Alpha Alumina with High Purity and High Relative Density, A Method for Its Production and Its Use

Abstract

The present invention relates to an alpha alumina having a high purity, a high density and a low surface area and particularly, to a method to produce such an alpha alumina as well as to the use of the alpha alumina in sapphire production or the production of composite and ceramic bodies.

Claims

1. An alpha alumina, wherein said alpha alumina has a purity of at least 99.99%, a relative density above 90%, and a BET surface area of less than 10 m.sup.2/g.

2. The alpha alumina as claimed in claim 1, wherein said alpha alumina has a relative density above 94%.

3. The alpha alumina as claimed in claim 2, wherein said alpha alumina has a relative density above 97%.

4. The alpha alumina as claimed in claim 1, wherein said alpha alumina further has a BET surface area of at least 2 m.sup.2/g.

5. The alpha alumina as claimed in claim 1, wherein said alpha alumina further has a BET surface area of at least 4 m.sup.2/g.

6. The alpha alumina as claimed in claim 1, wherein said alpha alumina has a BET surface area of less than 9 m.sup.2/g.

7. A body, wherein said body is prepared by applying at least heat and/or pressure to at least the alpha alumina of claim 1.

8. The body according to claim 7, wherein said body is a ceramic body or a composite body.

9. The body according to claim 7, wherein said body is a sapphire.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0017] As per the present invention by ensuring that the aged boehmite slurry produces boehmite particles that either have a difference in crystallite size along the (120) axis and the (020) axis of less than 1 nm, preferably bigger than 0 nm and smaller than 1 nm, more preferably bigger than 0.05 nm and smaller than 1 nm, even more preferably bigger than 0.1 nm and smaller than 1 nm and most preferably bigger than 0.5 nm and smaller than 1 nm or a crystallite size along the (120) axis of larger than 30 nm, preferably larger than 40 nm, one is able to obtain a pure alumina having a relative density of above 90% without using alumina seeds. To be clear, as per the invention either option a) of step ii) or option b) of step ii) is required to be followed. Alternatively, both option a) and option b) may be followed.

[0018] For option a) the length along the (120) axis of the boehmite particles (which means after applying the aging of step ii)) is preferably at least 7 nm and more preferably at least 10 nm and most preferably at least 13 nm.

[0019] The boehmite slurry may be obtained by known methods in the art of the invention such as by the hydrolysis of aluminium alkoxide for example as described in U.S. Pat. No. 3,394,990. Examples of commercial boehmite products produced from the hydrolysis of aluminium alkoxide include PURAL? and CATAPAL?.

[0020] Depending on the purity of the aluminium alkoxide used for the hydrolysis the purity of the boehmite is in the range of between 99.8000 and 99.9999%, preferably between 99.9900 and 99.9997%. This is the purity required in the field of ultra high purity alumina.

[0021] The boehmite slurry is preferably aged at a temperature of between 95 and 160? C. for a period of between 25 and 40 hours. The aging may take place under agitation. Agitation may be provided with a paddle stirrer at a circumferential velocity of 0.5 to 4 m/s, preferably 1 to 3 m/s, in particular for option a).

[0022] The crystallite size of the boehmite particles according to the invention are determined on the (120) and (020) reflexes using the generally known Scherrer formula (1):

[00001]$\begin{array}{cc}\mathrm{Crystallite}\mathrm{size}=\left(K?\mathrm{lambda}?57.3\right)/\left(\mathrm{beta}?\cos \mathrm{theta}\right);& \left(1\right)\end{array}$

where [0023] K (form factor): 0.992 [0024] Lambda (x-ray wave length): 0.154 nm [0025] beta (corrected line broadening of apparatus): reflex dependent [0026] theta: reflex dependent

[0027] A person skilled in the art knows that in the X-ray diffraction pattern of Boehmite the (120) reflex (reflex along the 120 axis) is located at 2 theta=28.2? and the (020) reflex (reflex along the 020 axis) is located at 2 theta=14.5?. Using these values, the corresponding crystallite size for each reflex can be calculated according to equation (1) from the corrected line broadening calculated by equation (2).

[00002]$\begin{array}{cc}\mathrm{beta}=\mathrm{half}\mathrm{width}\mathrm{of}\mathrm{the}\mathrm{sample}-\mathrm{half}\mathrm{width}\mathrm{of}\mathrm{standard}& \left(2\right)\end{array}$

[0028] With respect to option b) the crystallite size of the aged boehmite particles along the (120) axis is preferably between larger than 30 and 100 nm, more preferably between larger than 30 and 50 nm or between larger than 40 and 60 nm.

[0029] The drying step may be carried out in a spray dryer. Typical temperatures include an inlet temperature of between 300 and 500? C., preferably between 320 to 400? C. and an outlet temperature of between 100 and 150? C., preferably between 110 and 120? C.

