GB1563885A - Basic zinc compound flake like crystalline particle and method for preparation thereof - Google Patents

Description

(54) BASIC ZINC COMPOUND FLAKE LIKE CRYSTALLINE PARTICLE AND METHOD FOR PREPARATION THEREOF (71) We, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD., a Japanese Body Corporate, of 1006 Oaza Kadoma, Kadoma-shi Osaka-fu, Japan do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement :- The present invention relates to a material constituted by flake-like crystalline particles of a basic zinc compound which are suitable for employment in or as starting material for a crystal-oriented electronic device, and to a method of preparation of said material.

One requirement for improvement of characteristics of electronic devices is improvement of characteristics of materials constituting these devices, and for this reasons considerable attention has been given recently to crystal oriented material, which is known to give advantages in many electronic applications. For example, it is known that reduced costs and increased ease of manufacture may be achieved by use of crystal-oriented material rather than material constituted by a random array of single crystals in surface acoustic wave filters, thin film, or ceramics, for example, and that there is less absorption and scattering of light if fluorescent film material employed in a device such as a cathode ray tube is constituted by crystaloriented material. One form of crystal oriented material is material constituted by flake-like crystals. Known examples of such material include flake-like ferric oxide, Fe203, or manganese compounds, e. g., MnOOH, employed as hot-pressed material for magnetic heads in metering instruments or tape recording devices in which particularly good resistance to surface crumbling is demanded. Zinc compounds are known to provide advantages when employed in electronic devices, and there is accordingly a strong demand for crystaloriented zinc material, but it has not yet been made possible to produce such material on an industrial basis.

There have been published a great many reports relating to zinc oxide, ZnO, and to obtaining of crystal-oriented material. For example it is known that by cooling a concentrated solution of zinc sulfate, ZnSO"containing ZnO there can be obtained a small amount of flake-like material having a composition which is basically ZnSO4, 3Zn (OH) 2, 4H2O.

However, crystals thus obtained are very thin, and form and hence orientatability thereof are adversely affected if the material is converted to another material, in addition to which, since there is only a small yield, the process is unsuited to production on an industrial scale. According to another report, it is possible to obtain flake-like crystals by heating an aqueous solution of a zinc compound to 160 C in a closed tube.

However, this process, since it demands use of a special high-pressure container, presents problems of safety and cost in production on an industrial scale.

We have now developed a method for the preparation of a material constituted by flake-like crystals of a zinc compound suitable for employment in a crystaloriented electronic device. This material which is suitable for employment as, or as starting material for, material for a crystaloriented electronic device is constituted by flake-like crystals of a zinc compound having dimensions sufficiently great to permit resistance to change of from upon conversion-of said material to or combination thereof with another material.

It is another object of the invention to provide a method permitting easy largescale production of good quality crystaloriented material constituted by flake-like zinc compound crystals.

According to the present invention there is provided flake-like crystalline particles comprising basic zinc sulfate representable by the formula ZnSO,. XZn (OH) 2, wherein x has a value in the range of from 1. 7 to 4. 8, which particles have the following characteristics: (a) a ratio of thickness to diameter in the range of from 1: 5 to 1 : ; (b) a thickness of less than 30 Hm ; and (c) a diameter in the range of from I to 300 ssm.

The dimensions of the crystals ensure that the material may easily be combined with other materials or converted to corresponding oxides or sulfates without loss of its characteristics. Accordingly, the material of the invention has a high industrial value, since by suitable treatment and combination crystal-oriented ZnO, ZnS, or ferrites may be obtained, for example, the material being easily produced in comparatively low temperature conditions by a method which does not require any special pressure-resistant or similar equipment and which is described below.

The method of the invention for the preparation of the flake-like crystalline particles of the present invention comprises the steps of preparing a solution of a zinc salt containing 0. 1 to 6 mole per liter of zinc sulfate and having a pH in the range of from 0.5 to 6.0, heating the solution to a temperature in the range of from 50 to 100 C, and decreasing the hydrogen ion concentration of the solution, whereby a precipitate of flake-like crystalline particles is formed, and separating the precipitate from the solution.

In greater detail, a solution containing at least zinc ions and sulfate ions is heated to a temperature in the range of from 50 to 100 C and while maintained at this temperature is caused to react with an alkali in order to decrease the pH of the solution, the solution being stirred constantly during this reaction. At the start of the reaction a precipitate is produced which, however, is immediately dissolved as stirring of the solution continues. After a certain amount of alkali has been added stable deposition, i. e., deposition which is not followed by dissolution, of colorless, transparent flakelike hexagonal crystals having a diameter of up to 300, um commences.

