Dense molded bodies of polycrystalline aluminum nitride and process for preparation without use of sintering aids

The invention is a molded body of polycrystalline aluminum nitride having a density of at least 99.8% TD and comprising: PA1 at least 99% by weight AlN PA1 up to 0.35% by weight residual oxygen PA1 up to 0.35% by weight residual carbon and PA1 up to 0.03% by weight metallic impurities (Fe, Si, Ca, Mg). In the molded body, the aluminum nitride is present in the form of a single phase, homogeneous, isotropic microstructure having a maximum grain size of 5 .mu.m. The residual oxygen and the residual carbon are present in the form of a solid solution in the aluminum nitride lattice and caromografically not detectable as separate phase(s) up to a magnification of 2400 times. The molded bodies have a bending strength measured according to the 4-point method, at room temperature and up to about 1400.degree. C., of at least 500 N/mm.sup.2, a predominantly transcrystalline rupture modulus and a thermal conductivity at 300 K of at least 150 W/mK. The molded body is produced from a porous, deoxidized green body having a maximum density of 70% TD and the same chemical composition as the final product, by isostatic hot pressing in a vacuum sealed casing at a temperature of from 1700.degree. to 2100.degree. C. and a pressure of from 100 to 400 MPa in a high-pressure zone using an inert gas as a pressure transmitting agent.

Polycrystalline bodies of aluminum nitride are known. They are 
characterized by a combination of valuable properties such as high 
strength, resistance to oxidation, resistance to thermal shock, high 
thermal conductivity, low electrical conductivity and resistance to 
corrosion by liquid metals. Due to this combination of properties, they 
can be used in many different fields. They are particularly useful as 
construction materials in high-temperature machinery and as substrate 
materials in high-efficiency electronic devices. 
BACKGROUND OF THE INVENTION 
The known polycrystalline aluminum nitride molded bodies however, have in 
part less desirable properties than pure aluminum nitride. The properties 
depend to a great extent on the amount of impurities that are present in 
the molded body and particularly, the amount of oxygen, carbon and metals 
present. For example, the theoretical value of the thermal conductivity of 
pure, monocrystalline aluminum nitride is 320 W/mK, which drops to about 
50 W/mK as the oxygen content increases. Still lower values are obtained 
on further increase of the oxygen content or in two-phase aluminum nitride 
ceramics (see G.A. Slack in J. Phys. Chem. Solids (1973), Vol. 34, pp. 
321-335; ref. in C.A. Vol. 78 (1973), No. 129,310 r). 
The strength at high temperatures also depends on the impurities present in 
the molded body. The bending strength drops sharply at temperature above 
1000.degree. C. in comparison to the value measured at room temperature 
which is ascribed to the presence of oxygen-containing phases at the grain 
boundaries in the aluminum nitride sintered body. 
Pure aluminum nitride does not sinter easily because of its predominantly 
covalent bonding. In order to obtain high density bodies, it was deemed 
necessary either to start from aluminum nitride powders rich in oxygen or 
to add sintering aids preferably metal oxides which aid compression during 
hot pressing. It is possible to obtain from aluminum nitride powder having 
an oxygen content of 1.0% by weight, (2.1% by weight based on Al.sub.2 
O.sub.3) by axial hot pressing, a molded body of aluminum nitride with 98% 
theoretical density (herein-after abbreviated as % TD), which at room 
temperature has a bending strength of 265 N/mm.sup.2 and which at 
1400.degree. C. dropped to 125 N/mm.sup.2 (see DE-A-14 71 035 
corresponding to U.S. Pat. No. 3,108,887). 
By hot pressing commercially available aluminum nitride powders at 
2000.degree. C., sintering densities of 97 to 99% TD were obtained. The 
purest of the polycrystalline aluminum nitride molded bodies thus produced 
contained 0.9% by weight oxygen, had a density of 97% TD and a thermal 
conductivity of 66 W/mK (see G.A. Slack et al. in Amer. Ceram. Soc. Bull. 
(1972) Vol. 71, pp. 852 to 856; ref. in C.A. Vol. 78 (1973) No. 19686 K 
and DE-A-20 35 767). 
