Method for the manufacture of transparent aluminum oxide ceramic

A method is disclosed for manufacturing transparent aluminum oxide ceramic articles by sintering dry-pressed acid-containing aluminum oxide monohydrate in vacuo or in hydrogen. The method includes obtaining the acid-containing aluminum oxide monohydrate by peptisization of aluminum alkoxide with a mixture of water and acid vapor, with the acid bound to the formed monohydrate by electrostatic force. Such attachment allows ease in separation of the monohydrate crystals in water due to the polarizing effect of the acid. Subsequently, the acid-containing aluminum oxide monohydrate is gelled and dried, with the product, having an acid content of from 12 to 25%, being suitable for forming transparent ceramic articles without requiring calcination.

FIELD OF THE INVENTION 
The present invention relates to aluminum oxide ceramic. More particularly, 
the invention relates to a method for manufacturing transparent aluminum 
oxide ceramic from an acid-containing aluminum oxide monohydrate. 
BACKGROUND OF THE INVENTION 
Aluminum oxide is a starting material for manufacturing ceramics having 
particularly high hardness and good chemical corrosion resistance. 
Aluminum oxide ceramic is customarily manufactured by sintering mixtures 
of aluminum oxide and other oxides. Extremely high-quality formed parts of 
aluminum oxide ceramic are produced from practically pure aluminum oxide, 
to which may be added, as a sintering adjuvant, small fractions of MgO or 
TiO2, such that the resulting ceramic has an Al.sub.2 O.sub.3 content of 
99.7%. Metallurgical grade Al.sub.2 O.sub.3, produced by the Bayer-Process 
from bauxite, is typically unsuitable as a starting material for the 
manufacture of aluminum oxide ceramic, due to interfering impurities such 
as Na.sub.2 O--approx. 0.3-0.5%; SiO.sub.2 - approx. 0.02-0.05%; Fe.sub.2 
O.sub.3 - approx. 0.02-0.05%. Further, metallurgical grade aluminum oxide 
is too coarse for producing high quality ceramic, having a mediate grain 
diameter of about 50-100 um. To provide the desired properties in the 
ceramic, the aluminum oxide must possess uniform characteristics achieved 
from a relatively precise degree of calcination, combined with proper 
grinding These characteristics of the aluminum oxide are discussed further 
below. 
For certain applications, a transparent aluminum oxide ceramic having 
specific characteristics and prepared according to specific methods is 
required. Transparent aluminum oxide ceramic is particularly useful in the 
construction of sodium vapor high-pressure electric lamp tubes, because 
aluminum oxide ceramic provides a high chemical resistance to the 
aggressive vapor. 
The manufacture of transparent aluminum oxide ceramic requires an aluminum 
oxide of particularly high chemical purity. In particular, the 
concentration of SiO.sub.2, Fe.sub.2 O.sub.3, Mn.sub.2 O.sub.3 and other 
heavy metal oxide impurities in the aluminum oxide, is critical in 
determining the suitability of the aluminum oxide for the manufacture of 
transparent ceramic. The reason for this is that the heavy metal oxides 
and silicon oxides are reduced by sodium vapor under the operating 
conditions of the sodium vapor high-pressure lamps, resulting in the 
formation of elemental metal, which in turn leads to a reduction in light 
transmissibility of the ceramic lamp tube. 
A particular difficulty in the manufacture of transparent ceramic is that a 
sufficiently pure aluminum oxide starting material must be obtained. One 
known method for obtaining purified aluminum oxide proceeds by way of 
thermal degradation of ammonium alum, AlNH.sub.4 (SO.sub.4).sub.2. 
Ammonium alum can be purified easily because of its particularly good 
crystallization properties, resulting in a low impurities content. Other 
methods of purifying aluminum oxide are based on the 
fractionation/distillation of volatile aluminum compounds, such as 
aluminum alkoxide and aluminum alkyl, which can be subsequently degraded 
to aluminum oxides or its precursors, such as aluminum hydroxide or other 
aluminum salts. 
