved in a suitable aqueous solution of solvent at an appropriate temperature and pressure, and crystallization on a substrate is effected by gradual cooling or by transporting nourishment (material) through a temperature differential. The solvent should, therefore, be such that the starting material dissolves well in it and it should have little corrosive action on the apparatus used. A nitrate aqueous solution as a solvent is found to be ideal. 
     The invention is further described with reference to the following Examples. 
     EXAMPLE 1 
     As starting material, commercially available calcium carbonate of high purity was used. For hydrothermal synthesis an autoclaval test tube of stellite quality No. 25 was used. FIG. 1 shows the structure of the test tube having a pressure vessel body 1 shows the structure of the test tube having a pressure vessel body 1 with a cover 3 and a seal 2. 
     The inside temperature of the pressure vessel body was measured through a temperature measuring hole 4. A gold capsule, 33 mm-5 mm in diameter, was placed in the test tube. The starting material and solvent were poured into the test tube for hydrothermal synthesis. In this case, the pressure between the inside and the outside of the capsule was balanced by filling the inside of the pressure vessel with distilled water. 
     By using various solvents, the result of growing crystal for each solvent and the hydrothermal treatment conditions are shown in the following table I: 
     
                       TABLE I______________________________________Hydrothermal synthesis conditions                   Size* of     temperature  pressure crystalsolvent   [°C]  [kg/cm.sup.2 ]                           obtained______________________________________3 Mol     300          750      0.2-0.3 mmNaNO.sub.33 Mol     300          750      0.2-0.3 mmNaNO.sub.33 Mol     370          750      0.3-0.5 mmNaNO.sub.33 Mol     420          500      0.1-0.2 mmNaNO.sub.33 Mol     450          750      0.8-1.0 mmNaNO.sub.33 Mol     500          750      1.0-1.2 mmNaNO.sub.33 Mol     400          750      0.5-0.6 mmKNO.sub.33 Mol     400          1000     0.6-0.8 mmKNO.sub.33 Mol     380          750      0.3-0.5 mmLiNO.sub.33 Mol     420          750      0.6-1.0 mmLiNO.sub.33.5 Mol   180          1000     not grownCa(NO.sub.3).sub.23.5 Mol   200          750      0.1-0.2 mmCa(NO.sub.3).sub.24.0 Mol   240          750      0.3-0.4 mmCa(NO.sub.3).sub.23.5 Mol   280          1000     0.3-0.5 mmCa(NO.sub.3).sub.23.0 Mol   320          800      0.4-0.5 mmCa(NO.sub.3).sub.23.5 Mol   360          750      0.6-0.7 mmCa(NO.sub.3).sub.23.5 Mol   400          1000     0.8-1.0 mmCa(NO.sub.3).sub.23.0 Mol   440          750      0.8-1.2 mmCa(NO.sub.3).sub.21.0 Mol   100          1000     not grownNH.sub.4 NO.sub.31.0 Mol   120          1000     0.2 mmNH.sub.4 NO.sub.31.0 Mol   140          1000     0.4-0.6 mmNH.sub.4 NO.sub.30.8 Mol   170          1000     0.6-1.0 mmNH.sub.4 NO.sub.30.8 Mol   200          1000     0.8-1.2 mmNH.sub.4 NO.sub.30.5 Mol   240          750      0.8-1.1 mmNH.sub.4 NO.sub.30.5 Mol   280          750      0.9-1.2 mmNH.sub.4 NO.sub.30.5 Mol   320          1000     1.0-1.4 mmNH.sub.4 NO.sub.30.5 Mol   360          750      1.2-1.5 mmNH.sub.4 NO.sub.30.5 Mol   100          1000     0.2 mmNH.sub.4 NO.sub.30.5 Mol   120          1000     0.3 mmNH.sub.4 NO.sub.30.1 Mol   150          1000     0.3 mmNH.sub.4 NO.sub.30.02 Mol  170          1000     0.2 mmNH.sub.4 NO.sub.30.01 Mol  200          1000     0.2 mmNH.sub.4 NO.sub.30.01 Mol  240          800      0.3 mmNH.sub.4 NO.sub.30.01 Mol  280          500      0.4 mmNH.sub.4 NO.sub.30.01 Mol  360          500      0.4-0.6 mmNH.sub.4 NO.sub.30.02 Mol  320          500      0.4-0.7 mmNH.sub.4 NO.sub.33.0 Mol   100          300      0.2 mmNH.sub.4 NO.sub.33.0 Mol   120          300      0.4 mmNH.sub.4 NO.sub.33.0 Mol   140          200      0.3-0.5 mmNH.sub.4 NO.sub.33.0 Mol   170          100      0.3-0.6 mmNH.sub.4 NO.sub.33.0 Mol   200          100      0.3-0.6 mmNH.sub.4 NO.sub.33.0 Mol   240           50      0.4-0.6 mmNH.sub.4 NO.sub.33.0 Mol   280           50      0.5-0.7 mmNH.sub.4 NO.sub.33.0 Mol   320           20      0.5-0.7 mmNH.sub.4 NO.sub.33.0 Mol   360           20      0.7-0.9 mmNH.sub.4 NO.sub.3______________________________________ *All growth periods were 7 days. 
    