Polycrystalline aluminum nitride molded bodies produced by hot pressing 
commercially available aluminum nitride powder without sintering aids at a 
hot pressing temperature of 1700.degree. C., had 0% porosity, contained 
0.8% by weight oxygen and had a bending strength of 375 N/mm.sup.2, 
measured according to the 3-point method at room temperature, which at 
1300.degree. C. dropped to about 225 N/mm.sup.2 (see P. Boch et al. in 
Ceram, Int. 1982, Vol. 8 (1), pp. 34-40; ref. in C.A. Vol. 97(1982) No. 
59846 m). 
It has been reported from Japan, in relation to metallic impurities, that 
it is possible to hot press high-purity aluminum nitride powders without 
sintering aids at temperatures of 2000.degree. C. to form dense 
transparent bodies. Physical data were given only for monophase aluminum 
nitride bodies produced with an admixture of 0.5 percent by weight calcium 
oxide as a sintering aid and containing from 0.5 to 0.7% by weight oxygen. 
A hot pressed aluminum nitride body had a density of 99.6% TD, a thermal 
conductivity of 91 W/mK and a bending strength of 510 N/mm.sup. 2 measured 
at room temperature according to the 3-point method. A pressureless 
sintered aluminum nitride body had a density of 99.1% TD and a thermal 
conductivity of 95 W/mK (see N. Kuramoto et al. in J. Mater. Sci. Lett. 
1984, Vol. 3 (6), pp. 471-474; ref in C.A. Vol. 101 (1984), No. 42402s). 
Aluminum nitride molded bodies produced by conventional hot pressing 
methods, with biaxial application of pressure, have an anisotropic 
microstructure so that their properties depend on direction. 
Since only bodies of simple shape can be produced by a hot pressing 
process, pressureless sintering processes have been developed for the 
production of polycrystalline aluminum nitride molded bodies. Pressureless 
sintering processes for aluminum nitride require use of sintering aids to 
obtain high sintered densities. Numerous compounds have been tested for 
promoting sintering of aluminum nitride. Especially effective are oxides 
of elements from the 2nd and 3rd group of the Periodic System including 
the lanthanides (see K. Komeya et al. in Yogyo Kyokaishi; 1981, Vol. 89 
(6), pp. 330-336; ref. in C.A. Vol. 95 (1981), No. 155257 z). 
Due to the sensitivity of aluminum nitride to impurities, in particular 
oxygen impurities, it is necessary to use the least possible amounts of 
oxygen containing sintering aids or to reduce, by additional processing 
steps, the oxygen present in the aluminum nitride powder and/or the oxygen 
introduced by the sintering aids. 
According to the process disclosed in U.S. Pat. No. 4,435,513, a mixture of 
commercially available aluminum nitride powders having an oxygen content 
of not more than 5% by weight, together with up to 5.66% by weight of 
alkaline earth oxide sintering aids with up to 6.54% by weight carbon in 
the form, for example, carbon black or a carbonizable organic material 
such as sugar or phenolic resin were pressureless sintered at temperatures 
of up to 2000.degree. C. The carbon in the mixture prevents the formation 
of aluminum oxide nitride phases and the amount of oxygen present in the 
starting aluminum nitride powder is reduced. As can be seen from the 
examples, the aluminum nitride molded bodies produced had a density of 
98.5% TD and a thermal conductivity of 63 W/mK. By a subsequent hot 
isostatic pressing treatment, the density could be increased to 99% TD and 
the thermal conductivity to 71 W/mk. 
According to the process disclosed in EP-A-147 101, aluminum nitride 
powders containing 0.001 to 7% by weight oxygen mixed with 0.01 to 15% by 
weight oxides of rare earth metals were hot pressed or sintered without 
pressure. It is believed that the oxygen present in the aluminum nitride 
starting powder reacts with the oxides of the rare earth metals 
(preferably Y.sub.2 O.sub.3) forming compounds (phases) having a garnet or 
Perowskite structure so that oxygen does not diffuse into the aluminum 
nitride lattice with formation of mixed crystals or aluminum oxy-nitride 
phases (A1N polytypes). The garnet or Perovskite phases are formed during 
sintering at relatively low temperatures (1000.degree. to 1300.degree. 