Both methods generally provide acceptably low levels of impurities, as 
illustrated by the following limits: 
______________________________________ 
SiO2 max 20 ppm (parts per million) 
Fe2O3 max 10 ppm 
Cr2O3 max 5 ppm 
TiO2 max 10 ppm 
other elements max 
5 ppm 
______________________________________ 
The existing technology for the manufacture of transparent ceramic lamp 
tubes requires highly calcined aluminum oxides in order to attain the 
necessary properties required for further ceramic processing. In 
particular, calcination provides a BET surface area of 1-10 m.sup.2 /g 
(BET = nitrogen absorption according to Brunauer, Emmet & Teller (see ISO 
Standard, Ref.No.ISO-8008-1986(E)). 
The calcination of high-purity oxides is fraught with problems, since, when 
direct firing is used, impurities can be introduced into the product by 
the fuel used to produce the heat for calcination. Heating indirectly does 
avoid this contamination source. However, indirect heating is not readily 
adaptable to continuous operation, and during batch calcination, the 
danger exists that, due to the low powder density producing poor heat 
conductivity, a temperature gradient will occur across the material, 
creating large differences in the calcination state of the oxide. The 
calcination state determines the ceramic reactivity of the oxide, and 
consequently, to obtain uniform sintering, there should be only minor 
variations in the calcination state. Hence, the processing properties of 
the calcinated material is not generally acceptable for transparent 
aluminum ceramic manufacture. 
An additional critical aspect of oxide manufacture for use in transparent 
ceramics is the average particle size of the calcined oxides. The calcined 
oxides must be ground extremely fine, for the oxides to be subsequently 
successfully sintered, such that dense formed bodies can be obtained. If 
properly calcined material is obtained, it is generally hard, requiring 
contact grinding which presents the danger of contamination of the oxide 
material by particles abraded from the grinding substance. The 
contaminating particles can create faults in the sintered product and, 
consequently, increase the rate of rejects in the manufactured items of 
transparent ceramic. Therefore, the critical difficulties in the 
preparation of aluminum oxides for transparent ceramic lies in the 
calcination and grinding steps of the pure aluminum oxide. 
It is known that transparent bodies can be created by sintering aluminum 
oxide monohydrate gel. However, these bodies are also known to have a high 
degree of porosity, which precludes their use as lamp tubes, since they 
are not gas-impermeable. 
B. E. Yoldas, [Amer. Ceram. Soc. Buil. 54 (1975) 286 and Journal of 
Materials Science 10 (1975) 1856], has described that by the pyrolization 
of formed bodies, produced of aluminum oxide monohydrate gel, transparent 
but highly porous bodies are formed which may be used as catalyst 
carriers. 
The method developed by Yoldas has the disadvantage that the gel is formed 
into bodies, which are subsequently dried. During the drying process, 
there is a danger that tears might appear in the formed bodies due to 
shrinkage stresses. The tears formed in the bodies during drying do not 
heal in the calcination process. Accordingly, the method is suitable only 
for the manufacture of thin layers, for placement on carriers of different 
types, such as aluminum oxide or another ceramic material. Consequently, 
the search continues for compositions and methods for producing 
transparent ceramic while avoiding the known consistency and porosity 
problems. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method for preparing 
a starting material for transparent ceramic, without requiring calcination 
and contact grinding of the aluminum oxide. 
The foregoing, and other features and advantages of the present invention 
will become more apparent from the following description. 
DETAILED DESCRIPTION OF THE INVENTION 
According to the present invention, aluminum oxide monohydrate is used as a 
starting material for the preparation of a transparent ceramic. The 
aluminum oxide monohydrate is prepared by hydrolyzing and peptisizing an 
aluminum alkoxide with a mixture of water vapor and an acid vapor; 
removing byproduct alcohol from the mixture, leaving a remainder; mixing 
the remainder with water; heating the mixture until a homogeneous gel is 
achieved, and drying the gel which comprises an aluminum oxide monohydrate 
including about 12.5 to 25% by weight acid bound thereto, which, for 
acetic acid, may be measured as the organic carbon content. Of course, 
other acids, such as HNO.sub.3 cannot be determined by analysis of organic 
carbon and another appropriate method may be used. The dried gel is then 
ground to a fineness of, on average, 2 um (microns) before being 
processed. 