Zinc ions and sulfate ions (SO,-) must be present in the starting solution, whose hydrogen ion concentration, pH, is initially in the range of from 3.5 to 6.0. The pH of the solution increases gradually as alkali is added, and stable deposition of flake-like crystals commences when the pH of the solution is in the range of from 4.0 to 6.5.

For example, stable deposition starts when the pH of the solution is about 4. 6 if the starting solution employed is a zinc sulfate solution with a concentration of I mol/liter and the alkali added thereto is ammonia.

Once stable deposition has started there is . hardly any change of pH of the solution until deposition of product crystals is complete. This is because, with for example the alkali is added in the form of an ammonium compound, the sulfate ions in the starting solution and subequently added ammonium ions form ammonium sulfate which together with ammonia constitutes a buffer solution. This maintenance of pH of the solution at a generally constant level is an important aspect of the invention, since it makes it possible to produce large crystals, of diameter of up to 300 um, as noted above, and ensures uniformity of quality of the deposited crystals.

The starting solution is a zinc solution having a concentration of 0. 1 to 6 mol/liter of zinc sulfate. Crystals obtained when the concentration is less than 0. 1 mol/liter are too small and thin. The upper limit of the concentration can be increased if the zinc sulfate is made more soluble in water, but in terms of crystal production yield 6 mol/liter is a suitable maximum.

The crystals of the invention are still produced if a portion of the zinc sulfate of the starting solution is replaced by another zinc salt, for example zinc nitrate, zinc acetate, or zinc chloride. Up to 75 mol o of the zinc sulfate may be replaced by another zinc salt. The size of the flake-like crystals produced is more even and the diameter to thickness ratio of the crystals is increased when a portion of the zinc sulfate of the starting solution is replaced by zinc nitrate.

Substitution of zinc sulfate by zinc nitrate may be made up to a maximum value of 75 mol %. Higher proportions of zinc nitrate make it difficult to ensure production of flake-like crystals of uniform shape and hence are unsuitable. Replacing the zinc sulfate with up to 50 mol'0 zinc acetate or zinc chloride results in production of flakelike crystals having a more uniform shape.

In a preferred embodiment the zinc salt inclues zinc chloride in an amount of 25 mol % or less.

There are many possible alkalis which may be added to the starting solution, but in terms of availability, handling, etc. ammonia is one of the simplest alkalis, and is therefore preferred for use in the method of the invention, it being understood, of course, that the invention is not limited to use of this particular form of alkali.

Ammonia may be added in various ways.

For example if a solution of ammonia is employed, it is added in the form of successive drops, while the starting solution is continuously stirred. In this case, a white precipitate is deposited upon addition of the first drop, but this is immediately dissolved due to the effect of stirring the solution.

This process is repeated upon addition of further drops of ammonia solution up to a certain total amount of ammonia has been added, after which deposition of the precipitate is no longer followed by dissolution, and stable formation of crystals commences. Ammonia solution may be continued to be added until completion of deposition of crystals has occurred.

Subsequent filtration, and drying of the filtrate yields flake-like crystals of the basic zinc sulfate.

Another method of effecting adding ammonia is to make use hydrolysis of urea.

In this case, urea initially dissolved in a starting solution containing zinc sulfate is subsequently hydrolyzed to produce ammonia, the hydrolysis reaction being representable by the following formula.

(NHCO+H, 0-CO, +2NH, This hydrolysis reaction proceeds only very slowly at room temperature, but if temperature of the solution is raised to 50 to l00 C, ammonia is produced, the pH of the solution is decreased and flake-like crystals of basic zinc sulfate are deposited at a suitably rapid rate, precipitation of crystals first becoming noticeable when the solution temperature is 50 C, and being completed in a very short time if solution temperature is raised to 100 C, at which temperature hydrolysis of the urea proceeds at very fast rate, thereby resulting in the supply of a large amount of ammonia to the solution. In terms of mole ratio with respect to the zinc sulfate, the amount of urea initially included in the starting fluid may be as low as 0.1, but in order to achieve a satisfactory yield rate of crystals is preferably I or more, with an upper limit of 6, no advantage being achieved by the addition of urea in an amount greater than this upper limit.

Powder X ray diffraction analysis of crystals produced by the abovedescribed method showed that the crystals which were hexagonal, had good uniformity of shape, crystal thicknesses ranged from about I, um to 30, um, and that the ratio of thickness to diameter ranged from 1 : 5 to 1: 300, i. e., the crystals were definitely flake-like and suited to production of crystal-oriented material.