C.), they melt at high temperatures (1600.degree. to 1950.degree. C.) and 
induce a liquid-phase sintering that results in dense bodies. As can be 
seen from the examples, the best results with regard to thermal 
conductivity were obtained with aluminum nitride bodies prepared from 
aluminum nitride powders having oxygen contents of 0.3 to 1.0 percent by 
weight with admixture of 0.1 to 3.0% by weight Y.sub.2 O.sub.3 which were 
pressureless sintered at 1000.degree. C. For an aluminum nitride (AlN) 
body with the relatively high oxygen content of approximately 0.9% by 
weight (0.6% by weight from the AlN powder +about 0.3 by weight oxygen 
from the 1.5% by weight Y.sub.2 O.sub.3), the highest heat conductivity 
given in all the examples was 135 W/mK. By X-ray diffraction analysis 
there were detected in these bodies, together with the main aluminum 
nitride phase, small amounts of an Al-Y garnet phase and an aluminum 
oxynitride phase, which are present as oxidic impurities at the aluminum 
nitride grain boundaries. 
According to the process disclosed in EP-A-13 32 75 (corresponding to U.S. 
Pat. No. 4,478,785 and U.S. Pat. No. 4,533,645), it was disclosed that 
sintering aids were not necessary and only a carbon-containing material 
was used. Commercially available aluminum nitride powders of high purity, 
with regard to metallic impurities, and containing approximately 1.5 to 
3.0% by weight oxygen were partially deoxidized by adding carbon so that 
the aluminum nitride powders or the green body prepared therefrom still 
contained, after the deoxidation treatment by heating, a residual oxygen 
content of from about 0.35 to about 1.1% weight. The high residual oxygen 
content is necessary for pressureless sintering, at temperatures in the 
range of 1900.degree. to 2200.degree. C., to sintered densities of more 
than 85% TD in resulting sintered bodies. Accordingly, the finished 
aluminum nitride sintered bodies have a residual oxygen content in the 
range of about 0.35 to about 1.1% by weight and a residual carbon content 
in detectable amounts as low as about 0.2% by weight. The sintered bodies 
of aluminum nitride prepared by the process are stated to be free of 
secondary phases which is understood to mean that they contain less than 
about 1% by volume secondary phases (that is, phases other than AlN). As 
can be seen from the examples, however, the lack of sintering aids 
produces sintered bodies with final densities in the range of 91.6 to 
97.2% TD and relatively high residual oxygen contents. In addition, 
despite the use of an aluminum nitride starting powder of high purity in 
relation to metallic impurities, the thermal conductivity values at room 
temperature were at a maximum of only 82 W/mK. 
According to the process disclosed in EP-A-15 25 45, the improvement in 
thermal conductivity is obtained, by using for deoxidation of the aluminum 
nitride, an admixture containing yttrium such as yttrium metal, yttrium 
hydride and/or yttrium nitride instead of carbon. The yttrium reacts with 
the oxygen present in the aluminum nitride forming liquid phases 
containing yttrium and oxygen, which, at the same time act as sintering 
aids in the pressureless sintering step. After cooling, these phases 
remain in the aluminum nitride sintered body as secondary phases at the 
grain boundaries of the aluminum nitride. The composition according to 
point F in the phase diagram, has the smallest amount of these secondary 
phases, and corresponds to 1.6 equivalent percent Y and 3.2 equivalent 
percent oxygen, which corresponds to 6.2% by weight of a YAlO.sub.3 
secondary phase, or, differently expressed, to 1.81% by weight oxygen and 
3.36% by weight Y in the AlN sintered body. As it can be seen from the 
examples, the highest value for thermal conductivity was 174 W/mK for an 
AlN sintered body containing Y.sub.2 O.sub.3 and Y.sub. 4 Al.sub.2 O.sub.9 
as secondary phases. 
As can be seen from the extensive prior art, it has not hitherto been 
possible to produce polycrystalline aluminum nitride bodies having a high 
density that do not contain substantial amounts of impurities such as 
oxygen, carbon and/or metals, which unfavorably affect the thermal 
conductivity by changing the lattice parameters of the aluminum nitride 
crystals and/or unfavorably affect resistance to high temperatures by 
inclusion of impurity containing phases at the grain boundaries of the 
aluminum nitride crystals. 