[0030] Calcination of the dried boehmite preferably occurs at a temperature of between 1300 to 1400? C. and more preferably at a temperature of between 1350 and 1360? C. The time of calcination is preferably between 3 to 4 hours. The heating ramp up rate of the calciner is preferably between 2 and 8? C./min and more preferably between 4 and 8? C./min. The temperature, time of calcination and ramp up rate can be selected independently of one another.

[0031] The method of the invention produces an alpha alumina having a relative density higher than 90%, preferably a relative density higher than 94%, and a BET surface area less than 10 m.sup.2/g %.

[0032] According to a second aspect of the invention there is therefore provided an alpha alumina produced according to the method of the invention.

[0033] The alpha alumina produced according to the method of the invention may have a purity of above 99.99% and a relative density of above 90%, preferably a relative density of above 94%, and a BET surface area of less than 10 m.sup.2/g.

[0034] According to a third aspect of the invention, there is provided an alpha alumina having a purity of at least 99.99% and a relative density above 90%, preferably a relative density above 94%, and a BET surface area of less than 10 m.sup.2/g.

[0035] In a further preferred embodiment of the present invention, the alpha alumina of the invention is used as a raw material in powder form for the production of sapphires. For example, sapphire can be produced by charging alpha alumina powder in a crucible, heating and melting the alpha alumina powder. The alpha alumina powder can be charged in the crucible at a high bulk density and is suitable for producing sapphire having less voids.

[0036] According to a further preferred embodiment of the present invention, the alpha alumina of the invention is used as a raw material possibly together with further raw materials in the production of ceramic or composite bodies which are typically produced in a mould by applying heat and/or pressure.

[0037] The invention will now be described with reference to the following non-limiting examples.

EXAMPLES

[0038] Crystallite size was measured as per the Scherrer equation described above.

[0039] The purity of the high purity alpha alumina was measured by ICP atomic emission i.e. the contents of Ca, Fe, Na, Si and Ti were measured by an ICP atomic emission spectrometer after dissolution using Spectrosolv? PSS with microwave heating.

[0040] The purity is calculated according to equation (3):

[00003]$\begin{array}{cc}\mathrm{Purity}\mathrm{in}\%=100\left[1-\left(\mathrm{total}\mathrm{amount}\mathrm{of}\mathrm{impurities}\mathrm{in}\mathrm{ppm}\right)/{10}^6\right]& \left(3\right)\end{array}$

[0041] % means wt. % with respect to the metals comprised in the composition.

[0042] Further, the content of sulphate, chloride and phosphate is in total less than 200 ppm.

[0043] The Relative Density is calculated from the Particle Density using equations (4)-(6):

[00004]$\begin{array}{cc}\mathrm{Closed}\mathrm{Pore}\mathrm{Volume}\left({\mathrm{cm}}^3/g\right)=\left(1/\mathrm{Particle}\mathrm{Density}\right)-\left(1/3.98\right)& \left(4\right)\end{array}$$\begin{array}{cc}\mathrm{Sintered}\mathrm{Density}\left(g/{\mathrm{cm}}^3\right)=1/\left[\left(1/3.98\right)+\mathrm{Pore}\mathrm{Volume}+\mathrm{Closed}\mathrm{Pore}\mathrm{Volume}\right]& \left(5\right)\end{array}$$\begin{array}{cc}\mathrm{Relative}\mathrm{Density}\left(\%\right)=\left(\mathrm{Sintered}\mathrm{Density}/3.98\right)?100& \left(6\right)\end{array}$

[0044] The Particle Density (equation 4) is determined using the ULTRA PYCNOMETER 1000 T (Quantachrome).

[0045] The XRD-measurements are carried out using a Philips XRD X'pert apparatus.

[0046] The Pore Volume (equation 5) is determined by the mercury penetration method in accordance to DIN 66133 within a pore radius range of 1.8 to 1000 nm.

[0047] BET surface area refers to the Brunauer-Emmett-Teller method for the determination of specific surface area by N.sub.2 adsorption. using typical volumetric devices like the Quadrasorb from Quantachrome at temperature of liquid nitrogen. The surface area is determined using DIN ISO 9277:2003-05.

Comparative Example 1 (Related to Option a) of Step ii) of the Method of the Invention)

[0048] 500 g of a suspension of PURAL? in water (10.5 wt. % Al.sub.2O.sub.3, pH=9) were added into an autoclave which was heated to 110? C. (1? C./min.). After the reaction conditions had been adjusted the slurry was allowed to age for 24 hours at 110? C. using a standard agitator running at a peripheral velocity of 1.6 m/s corresponding to an agitator speed of 500 r.p.m. After cooling down to room temperature the aged slurry was spray dried (inlet temperature: 350? C., outlet temperature: 110? C.). The crystallite sizes were 14.1 nm along the 120 axis and 10.6 nm along the (020) axis. The particles were then calcined at 1350? C. for 4 hours (heating rate 1? C./min.) Pure alpha alumina having a purity of 99.9990% was obtained and the relative density was 59.6%. The BET surface area was 16 m.sup.2/g.