"Peptisizing" is a process for forming a gel of the monohydrate which uses 
acid molecules to form a molecular layer on the monohydrate, which makes 
the crystals easily separated by water from each other, preferably at 
elevated temperatures. The resulting gel is opaque and stable, i.e. the 
peptisized particles do not precipitate from the gel. The acid is "bound" 
to the monohydrate, which is defined as bonding to the monohydrate by weak 
forces such as Van Der Waals forces which are electrostatic. The acid does 
not react with the OH-groups to form aluminum acetate. This electrostatic 
interaction leads to polarization of the shell of the acid molecules 
covering the monohydrate, causing the shells to repel each other and 
supporting separation of the particles in water. 
Peptisizing may be achieved by reaction of the monohydrate with any acid. 
Preferred acids are monobasic acids which disassociate in water to yield 
only one proton. Of course, various processing differences will be 
required with some acids. For example, nitric acid or hydrochloric acid 
could be used, yet these acids may yield corrosive or toxic gasses during 
sintering. Consequently, an acid such as acetic acid is preferred which 
yields carbon dioxide during sintering. While acetic acid will discussed 
hereinafter, it will be understood that the invention is not limited 
thereby. 
Organically bound carbon is used as the indicator of the acid content of 
the monohydrate because the acetic acid is bound so strongly to the 
aluminum oxide monohydrate that direct analysis by titration is not 
possible. Generally, carbon contents ranging from 5 to 10% correspond 
linearly to acetic acid contents ranging from 12.5 to 25%. While this 
indirect method is illustrative, it will be understood that other analytic 
methods may be employed to determine the bound acid content. 
The starting material is an aluminum alkoxide, for example, aluminum 
isopropylate, aluminum butylate, or aluminum ethylate, with the alkoxide 
hydrolized and peptisized in one step to produce the acid-containing 
aluminum oxide monohydrate. Of course, the choice of starting material may 
vary beyond the examples disclosed, as will be evident to one skilled in 
the art. 
An advantage of the present invention is that grinding can be accomplished 
using non-contact jet mills, which yield an essentially non-contaminated 
product. Jet mills are suitable for use with the acid-containing aluminum 
oxide monohydrate, because compared to calcined aluminum oxide, aluminum 
oxide monohydrate is very soft, and therefore, can readily be reduced to 
the desired degree of fineness with a jet mill, such as an Alpine-air jet 
mill which uses dry air or another inert gas for milling. However, other 
milling apparatus could also be used without product quality loss, so long 
as the gel is sufficiently ground, to a median grain diameter of about 2 
um, without contamination. 
The ground acid-containing aluminum oxide monohydrate is therefore suitable 
for processing according to a "dry-press" method, without further 
pre-treatment, to supply green (unsintered) bodies, which can then be 
processed in a vacuum or hydrogen kiln to yield transparent aluminum oxide 
ceramic bodies. If necessary or desired, sintering adjuvants such as are 
customarily added in the manufacture of transparent aluminum oxide 
ceramic, for example, MgO, can be added to the aluminum oxide monohydrate 
before dry-press processing. Any hydrolyzable magnesium compound, such as 
magnesium ethylate, can be added to the aluminum alkoxide before the 
solvolysis, so that the sintering adjuvant becomes distributed 
homogeneously in the peptized aluminum oxide monohydrate. Magnesium oxide 
offers the additional advantage of directing crystal growth during 
sintering, building an even structure, suitable for producing, for 
example, a sintered transparent high-pressure sodium vapor lamp tube. Of 
course, other additives could also be used. 