BRIEF SUMMARY OF THE INVENTION 
The problem is to provide molded bodies of polycrystalline aluminum nitride 
which are dense, substantially poreless and of high purity which have 
improved thermal and mechanical properties and therefore have a broad 
range of uses as construction materials in high-temperature machine 
construction and as substrate materials in high-efficiency electronics. In 
addition, a process whereby those molded bodies can be economically and 
reproducibly manufactured with the desired properties without use of 
sintering aids is required. 
According to the invention, the problem is solved by providing a dense 
substantially non-porous molded body of polycrystalline aluminum nitride 
having a density of at least 99.8% TD calculated on the theoretically 
possible density of pure aluminum nitride and consisting of 
at least 99% by weight aluminum nitride, 
up to 0.35% by weight residual oxygen, 
up to 0.35% by weight residual carbon and 
up to 0.30% by weight total of metallic impurities (Fe, Si, Ca, Mg) 
wherein the aluminum nitride is present essentially in the form of a 
single, homogeneous, isotropic microstructure with a grain size not larger 
than 5 .mu.m; the residual oxygen and residual carbon are present in the 
form of a solid solution in the AlN lattice and at an magnification of up 
to 2,400 times, they are not ceramically detectable as separate phase(s), 
having the following properties: a bending strength (measured according to 
the 4-point method) from room temperature to about 1400.degree. C. of at 
least 500 N/mm.sup.2, a predominantly transcrystalline rupture modulus and 
thermal conductivity at 300 K of at least 160 W/mK. 
The molded bodies according to the invention are prepared from porous 
deoxidized green bodies having a maximum density of 70% TD and consisting 
of 
at least 99% by weight aluminum nitride, 
up to 0.35% by weight residual oxygen, 
up to 0.35% by weight residual carbon and 
up to 0.30% by weight total of metallic impurities (Fe, Si, Ca, Mg) 
by isostatic hot pressing in a vacuum sealed casing at a temperature of 
from 1700.degree. to 2100.degree. C. and a pressure of from 100 to 400 MPa 
in a high-pressure zone using an inert gas as a pressure-transmitting 
agent. 
Since nothing can escape during the isostatic hot-pressing operation due to 
the gas tight casing present, the molded bodies according to the invention 
have at least 99.8% TD, preferably 100% TD, and the same chemical 
composition as the porous deoxidized green bodies with a maximum 70% TD. 
DETAILED DESCRIPTION OF THE INVENTION 
The molded bodies according to the invention, made of polycrystalline 
aluminum nitride, have a single phase microstructure in which the 
individual AlN grains having a maximum grain sizes of 5 .mu.m are 
distributed uniformly, that is, homogeneously and independently of 
direction. The residual oxygen and residual carbon are essentially in the 
form of a solid solution in the aluminum nitride lattice and cannot be 
detected as a separate phase or phases by x-ray diffraction or by 
ceramographic techniques at magnifications of up to 2400 fold. The 
metallic impurities, however, can be detectable in the form of particlate 
segregations in sizes of .ltoreq.0.5 .mu.m. 
For the production of the deoxidized green bodies, it is preferable to use 
as the AlN starting material, a powder having a maximum particule size of 
5 .mu.m, preferably 2 .mu.m, and an average particle size of &lt;1 .mu.m, 
preferably &lt;0.5 .mu.m, with a specific surface of from 4 to 10 m.sup.2 /g 
(measured according to BET) and a purity of at least 99.8%, preferably 
99.9%, calculated on the metallic impurities. Metallic impurities are to 
be understood to mean all metallic elements (essentially Fe, Si, Ca and 
Mg), with the exception of the aluminum present in bonded form, which can 
be present in AlN powders. 
The adherent carbon present in commercially available AlN powder can be 
tolerated to a maximum of 0.2% by weight. The residual oxygen which as a 
result of the known tendency of the finely divided AlN powder to hydrolyze 
(according to AlN+3H.sub.2 O .fwdarw.NH.sub.3 +Al(OH).sub.3) is present as 
the main impurity mostly in the form of the hydrolysis product 
Al(OH).sub.3, can be tolerated up to a maximum of 4.0% by weight. 
The aluminum nitride starting powders in admixture with small amounts of 
free carbon or a material which forms carbon on heating is compacted to 
form pre-molded green bodies and then subjected to a heat treatment, which 
is a purifying deoxidizing annealing at 1600.degree. to 1800.degree. C. to 
a nitrogen atmosphere, to form deoxidized green bodies with a maximum 
density not exceeding 70% TD. 