     As shown in the above table I, it was found that crystals were grown by using any one of NaNO 3 , KNO 3 , LiNO 3 , Ca(NO 3 ) 2  and NH 4  NO 3  as the solvent. In the case of using the alkali nitrate aqueous solutions, namely NaNO 3 , KNO 3  and LiNO 3 , it is possible to grow crystal in the range of 300° to 500° C., and the preferable temperature range of the hydrothermal synthesis is in the range of 370° to 420° C. As for pressure, unlike the case of using an alkali carbonate aqueous solution, it is possible to grow crystals at a pressure lower than 1000 kg/cm 2 , and a very good crystal growth at pressures of around 750 kg/cm 2  is achieved. By making the concentration of each alkali nitrate aqueous solution large, good crystal can be grown, however it is preferable to arrange the molar concentration to about 3 Mol in view of the relation with pressure because it is easier in handling. In the case of using Ca(NO 3 ) 2  aqueous solution, the preferable temperature range of the hydrothermal synthesis is in the range of 200° C. to 400° C. If the temperature is over 400° C., it is still possible to grow crystals, however, there is the possibility of reduced quality. In the case of NH 4  NO 3  aqueous solution, the preferable temperature range of the hydrothermal synthesis is in the range of 100° to 360° C. If the temperature is over 360° C., it is possible to grow crystals, however, there is the possibility of reduced quality and corrosion of the pressure vessel. As for pressure, it is possible to grow crystals at pressure lower than 300 kg/cm 2  by selecting other hydrothermal synthesis conditions, e.g., the concentrations of the solvent or growth temperature. The NH 4  NO 3  aqueous solution comprises preferably 0.01 to 3 Mol aqueous solution of NH 4  NO 3 . It was identified by X-ray diffraction that in each case, the crystals grown were calcium carbonate single crystal. 
     EXAMPLE 2 
     FIG. 2 is a sectional view illustrating a typical test tube used in this Example. The test tube was made of stellite 25 as in Example 1, however the inside was lined with platinum in order to avoid contamination by pollutants. A pressure vessel body 5 was sealed with a cover 7 through a sealing ring 6. At the bottom of the pressure vessel body 5, starting material 8 for the crystals to be grown was placed. The starting material was refined and recrystallized in nitrate solvent according to the method of Example 1 and was powdered. A crystal support frame 9 carried thereon a species or seed crystal 10 over the starting material 8 at an upper portion of the pressure vessel. The seed crystal 10 was a calcite with (0001) faces of natural optical grade. As the seed crystal, calcite with (1011) faces of natural optical grade may be used. It was necessary to choose a seed crystal with few internal inclusions, and little lattice displacement, etc. so that defects in the single crystal to be grown thereon may be avoided. A baffle 11 was provided between the starting material 8 and the seed crystal 10 to produce a temperature difference therebetween and was supported on the frame 9. The inside of the pressure vessel body 5 was filled up with nitrite aqueous solution, e.g., NaNO 3 , KNO 3 , LiNO 3 , Ca(NO 3 ) 2  and NH 4  NO 3 , as a solvent at such a filling-up rate as to establish a predetermined temperature and pressure. 
     By using various solvents, the result of growing crystal for each solvent and the hydrothermal treatment are shown in the following table II: 
     
                                           TABLE II__________________________________________________________________________                         Thickness Temperature        Temperature      of grown of seed        of starting Growth                         layer GrowthSolvent crystal        material               Pressure                    period                         or film                               rate__________________________________________________________________________3 Mol .sup.  370°C.        .sup.  420°C.               750  50   6.6   132NaNO.sub.3          kg/cm.sup.2                    day  mm    μm/day3.5 Mol 360    400    750  50   7.0   140Ca(NO.sub.3).sub.23.5 Mol 280    320    750  50   3.8    76Ca(NO.sub.3).sub.23.5 Mol 200    260    750  50   2.2    44Ca(NO.sub.3).sub.20.5 Mol 280    320    750  50   10    200NH.sub.4 NO .sub.30.5 Mol 220    260    750  50   8.9   178NH.sub.4 NO.sub.30.8 Mol 160    200    1000 50   8.6   172NH.sub.4 NO.sub.33 Mol 280    320     20  50   7.3   146NH.sub.4 NO.sub.33 Mol 220    260     50  50   7.6   152NH.sub.4 NO.sub.33 Mol 160    200    100  50   7.0   140NH.sub.4 NO.sub.30.01 Mol 280    320    500  50   6.5   130NH.sub.4 NO.sub.30.01 Mol 220    260    800  50   5.5   110NH.sub.4 NO.sub.30.02 Mol 160    200    1000 50   4.0    80NH.sub.4 NO.sub.3__________________________________________________________________________ 
    
     The characteristics of the grown layer or film in each case were those of calcium carbonate single crystal and were identified by X-ray diffraction. Its optical characteristics were the same as those of natural calcite (with respect to permeability rate, compound refractive index, etc.). 
     Having described a specific embodiment of our invention, it is believed obvious that modification and variation of our invention is possible in light of the above teachings. 
     From the above discussion, it will be appreciated that it is easier for growth of calcium carbonate single crystals to be industrialized by hydrothermal synthesis because a pressure less than 1000 kg/cm 2  is used and, at the same time, defects inside the grown crystals are reduced as, compared with the prior art growth methods of calcium carbonate single crystal using alkali carbonate aqueous solutions. 
     The growth rate of the crystals is over twice that achieved by the prior art method and this is a very favorable feature in view of industrialization. In the case of using NH 4  NO 3  aqueous solution, CaCO 3  single crystal is easily industrialized because of growth by lower pressure and lower temperature, e.g., lower than 300 kg/cm 2  pressure and temperature in the range of 100° to 360° C. It is thus possible for calcium carbonate single crystals of optical grade quality equal to that of natural calcite to be made industrially by the same technology as that for the current artificial crystals. To be industrially able to produce such crystals equal in quality to natural optical grade calcite will mean that it will be possible always to provide a market with such crystals with the same quality. Because of the dependence upon natural calcite, there has been no guarantee of either a regular supply or consistent quality. Industrialization of calcium carbonate single crystal will achieve such consistency and enhance their use in optical elements and parts, etc. used in a whole range of apparatus and will enable their characteristics to be improved.