The ground, peptized, and dried acid-containing aluminum oxide monohydrate 
can be processed with compacting machines customarily employed in the 
ceramic industry for shaping and densifying powders to form green bodies 
by the dry-press method. The pressing pressures required for green 
densification are of the same order of magnitude as are customary for 
ceramic, generally about 50-150 N/mm.sup.2 
Surprisingly, bodies formed from the- acid-containing monohydrate can be 
sintered to form dense bodies without intermediary steps of drying or 
dehydration, with the bodies becoming transparent during sintering. This 
is all the more surprising, since the reduced green density, which is 
achieved in dry pressing, is approximately 1 g/cm.sup.3 and consequently, 
based on general experience, it would not be expected that these green 
bodies could be sintered, practically to the theoretical density of the 
aluminum oxide, i.e. 3.98 g/cm.sup.3 Generally, in processing aluminum 
oxide into ceramic, commercial oxides must be green densified by at least 
50% of the theoretical pure (unalloyed) density so that pure density can 
be reached during sintering. At a lower green density, it was believed 
that pure density could not be reached during sintering. It should be 
noted that the sintering is carried out in a conventional fashion, 
generally in a vacuum or hydrogen kiln, and that the times and 
temperatures discussed are illustrative only.

The invention is explained in greater detail in conjunction with the 
following examples. 
EXAMPLE 1 
200 kilograms (kg) of isopropyl alcohol and 90 kg of aluminum isopropylate 
were added to a vessel and heated with stirring to 80.degree. C.. 
Magnesium ethylate, produced by Dynamit Nobel, Troisdorf, West Germany, 
was added to the mixture to yield a weight ratio of MgO to Al.sub.2 
O.sub.3 of about 8:10,000. A mixture of acetic acid vapor and water vapor 
was introduced through an immersion tube into the mixture contained in the 
stirred vessel. The vapor was produced by vaporization of a mixture of 3.9 
kg of acetic acid and 15.8 kg of water in an evaporator. The rate of 
inflow of the vapor mixture was set by regulating the heating power of the 
evaporator in such a way that the entire fluid quantity is introduced to 
the stirred vessel within two hours. During this time, the reaction 
mixture was maintained at a temperature of 80 C. After the vapor had been 
completely introduced, stirring continued for an additional three-hour 
period. Subsequently, both the isopropanol used as a solvent and the 
isopropanol released during the reaction were removed by distillation from 
the reaction mixture, leaving a powder residue (remainder). 
The powder remaining after distillation was mixed with 50 kg of water. Any 
remaining isopropanol released was again removed from the stirred vessel 
by distillation. The stirred vessel was heated, raising the temperature to 
the boiling temperature of water, approximately 100.degree. C. The 
reaction mixture was then stirred for five hours to produce a homogeneous 
mixture. Heating was done in such a way that a weak reflux was maintained. 
After boiling for 5 hours with weak reflux, the water was distilled off 
from the vessel at about 100 to 120.degree. C., until a solid dry residue 
of peptisized and dried aluminum oxide monohydrate containing from 5-10% 
organically bound carbon was formed. The peptized and dried aluminum oxide 
monohydrate was removed from the vessel and reduced by milling, preferably 
with an air jet mill, to an average grain diameter of 2 um. 
On a hydraulic press, test samples having the dimension 1 cm.times.1 
cm.times.0.4 cm were formed from the powder at a pressing pressure of 100 
N/mm.sup.2. The formed bodies were brought to a temperature of 
1850.degree. C over a period of 24 hours in an inductively heated vacuum 
kiln, such as a Balzers kiln model USE 02, with this temperature 
maintained for five hours. After the heat was turned off, the test samples 
were cooled in the kiln to about room temperature. 
The samples taken from the kiln were transparent and under microscopic 
examination were determined to have a very uniform structure. Flaking or 
tearing was not detected on the bodies. 