The carbon-containing admixture for the preparation of the deoxidized green 
bodies can be formed in any manner that ensures a uniform distribution of 
the carbon in the AlN-C mixture, for instance, by admixture of the AlN 
with particulate carbon black or colloidal graphite with a specific 
surface in the range of from 10 to 400 m.sup.2 /g. To obtain good pressing 
properties of the powder mixtures containing carbon black or colloidal 
graphite, it is preferable to use small amounts of a temporary binder such 
as camphor or stearic acid. The temporary binders are preferably used in 
amounts of up to a maximum of about 3% by weight calculated on the 
resulting mixture. The admixture containing carbon preferably contains an 
organic material that can be carbonized at temperatures of up to about 
1000.degree. C. forming carbon. Examples of preferred organic materials 
are condensation products of phenol formaldehyde of the Novolak and Resole 
type which are carbonized in the range of from 100.degree. to 900.degree. 
C. forming amorphous carbon in yields of 35 to 50%. 
For determining the amount of carbon to be admixed with the starting 
aluminum nitride powder, the free carbon present in the aluminum nitride 
starting powder has to be taken into consideration. The total amount of 
the free carbon present in the compacted powder mixture, after carbonizing 
the organic material if an organic material is used, is critical for 
carrying out the process and for obtaining the advantageous properties of 
the sintered bodies of the invention. It has been found that more carbon 
must be used than is stoichiometrically required for deoxidation of the 
oxygen impurities present in the aluminum nitride powder. As a calculation 
basis for determining the stoichiometrically required amount of carbon, 
the following chemical equation for the carbothermal reduction of aluminum 
hydroxide in a nitrogen atmosphere can be used. 
EQU 2 Al(OH).sub.3 +3 C+N.sub.2 .fwdarw.2 AlN+3 H.sub.2 O+3 CO. 
However, the value thus calculated is an approximate value since the oxygen 
in the aluminum nitride powder as a rule is not present completely as 
Al(OH).sub.3 but partly as physically or chemically absorbed water, as 
oxygen dissolved in the aluminum nitride lattice and as aluminum oxide 
(Al.sub.2 O.sub.3). 
The amount of the carbon admixed with the AlN powder is preferably used in 
an amount sufficient to lower the oxygen content in the preoxidized green 
body to less than 0.35% by weight residual oxygen but which at the same 
time does not increase the carbon content of the deoxidized green body to 
more than 0.35% by weight residual carbon. The addition of a insufficient 
amount of carbon results in deoxidized green bodies having more than 0.35% 
by weight oxygen; the addition of too large amounts of carbon results in 
deoxidized green bodies having more than 0.35% by weight carbon; in either 
case, the properties of the dense aluminum nitride molded bodies produced 
from the deoxidized green bodies are adversely effected. The optimal 
amount of the carbon admixed with aluminum nitride starting powder of 
specific grain fineness and a given content of oxygen and carbon can be 
easily ascertained by performing deoxidation tests. 
The process for producing deoxidized green bodies is as follows: 
The AlN powder is first homogeneously mixed with the carbon-containing 
material which is preferably obtained by dissolving a carbon containing 
organic material in a solvent and dispersing the AlN powder in the 
solution. When free carbon per se is used, the AlN powder together with 
the elementary carbon can be dispersed in a solution of a temporary 
binder. Useful organic solvents, include acetone and lower alcohols having 
1 to 6 C atoms. The dispersion can be carried out by stirring a dilute 
suspension in a plastic container using a stirrer or by kneading a viscous 
suspension in a kneading apparatus. The solvent can be removed, for 
instance, in the case of a dilute suspension, by spray drying or, in the 
case of a viscous suspension, by evaporation during the kneading 
operation. Generally, the dried material is milled in a jet mill, pin 
beater mill or ball mill to disintegrate agglomerates to ensure 
homogeneous distribution of the carbon-containing material in the 
admixture. 
The starting powder mixtures are compacted by molding to form pre-molded 
green bodies. The molding can be effected by means of conventionally known 
steps such as die pressing, isostatic pressing or slip casting. In case of 
die pressing or isostatic pressing, a pressure between 10 and 200 MPa, 
preferably 50 to 100 MPa is generally applied. 