Further, thermogravimetric analysis of the crystals obtained showed that there is a reduction of weight of the crystals which occurs in 3 tQ 4 stages in the temperature region 50 to 400 C. This reduction of weight is presumed to be due to dehydration which occurs in the noted temperature region, as may be expected since immediately subsequent to production thereof the crystals have considerable water content.

When the crystals were further heated, there was found to be a further loss of weight due to loss of sulfur trioxide gas, SOZ in the temperature region 700 to 900 C. The final product remaining is zinc oxide, ZnO, and by calculation it is found that the original composition of the product crystals was ZnSO, XZn (OH),, with X in the range 1.7 to 4.8, as noted earlier.

The abovedescribed material was further treated by suitable processes to convert it to zinc oxide and zinc sulfate, and it was found that even after such conversion crystals still remained generally hexagonal and flake- like. In other words, flake-like basic zinc sulfate crystals according to the invention are suitable as starting material for other zinc compounds for use as crystal-oriented material such as required in electronic devices. It will also be noted that the method of the invention does not require special equipment, and that starting solution does not require to be heated above 100 C, which is easily, and economically achievable.

The description of the invention continues below in reference to several specific examples thereof.

Example 1.

288 g of zinc sulfate was dissolve in 1 liter of water. While being stirred, this solution was heated to 87 C and had added thereto ammonia water of 10 /o concentration, which was added drop-wise from a pipette.

The first drop of added ammonia water resulted in deposition of a precipitate, which, however, was immediately dissolved, this process being repeated upon further addition of ammonia water up to a certain amount, after which dissolution of precipitate stopped, and stable deposition of flake-like crystals commenced. Drop-wise addition of ammonia water was continued until deposition of flake-like crystals was completed. pH of the solution at start of stable deposition of crystals was 4.5, and remained more or less constant while addition of ammonia water was continued.

The precipitate obtained was filtered washed in water, and then dried at a temperature of 80 C, the resultant product being a zinc hydroxide powder constituted by colorless, transparent, flake-like hexagonal crystals having an average particle size of I um, and thickness of approximately 0.15 um. Thermogravimetric analysis showed that the product contained approximately 1 2 by weight of sulfate ions (SO,--). The product was subsequently heat-treated at 900 C for (houer, and it was found that the flake-like hexagonal structure still remained.

Examples 2 to 12.

Zinc sulfate and urea were dissolved in I liter of water, and this solution was heated, while being stirred, to cause hydrolysis of the urea. Deposition of a white precipitate commenced when the solution was at a temperature of 50 to 100 C, and with stirring continued and temperature maintained constant, deposition of precipitate was completed in 15 to 20 minutes. The precipitate was then filtered, washed, and dried at a temperature of 80 C.

The product was a white powder constituted by colorless, transparent, flake-like hexagonal crystals and having a maximum particle size of 100 um. Thermogravimetric analysis showed that, discounting crystal water and other water components, the chemical composition of this product also was ZnSO4 XZn (OH) 2, with X in the range 1. 7 to 4.8. A portion of the product was dried in air for 1 hour at 900 C, and there was obtained a large amount of orientable zinc oxide retaining a flake-like hexagonal structure. Also, by drying the product in a sulfurizing atmosphere there was obtained zinc sulfate powder retaining a flake-like hexagonal structure and hence suitable as crystal oriented material.

Further details relating to Examples 1 to 12 are given in Table 1, which indicates zinc sulfate and urea concentration in the starting solution, temperature at which stable deposition of precipitate takes place pH of the starting solution at commencement of stable deposition of precipitate, average diameter of crystals obtained, thickness to diameter ratio, and value of X in the formula ZnSO4 XZn (OH), of the crystals obtained, as determined by thermogravimetric analysis. In Table 1, and also Table 2, since there was a large variation of particle diameter, approximate average volumes of diameter are given, and similarly approximate average values of thickness to diameter ratio are given.

Examples 13 to 22., In these Examples a portion of the zinc sulfate in the starting solution was replaced by other zinc salts, and the process was otherwise the same as for Examples I to 12, there being similarly produced, flake-like zinc compound crystals. Details relating to these examples are given in Table 2, which indicates the same items as Table 1.