Comparative Example 2 (Related to Option b) of Step ii)

[0049] 500 g of a suspension of PURAL? in water (7.5 wt. % Al.sub.2O.sub.3) were added into an autoclave which was heated to 140? C. (1? C./min.). The autogenous pressure was 5 bar. After the reaction conditions had been adjusted the slurry was allowed to age at 140? ? C. for 30 hours using a standard agitator running at a peripheral velocity of 3.0 m/s corresponding to an agitator speed of 265 r.p.m. After cooling down to room temperature the aged slurry was spray dried (inlet temperature: 350? C., outlet temperature: 110? C.). The crystallite size was 24.7 nm along the 120 axis. The particles were then calcined at 1350? C. for 4 hours (heating rate 1? C./min.) Pure alpha alumina having a purity of 99.9992% was obtained and the relative density was 85.2%. The BET surface area was 11 m.sup.2/g.

Example 1 (Related to Option a) of Step ii) of the Method of the Invention)

[0050] 500 g of a suspension of PURAL? in water (10.5 wt. % Al.sub.2O.sub.3, pH=9) were added into a beaker which was heated to 98? C. (1? C./min.). After the reaction conditions had been adjusted the slurry was allowed to age for 30 hours at 98? C. using a standard agitator running at a peripheral velocity of 1.6 m/s corresponding to an agitator speed of 500 r.p.m. After cooling down to room temperature the aged slurry was spray dried (inlet temperature: 350? C., outlet temperature: 110? C.). The crystallite sizes were 13.8 nm along the 120 axis and 13.1 nm along the 020 axis. The particles were then calcined at 1350? C. for 4 hours (heating rate 1? C./min.) Pure alpha alumina having a purity of 99.9996% was obtained and the relative density was 97.9%. The BET surface area was 4 m.sup.2/g.

Example 2 (Related to Option b) of Step ii) of the Method of the Invention)

[0051] 500 g of a suspension of PURAL? in water (7.5 wt. % Al.sub.2O.sub.3) were added into an autoclave which was heated to 160? C. (1? C./min.). The autogenous pressure was 10 bar. After the reaction conditions had been adjusted the slurry was allowed to age for 30 hours at 160? C. using a standard agitator running at a peripheral velocity of 3 m/s corresponding to an agitator speed of 265 r.p.m. After cooling down to room temperature the aged slurry was spray dried (inlet temperature: 350? C., outlet temperature: 110? C.). The crystallite size was 35.8 nm along the 120 axis. The particles were then calcined at 1350? C. for 4 hours (heating rate 1? C./min.) Pure alpha alumina having a purity of 99.9994% was obtained and the relative density was 94.1%. The BET surface area was 8 m.sup.2/g.

Example 3 (Related to Option b) of Step ii) of the Method of the Invention)

[0052] 500 g of a suspension of PURAL? in water (7.5 wt. % Al.sub.2O.sub.3) were added into an autoclave which was heated to 180? C. (1? C./min.). The autogenous pressure was 15 bar. After the reaction conditions had been adjusted the slurry was allowed to age for 30 hours at 180? C. using a standard agitator running at a peripheral velocity of 1.6 m/s corresponding to an agitator speed of 500 r.p.m. After cooling down to room temperature the aged slurry was spray dried (inlet temperature: 350? C., outlet temperature: 110? C.). The crystallite size was 41.8 nm along the 120 axis. The particles were then calcined at 1350? C. for 4 hours (heating rate 1? C./min.) Pure alpha alumina having a purity of 99.9996% was obtained and the relative density was 97.4%. The BET surface area was 7 m.sup.2/g.

[0053] The results of the Experiments are outlined in the Tables hereunder:

TABLE-US-00001 TABLE 1 Option a) of step ii) of the method of the invention: Crystallite Crystallite Total Surface size size Difference = Relative impurity Area [120] [020] ([120] ? [020]) Density content BET Comparative 14.1 nm 10.6 nm 3.5 nm 59.6% 10 ppm 16 m.sup.2/g Example 1 Example 1 13.8 nm 13.1 nm 0.7 nm 97.9% 4 ppm 4 m.sup.2/g

TABLE-US-00002 TABLE 2 Option b) of step ii) of the method of the invention: Total Surface Crystallite Size Relative impurity Area [120] Density content BET Example 2 35.8 nm 94.1% 6 ppm 8 m.sup.2/g Example 3 41.8 nm 97.4% 4 ppm 7 m.sup.2/g Comparative 24.7 nm 85.2% 8 ppm 11 m.sup.2/g Example 2

[0054] As can be seen by the Examples when compared to the Comparative Examples, the method of the present invention produces an alpha alumina having a purity of above 99.99% and a relative density of above 90%, preferably a relative density of above 94%, and a BET surface area lower than 10 m.sup.2/g.