CA1110037A - Method for preparation of basic zinc compound flake like crystalline particle - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Basic zinc compound flake-like crystalline material suited for use in crystal-oriented electronic devices and method for preparation thereof are described.
The method comprises the steps of preparing a solution including at least zinc ions and sulfate ions having a temperature in the range of 50 to 100°C causing precipitation of zinc sulfate crystals by reaction of components of said solution with alkali to lower the pH of the solution to the range 4 to 6.5, and separating the precipitate from the solution.

Description

tQ3~

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-orientated 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 reason considerable attention has been given recently to crystal oriented materials, which are 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 materials rather than materials 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 the fluorescent film material employed in a device such as a cathode ray tube is constituted by a crystal-oriented material. One form of crystal oriented material is a material constituted by flake-like crystals. Known examples of such material include flake-like ferric oxide, Fe2O3, or manganese compounds, e.g., MnOOH, employed as hot-pressed material for magnetic heads in metering instruments or tape rerecording 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 crystal-oriented zinc materiai, but it has not yet been 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, ZnSO4, 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 hence orientatability thereof is 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 sclution of a zinc compound to 160C 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 crystal-oriented electronic device. This material which is suitable for employment as, or as starting material for, material for a crystal-oriented electronic device is constituted by flake-like crystals of a zinc compound having dimensions sufficiently great to permit resistance to change of form upon conversion of said material to or combination thereof with another material.

~lQQ37 According to the present invention there is provided a method for preparing flake-like crystalline particles of the formula ZnSO4.xZn(OH)2, wherein x = 1.7 - 4.8, having a ratio of thickness to diameter of 1:5 to 1:300, a thickness of less than 30 ~m and a diameter in the range of from 1 to 300 ~m which comprises: (a) preparing a solution of 0.1 to 6 moles per liter of no more than two zinc salts selected from the group consisting of zinc nitrate, zinc sulfate, zinc acetate, zinc chloride and zinc iodide and wherein at least 25 mol percent of said zinc salt is zinc sulfate; (b) heating said solution to a temperature in the range of from 80C to 100C, (c) decreasing the pH of the solution to 4.0 to 6.5 by the gradual addition of urea to said solution, whereby deposition of a precipitate of flake-like crystalline particles is caused, and (d) separating said precipitate from said solution.
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 re~uire any special pressure-resistant or similar equipment and which is described below.

In greater detail, a solution containing at least zinc ions and sulfate ions is heated to a temperature in the range of from 80 ~o 100C and while maintained at this temperature is caused to react with urea in order to increase 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 con-tinues. After a certain amount of urea has been added stable deposition, i.e., deposition which is not followed by dissolution, of colorless, transparent flake-like hexagonal crystals having a diameter of up to several 100 ~m commences.
Zinc ions and sulfate ions (SO4 -) must be present in the starting solution, whose hydrogen ion concentration, as pH, is initially usually in the range of from 3.5 to 6Ø The pH of the solution increases gradually as urea 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. Once stable deposi-tion has started there is hardly any change of pH of the solution until deposition of product crystals is complete.

n~3!37 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 several 100 ~m, 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 % 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 %. ~igher 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 % zinc acetate or zinc chloride results in production of flake-like crys~als having a more uniform shape.

The urea is particularly preferred for decreasing the hydrogen ion,concentration of the solution, and is superior to ammonia water which tends to produce fine crystals unless the ammonia water is continuously added dropwise.
The urea initially dissolved in a starting solution containing zinc sulfate i-s subsequently hydrolyzed to produce ammonia, the hydrolysis reaction being represented by the following formula.

( 2)2C + H20 ~ C02 + 2NH3 This hydrolysis reaction proceeds only very slowly in the vicinity of room temperature, but if the temperature of the solution is raised to 50 to 100Cr :1~37 ammonia is produced, the pH of the solution is increased and flake-like crystals of basic zinc sulfate are deposited at a suitably rapid rate. The precipitation of crystals first becomes noticeable when solution temperature is 50C, and is completed in a very short time if solution temperature is raised to 100C, at which temperature hydrolysis of the urea proceeds at very fast rate, thus resulting from the supply of a large amount of ammonia into 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 crystal formation, it is preferably 1 or more, with an upper limit of 6, no advantage being achieved by addition of urea in an amount greater than this upper limit.
Powder X-ray diffraction analysis of crystals produced by the above-described method showed that the crystals which were he~agonal, had good uniformity of shape, crystal thickness ranged from about 1 ~m to 30 ~m, and that 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 materials.
Further, thermogravimetric analysis of the crystals obtained showed that there is a reduction of weight of the crystals which occurs in 3 to 4 stages in the temperature region ~0 to 400C. 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 ~rtQ37 heated, there was found to be a further loss of weight, due to loss of sulfur trioxide gas, SO3, in the temperature region 700 to 900C. The final product remaining is zinc oxide, ZnO, and by calculation it is found that the original composition of the product crystals was ZnSO4.XZn(OH)2, with X in the range 1.7 to 4.8, as noted earlier.
The above-described 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 materials for other zinc compounds for use as crystal-oriented materials 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 is not required 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 dissolved in 1 liter of water. While being stirred, this solution was heated to 87C and had added thereto ammonia water of 10~ concen-tration, which was added drop-wise from a pipet. The first drop of added ammonia water resulted in deposition of a precipitate, which, however, was immediately dis-solved, this process being repeated upon further addition of ammonia water up to a certain amount, after which dissolution of precipitate stopped, and stable deposition g _ ~1nQ37 of flake-like crystals commenced. Drop-wise addition of ammonia water was continued until deposition of flake-like crystals was completed. The pH of the solution at the 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 80C, the resultant product being a zinc hydroxide powder constituted by colorless, transparent, flake-like hexagonal crystals having an average particle size of 1 ~m, and a thickness of approximately 0.15 ~m. Thermo-gravimetric analysis showed that the product contained approximately 12% by weight of sulfate ions (SO4 ).
The product was subsequently heat-treated at 900C for 1 hour, and it was found that the flake-like hexagonal structure still remained.
Examples 2 to 12 Zinc acetate and urea were dissolved in 1 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 100C, 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 80C. The product was a white powder constituted by colorless, transparent, flake-like hexa-gonal crystals and having a maximum particle size of 100 ~m. 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 900C, 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 a crystal ~riented 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)2 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 values of diameter are ~1(;~;37 given, and similarly approximate average values of thick-ness 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 1 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.