TABLE 1

Conc.mol/liter Deposition Starting temperature Deposition diameter Thickness-diameter Value of X Example solution Alkali ( C) pH ( m) ratio [ZnSO4.XZn(OH)2] # 1 ZnSO4-1.0 Ammonia water of 87 4.8 about 1.0 1 7 3.8 10% concentration 2 " -0.2 Urea-0.2 82 5.6 15 1:15 4.8 3 " -1.0 " -1.0 89 4.6 30 1.10 1.7 4 " -1.0 " -3.0 90 4.7 50 1 20 4.6 5 " 0.12 "-1.2 95 5.7 70 1 50 3.8 6 " -8 " -24 100 - Not flaky - 7 " -1 " -3 60 4.7 30 1 270 2.5 8 " -1 " -3 80 4.7 60 1 50 2.4 9 " -1 " -3 100 4.7 120 1 100 2.9 10 " -0.1 " -3 100 5.8 60 1 40 2.5 11 " -1 " -0.1 100 - 10 1 20 2.6 12 " -1 " -3 50 4.6 5 1 50 2.5 TABLE 2

Conc.mol/liter Deposition Particle temperature diameter Thickness-diameter Value of X Example Starting solution Alkali ( C) ( m) ratio [ZnSO4~XZn(OH)2] # 13 ZnSO40.5+Zn(NO3)2 0.5 Urea -1 100 80 1 8 2.5 14 " 0.75+ " 0.25 " -1 100 70 1 7 2.5 15 " 0.25+ " 0.75 " -1 100 40 1 30 2.5 16 ZnSO4 0.5+Zn(NO3)2 0.5 " -3 94 100 1 10 2.4 17 " 0.5+Zn(CH3COO)2 0.5 " -3 98 120 1 13 4.8 18 " 0.18+ " 0.04 " -0.6 98 90 1 8 2.9 19 ZnSO40.75+ZnCl2 0.25 " -3 94 75 1 20 3.5 20 ZnSO4 0.75+Znl2 0.25 " -3 85 30 1 20 2.7 21 ZnSO4 0.5+(NH4)2Zn(SO4)2 " -1 90 70 1 10 2.3 22 ZnSO4 0.5+Zn(HCO2)2 " -3 92 30 1 30 3.8

Claims (14)

1. WHAT WE CLAIM IS:** 1. Flake-like crystalline particles comprising basic zinc sulfate representable by the formula ZnSO4~XZn(OH)2, wherein x has a value in the range of from 1.7 to 4.8, which particles have the following characteristics: (a) a ratio of thickness to diameter in the range of from 1:5 to 1:300; (b) a thickness of less than 30 m; and (c) a diameter in the range of from 1 to 300 m.
2. 2. Flake -like crystalline particles as claimed in claim 1 substantially as hereinbefore described with reference to any one of Examples 1 to 5, or 7 to 22.
3. 3. A method for preparing flake-like crystalline particles as claimed in claim I which process comprises the steps of preparing a solution of a zinc sulfate containing 0.1 to 6 mole per liter of zinc sulfate and having a pH in the range of from 0.5 to 6.0, heating the solution to a temperature in the range of from 50 to 100 C, and decreasing the hydrogen ion concentration of the solution, whereby a precipitate of flake-like crystalline particles is formed, and separating the precipitate from the solution.
4. 4. A method as claimed in claim 3 wherein the step of decreasing the hydrogen ion concentration of the solution is effected by including urea in the solution.
5. 5. A method as claimed in claim 4 wherein the mole ratio of urea to zinc salt in the solution is in the range of from 0.1 to 6.
6. 6. A method as claimed in claim 3 wherein the step of decreasing the hydrogen ion concentration of the solution comprises the step of adding a solution of ammonia water to the solution.
7. 7. A method as claimed in any one of claims 3 to 6, wherein at least 25 mol % of the zinc salt is zinc sulfate.
8. 8. A method as claimed in claim 7 wherein in addition to zinc sulfate the zinc salt also includes zinc nitrate in an amount of 75 mol % or less.
9. 9. A method as claimed in claim 7 wherein in addition to zinc sulfate the zinc salt also inclues zinc acetate in an amount of 50 mol % or less.
10. 10. A method as claimed in claim 7 wherein in addition to zinc sulfate the zinc salt also includes zinc chloride in an amount of 25 mol % or less.
11. ll. A method as claimed in claim 3 of preparing flake-like crystalline particles substantially as described herein.
12. 12. A method as claimed in claim 3 of preparing flake-like crystalline particles substantially as described herein in anyone of Examples l to 5, or 7 to 22.
13. 13. Flake-like crystalline particles whenever produced by a method as claimed in any one of claims 3 to 12.
14. 14. A crystal-oriented electronic device comprising a flake-like crystalline particle as claimed in any one of claims 1, 2 or 13.