The pre-molded green bodies are then subjected, according to the invention, 
to an deoxidation annealing under a nitrogen atmosphere at 1600.degree. to 
1800.degree. C. The indicated temperature range is critical for obtaining 
the desired properties of the final product. It has been shown that under 
equivalent conditions but at lower temperatures, insufficient deoxidation 
was obtained and the residual oxygen content was above 0.35% by weight 
whereas at higher temperatures especially 1900.degree. C., as a 
consequence of partial sintering, a noticeable grain enlargement occurred 
which is associated with a deterioration of the strength properties of the 
final product. 
The deoxidation annealing of the pre-molded green bodies can be carried out 
in any desired high-temperature apparatus such as a graphite tube 
resistance furnace (Tammann furnace) or in an inductively heated furnace 
with graphite suszepfor. For continuous operation, there can be 
advantageously used a horizontal pusher or band-type furnace in which the 
pre-molded green bodies are transported through the hot zone of the 
furnace in a manner such that they can each be held at the desired 
temperature for a predetermined period of time. The time intervals for 
heating up and dwelling at the final temperature are here dependent on the 
size of the pre-molded green bodies to be deoxidized. The pre-molded green 
bodies are conveniently accommodated in graphite containers and surrounded 
by coarse-grained aluminum nitride powder to prevent carburization from 
the graphite container. But the pre-molded green bodies are preferably 
loaded in containers of aluminum nitride without using the surrounding 
powder bed of aluminum nitride. Nitrogen, optionally mixed with carbon 
monoxide, is used as the gaseous atmosphere. The deoxidation is 
advantageously carried out in a flowing nitrogen atmosphere that is, under 
a nitrogen pressure of about 0.1 MPa; but it can also be carried out under 
reduced N.sub.2 pressure, a pressure of about 5000 Pa having proved 
especially satisfactory. The deoxidized green bodies obtained after the 
deoxidation annealing have, as a rule, a density of 55 to 65% Td, but in 
all cases .ltoreq.70% TD, that is, they are porous with open porosity 
whereby it is to be understood that they have canal pores which are 
intercommunicating and open to the surface of the article. 
According to the present invention, these deoxidized green bodies consist 
of at least 99% by weight AlN, with residual oxygen and residual carbon 
contents preferably of less than 0.35% by weight each and with unavoidable 
metallic impurities preferably amounting to less than 0.3% by weight. The 
metallic impurities present in the aluminum nitride powder are exclusively 
from the preparation and further processing of the powder since no 
metal-containing sintering aids are intentionally added to the carbon 
containing aluminum nitride powder mixtures. 
The deoxidized green bodies are used for the preparation of the 
substantially non-porous dense molded bodies of polycrystalline aluminum 
nitride. According to the invention, the deoxidized green bodies are 
isostatically hot pressed in an gastight casing at a temperature of from 
1700.degree. to 2100.degree. C. and a pressure of from 100 to 400 MPa in a 
high-pressure autoclave using an inert gas as a pressure transfer medium. 
For carying out the process of the invention, for preparation of the molded 
bodies, the deoxidized green bodies must be provided with a gastight 
casing before being introduced into the high-pressure zone so as to 
prevent the gas, used as the pressure-transmitting agent, to penetrate 
through the open pores into the body thereby hindering compression. 
The casing must be formed from materials that can be sealed gastight and 
which at the applied pressing temperatures in the range of from 
1700.degree. to 2100.degree. C. neither melt nor react with the deoxidized 
green bodies that is, they must remain inert in respect to the deoxidized 
green bodies. The casing must be sufficiently plastic at the pressing 
temperature used to adapt to the shape of the body without cracking to 
ensure that the gaseous pressure is uniformly transmitted via the casing 
to the body. 
Examples of suitable casing materials that meet the requirements include 
high-melting glasses such as pure quartz glass, high-melting ceramics or 
high-melting metals and metal alloys like molybdenum, tantalum or 
tungsten. These materials can be used in the form of pre-fabricated 
casings or capsules in which the deoxidized green bodies are introduced. 