COMISON EXAMPLE 1: 
200 kg of isopropyl alcohol and 90 kg of aluminum isopropylate were added 
to a vessel and heated with stirring to 80.degree. C.. Magnesium ethylate 
was added to the mixture in such a way that a weight ratio of MgO to 
Al.sub.2 O.sub.3 of about 8:10,000 resulted. A mixture of acetic acid 
vapor and water vapor was introduced through an immersion tube into the 
mixture contained in the stirred vessel. The vapor mixture was obtained by 
evaporating a mixture of 3.9 kg acetic acid with 15.8 kg water. The rate 
of inflow of the vapor mixture into the vessel was set by regulating the 
heating power of the evaporator so that the entire given fluid quantity 
was introduced over a two hour period. The reaction mixture was maintained 
at a temperature of 80.degree. C. during this time. After the vapor inflow 
had been completed, the reaction mixture was stirred for another three 
hours. Subsequently, the isopropanol used as solvent and that freed in the 
reaction were removed from the reaction mixture by distillation. 
The powder was mixed with 50 kg water, freeing any residual isopropanol 
which was likewise removed from the vessel by distillation. By heating the 
stirred vessel, the temperature was raised to the boiling temperature of 
water, approximately 100.degree. C., and the reaction mixture was stirred 
for five hours until a homogeneous mixture was produced. Heating was 
conducted in such a way that a weak reflux was maintained. 
After completion of the boiling step, the water was distilled from the 
stirred vessel at 160 to 180.C until a solid dry residue of peptisized and 
dried aluminum oxide monohydrate was obtained. The powder remaining after 
distillation was amber in color. The higher temperature pyrolyses a 
portion of the acetic acid reducing the amount of acetic acid bound to the 
aluminum oxide monohydrate. The peptized and dried aluminum oxide 
monohydrate was taken from the vessel and reduced by crushing or milling 
with an air jet mill to an average grain diameter of 2 um. 
Test samples with the dimensions 1 cm.times.1 cm.times.0.4 cm were prepared 
from the powder using a hydraulic press at a pressure of 100 N/mm.sup.2 
The formed bodies were heated to a temperature of 1850.degree. C. over a 
24 hour period, in an inductively heated vacuum kiln. This temperature was 
maintained for five hours. Subsequently, after the heat was turned off, 
the samples were allowed to cool in the kiln. 
The bodies taken from the kiln were opaque and exhibited signs of flaking. 
The peptisized and dried aluminum oxide monohydrates obtained in Example 1 
and comparison Example 1 have the following composition: 
______________________________________ 
Relative to AlO3 
Example 1 Comparison Example 1 
______________________________________ 
organically bound 
7% 4% 
carbon 
MgO 800 ppm 800 ppm 
Fe2O3 9 ppm 7 ppm 
SiO2 14 ppm 17 ppm 
Cr.sub.2 O3 5 ppm 5 ppm 
______________________________________ 
The difference between the processes of Example 1 and comparative Example 1 
is that the water is distilled at a higher temperature (160-180 versus 
100-120), causing some pyrolysis of the acetic acid. Consequently, the 
amount of acid bound to the monohydrate is reduced. This is reflected in 
the difference in bound organic carbon, reduced from 7 to 4 percent. This 
equates to a reduction in bound acetic acid from 17.5 to 10%. While a 
relatively small difference, the effect in the final product is dramatic, 
as a transparent aluminum oxide ceramic is not achieved. 
COMISON EXAMPLE 2: 
Aluminum oxide monohydrate, product no. NB 400 produced by VAW, was 
calcined in an inductively heated cylindrical rotary kiln manufactured by 
Smidt Ovens, Netherlands in a continuous operation at a temperature of 
1180.degree. C. to yield an aluminum oxide having an effective specific 
BET surface area of 125 m.sup.2 /g (square meters per gram). The oxide was 
subsequently ground in an air jet mill to an average grain diameter of 2 
um. The low degree of calcination allowed use of the jet mill in this 
example. From this powder, test samples were produced as described in 
Example 1 which were sintered in a vacuum kiln, as in Example 1. 
The bodies taken from the kiln were opaque and exhibited signs of flaking. 