1~1~37 . ~ _ _ _ __ O Coo X I~ `D 00 u~ ~ O~u~ ~O U~
~JJ ~ ~ ~ _~ ~ ~'t N N N N ~i N .

N. . .

~ O O
U~ O O . O I~ O O O O O
rA 1~ _~ _~ N ~ N Ir7 _I ~ N It~
~U .. .. .. .. , ..... .- ... .. .. .. .. . ' ~ ____ _ _ _ _l _l ~ __~

O U~ O O O C~ ~ O O O O U~
~_ ~ _1 C~'l U'l ~ ~1 CO ~O ~`J ~O _ D Z _ _ C
V ~~O ~ I~ ~_ I~ r~ I~ 0~ ~D
~ :c ~:~ u~ ~ ~r ~ ~:i ~;i ~ u~ ~
.~
_ _ __ ~g~
~rl ~ ~-- N 0~ O 1~ OO O O O O O
,~ 00 CC~ 0~ ~ O O ~ oo O _~ O 1/'1 _ :1 V C _ _ _ __ V r ~ V V C~l O O C`~ ~I
_I~ C ~ ~ ~ ~ ~t .
~S~ ~ O ~1 ~ ~1 N ~7 t~t t'~ ~'t O
S~ ¢ ,~ 1: ~1 l l l l l l l l l ~ ~ ~ :~ - ~- - - - - - - - -O IO N O I O --i I ~
~ o~ c l ~ ~ ~ l ~
~ o l --l o ~ l o o~ --l ~ ~l l o ~
c ~ ' 1 ' 1 ' 1 ' l l l ' 1 ' ' 1 ' ~ L ~ I ~ - - - - -1- ~

~ ~ ! ~ ~ ~) ~D I~ CO _ ¦ O ~
I i I ~ I _ l ..,~

l~laQ~7 ~rm m~ I
~o~ c~ ~ ~ ~ ~ ~ ~ ~

b ~ _ + _ O O ~ O O O O
l CO ~ ~ _1 _1 X ~ e~l ,_1 ~') G .. .. .. .. .. .. .. .. .. ..
Co _~ _l _l _l _~ _l _~ _l _( _~
_ ~_ _ _ .~ I I
I
.
~ ~ O O O O O O ~ O O O
~ cO ,~ ~ _( ~ a~ ~ ~ ,~ ~, _ ~ ~ -- -- - , o , ~v~ O O O ~ a~ c~ ~ u~ O
8 ~ ~o ,, o a~ o~ ~ co o~ o~
_, ~ _ I _~ .
~7 j h ul 1~ n ~ J O O
~ . ~ I~ . ~, ~ cl~
o . . o o ~ ~ u~ ~_ c~
~ ~ o o e~ o ! . ~ I ~ _`
-~ ~ I I --~ I O I ~
-- ~1 1 1~7 ~1 ~O ' C`l O I
_1O I - 1 : 10 ~ 1 : ~s~3 . ^ ~
~ oo ~ z I j I z c~ I + +- I æ ~ I
tJ ~ o + I f I + l + I + I o~) L~ u~ I + + i a .~.~ ~ ~ ~ I
c~ ~ o~ ~ ~ O ~ o~ o I o ~ o~ ~ I o~ ~ I :
u~ o l ~ i o~ o o l o 3 ~ : I C j ~
~ 11 1 1 1 1 1 1 i I
_1 ~ ~ ~ I o _l I ~ i . . :

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for preparing flake-like crystalline particles of the formula ZnSO4?xZn(OH)2, wherein x =
1.7 - 4.8, having a ratio of thickness to diameter of 1:5 to 1:300, a thickness of less than 30µ m and a diameter in the range of from 1 to 300µ m which comprises:
(a) preparing a solution of 0.1 to 6 moles per liter of no more than two zinc salts selected from the group consisting of zinc nitrate, zinc sulfate, zinc acetate, zinc chloride and zinc iodide and wherein at least 25 mol percent of said zinc salt is zinc sulfate;
(b) heating said solution to a temperature in the range of from 80°C to 100°C, (c) decreasing the pH of the solution to 4.0 to 6.5 by the gradual addition of urea to said solution, whereby deposition of a precipitate of flake-like crystalline particles is caused, and (d) separating said precipitate from said solution.

2. A method as claimed in claim 1 wherein the mole ratio of urea to zinc salt in the solution is in the range of from 0.1 to 6.

3. A method as claimed in claim 1 wherein in addition to zinc sulfate the zinc salt also includes zinc nitrate in an amount of 75 mol % or less.

4. A method as claimed in claim 1 wherein in addition to zinc sulfate the zinc salt also includes zinc acetate in an amount of 50 mol % or less.

5. A method as claimed in claim 1 wherein in addition to zinc sulfate the zinc salt also includes zinc chloride in an amount of 25 mol % or less.