The casings together with the contents are then evacuated and sealed 
gastight. The casings can also be formed on the deoxidized green bodies by 
direct coating, for instance, by electroless deposition of metals or by 
applying a glass-like or ceramic-like composition, which is then melted or 
sintered in vacuum to form the gastight casing. In addition, it is 
preferred to apply between the casing and the deoxidized green body to be 
compressed, an intermediate layer. The intermediate layer can comprise 
inert powders, fibers or felts, for example, graphite felts and/or boron 
nitride powder. In addition, bodies provided with a casing of high-melting 
glass can be embedded in a powder bed of fine particulate material that 
serves to reinforce the glass casing from the outside. The expression 
"vacuumtight sealed casing" is intended to refer to a casing which is 
impervious to the pressure gas acting from the outside and which contains 
no residual gases that disturb the compression operation. 
The deoxidized green bodies provided with gas-tight sealed casings are 
conveniently housed in graphite containers, then introduced in to the 
high-pressure zone and heated to the required compression temperature of 
at least about 1700.degree. C. It is convenient here to separately 
regulate pressure and temperature that is, to raise the gaseous pressure 
only when the casing material starts to plastically deform at the elevated 
temperature. Argon or nitrogen are preferably used as inert gases for the 
transmission of pressure. The gaseous pressure applied is preferable 
within the range of 150 to 250 MPa reached by slow intensification at the 
final temperature used which is preferably within the range of 
1800.degree. to 2000.degree. C. The optimal temperature within the range 
of 1700.degree. to 2100.degree. C. depends on the fineness and purity of 
the aluminum nitride starting powder used and on the chemical composition 
of the deoxidized green bodies. The maximum temperature of about 
2100.degree. C. should not be exceeded since there is danger that the 
non-porous molded bodies formed would acquire a "secondary recrystallized 
microstructure" that reduces the strength and is no longer homogeneous 
since some grains become larger than the rest. 
After lowering the pressure and temperature, the cooled bodies are removed 
from the high-pressure autoclave and the casing is removed from the dense 
body. The casing can be removed by milling or fusing the metal casing by 
sandblasting glass or ceramic casings or by chemical corrosion. 
The molded bodies thus produced are substantially non-porous with a density 
of at least 99.8% TD and are substantially texture-free as result of the 
uniform multidirectional action of pressure that is, they have an 
isotropic microstructure and their properties are not dependent on 
direction but are substantially the same in all directions. The bending 
strength used for characterizing the high-temperature strength is not 
unfavorably affected by secondary phases at the grain boundaries by 
sintering aids. Bending strength values have been obtained which are &gt;500 
N/mm.sup.2, preferably &gt;600 N/mm.sup.2 that are not substantially reduced 
up to 1400.degree. C. 
The absence of texture, the extremely fine-grained microstructure which is 
practically single phase with maximum grain size of 5 .mu.m, preferably 2 
.mu.m, and the occurrence of a transcrystalline fracture mode are 
responsible for the excellent mechanical strength properties. 
The fracture mode of the molded bodies is transcrystalline up to a 
temperature of about 1370.degree. C. It is thus ensured that the grain 
boundaries do not provide a defect area for reducing strength that is, 
under stress at elevated temperature sliding of the grains at the grain 
boundaries is suppressed so that the molded bodies have high strength 
under long-term stress and a high creeping resistance. As result of their 
purity and of the practically 100% theoretical density, the molded bodies 
possess an extraordinary electrical insulating capacity corresponding to a 
specific electrical resistance of 10.sup.14 ohm.times.cm together with 
excellent thermal conductivity of at least 150 W/mK, preferably &gt;200 W/mK. 
Accordingly, molded bodies of polycrystalline AlN of the invention have a 
better range of properties than those produced according to known 
processes of pressureless sintering or hot pressing with or without use of 
sintering aids. The process for production of molded bodies by hot 
isostatic pressing is not as limited with regard to molding possibility as 
conventional hot pressing. A high pressure autoclave can contain a large 
furnace zone where numerous encased samples of any desired shape can be 
simultaneously hot isostatically pressed. From the hot isostatically 
pressed AlN bodies, thin substrate wafers can be produced at reasonable 
cost by conventional machining methods such as with an inner-hole saw. In 
particular, the combination of the following properties: high thermal 
conductivity, high electrical insulating capacity, low expansion 
coefficient and high thermal-shock resistance, recommends the material 
according to the invention made of dense, pure AlN for use as a substrate 
in high-efficiency electronics as, for example, as small mounting plates 
for semi-conductor elements or electronic circuits. The low specific 
weight, the good resistance to high temperatures and good thermal 
conductivity also make possible, however, their use as structural 
materials in high-temperature machinery such as engine construction.