COMISON EXAMPLE 3: 
Aluminum oxide monohydrate, (NB 400) was calcined in an indirectly heated 
cylindrical rotary kiln to yield an aluminum oxide with a effective 
specific BET surface area of 16 m.sup.2 /g. The oxide was ground in an air 
jet mill to an average grain diameter of 2 um. From the powder, test 
samples were produced which were subsequently fired under the same 
conditions as the test samples in Example 1. 
The test samples taken from the kiln were opaque and splintering was 
indicated. 
COMISON EXAMPLE 4: 
Aluminum oxide monohydrate (NB 400) was calcined in an indirectly heated 
cylindrical rotary kiln to yield an aluminum oxide with a effective 
specific BET surface area of 7 m.sup.2 /g. The oxide stuck to the kiln 
wall, indicating that this material achieved the highest degree of 
calcination obtainable with this equipment. 
The oxide was subsequently ground in contact grinder to an average grain 
diameter of 2 um. The ground oxide was contaminated by abraded material 
during grinding From the powder, test samples were produced and subjected 
to the same sintering conditions as the test samples of Example 1. 
The test samples taken from the kiln were not transparent and exhibited 
signs of flaking. In addition, the abraded material from the grinding step 
caused dark spots in the test samples. 
COMISON EXAMPLE 5: 
15 kg. of aluminum oxide monohydrate (NB 400) was added in small portions, 
of approx. 1 kg. each, to 85 kg. of water in a stirrer vessel over a one 
hour time period. A suspension formed, to which was added 0.75 kg. of 
acetic acid. The mixture was then agitated for an additional five hours. 
The agitator was then stopped. The mixture was opaque and solids did not 
settle out. Subsequently, the suspension was processed in a spray dryer to 
form a dry granulate, with a mean grain diameter of about 100 um. 
The granulate was processed in the same manner as described in Example 1 to 
form test samples, which were sintered in vacuo in the same way as 
described in Example 1. The test samples taken from the kiln were opaque 
and exhibited signs of flaking and tearing. 
In the table below the results of the example and the comparison examples 
are listed. It is evident that the product manufactured according to the 
invention is markedly better suited for the manufacture of transparent 
ceramic than the materials investigated as comparisons. 
______________________________________ 
green density 
sinter lin. 
at 100 N/mm.sup.2 
density shrinkage 
trans- 
g/cm.sup.3 
g/cm.sup.3 
% parency 
______________________________________ 
Example 1 1.20 3.93 37.5 0.73 
Comp. Example 1 
1.26 3.90 34.2 0.27 
Comp. Example 2 
0.96 3.90 36.7 0.43 
Comp. Example 3 
1.40 3.89 23.3 0.33 
Comp. Example 4 
1.50 3.79 21.3 0.40 
Comp. Example 5 
1.01 3.68 33.8 0.48 
______________________________________ 
The transparency was measured with a fully automatic computerized 
microscope, which performs a quantitative picture analysis (TAS-Plus, 
produced by TAS/Leitz Co.). Principally, it is a microscope with a TV 
camera which uses a digitized image and statistical analysis to determine 
variations in the light intensity of the test samples. A transparency of 
"1" indicates that the sample allows all irradiated light to pass through 
completely, while a value of "0" indicates that the sample is impermeable 
to light. 
Producing an acid containing aluminum oxide monohydrate in one step by 
combining hydrolysis of an aluminum alkoxide with the peptisizing of the 
resultant monohydrate offers substantial benefits in terms of manufacture 
of a purified material suitable for forming transparent ceramic articles 
without calcination. The material is soft and easily ground with 
non-contact grinding means, avoiding contamination. In addition, the 
overall yield of transparent articles is increased as variations in part 
quality are reduced. The fact that the acid containing aluminum oxide 
monohydrate is readily processed to a solid transparent ceramic is 
surprising in view of the Yoldas teaching that high porosity is required 
to achieve transparency. 
While the invention has been described using acetic acid, it will be 
understood by those skilled in the art that various other acids could be 
used with the present invention. In addition, other modifications to the 
processing steps could be made without varying from the scope of the 
present invention.