The process for producing the molded bodies according to the invention is 
explained in detail with reference to the examples that follow. The 
relative densities in % TD given in the specification and in the examples 
for the molded bodies have been calculated on the basis of the theoretical 
density of 3.26 g/cm.sup.3 of the aluminum nitride. 
EXAMPLE 1 
A technically pure AlN powder having a specific surface of 8.9 m.sup.2 /g 
was used as the starting material. The chemical analysis of this powder, 
which had a maximum particle size of 1 .mu.m, is disclosed in Table 1. A 
commercially available, powdery phenol formaldehyde resin of the Novolak 
type (for instance ALNOVOL.RTM. of the firm Hoechst AG) was used as the 
carbon-containing material. To each 100 parts by weight of the AlN powder, 
1.75 parts by weight of Novolak powder in the form of a solution in 
acetone were added and the viscous slurry was dried in air until the 
solvent had evaporated. The crumbly powder obtained after kneading was 
deglomerated by dry grinding in a jet mill and then isostatically 
compressed under a pressure of 100 MPa to cylinders 30.phi..times.50 mm 
(30 mm diameter.times.50 mm height). 
The cylindrical blanks were then annealed for two hours at 1800.degree. C. 
under a flowing nitrogen atmosphere under a gaseous pressure of 0.1 MPa in 
an AlN crucible that had been introduced in the hot zone of a graphite 
furnace of the Tammann type. The annealing was carried out according to 
the following temperature schedule: 
20.degree.-400.degree. C. : 60 min. 
400.degree.-1800.degree. C. : 120 min. 
kept at 1800.degree. C. : 120 min. At the end of the dwell period, the 
furnace was switched off and the deoxidized green bodies were cooled in 
the furnace to room temperature. The deoxidized green bodies had a green 
density of an average 66% TD, a residual oxygen content of 0.29% by weight 
and a residual carbon content of 0.23% by weight. As shown in Table 1 
which sets forth the analyses of the AlN starting powder and the 
deoxidized green body, practically no change occurs with regard to carbon 
content and metallic impurities whereas the oxygen content is drastically 
lowered from 1.80 to 0.29% by weight that is, above 80% based on the 
oxygen content of the starting powder. 
TABLE 1 
______________________________________ 
Analysis of the AlN sintering powder and 
of the green body deoxidized at 1800.degree. C. 
AlN sintering powder 
deox. green body 
Elements (% by weight) (% by weight) 
______________________________________ 
N 32.9 33.8 
O 1.80 0.29 
C 0.21 0.23 
Fe 0.139 0.210 
Si 0.029 0.031 
Ca 0.006 0.005 
Mg 0.003 0.004 
______________________________________ 
The deoxidized green bodies were then introduced into prefabricated quartz 
glass casings and the space between the inrer side of the casing and the 
green body filled with finely divided boron nitride powder. The casing 
together with the contents were then evacuated, heated to 1000.degree. C. 
in vacuum and sealed gastight by melting in an oxyhydrogen burner. The 
encased samples were then hot isostatically compressed at 1800.degree. C. 
in a high-pressure autoclave under an argon gaseous pressure of 200 MPa. 
The hot isostatic pressing was scheduled according to the following 
temperature/pressure program: 
20.degree.-800.degree. C./0.1 MPa : 60 min. 
800.degree.-1400.degree. C./0.1 MPa : 60 min. 
1400.degree.-1600.degree. C./0.1-125 MPa : 120 min. 
1600.degree.-1800.degree. C./125-200 MPa : 120 min. 
kept at 1800.degree. C./200 MPa : 60 min. 
1800.degree.-1350.degree. C./200 MPa : 60 min. 
1350.degree.-1250.degree. C./200-5 MPa : 30 min. 
After decompressing and cooling the molded bodies to room temperature, the 
samples were removed from the hot isostatic pressing equipment and the 
glass casings removed by crushing and sandblasting. The AlN molded bodies 
thus produced had a density totally of 3.26 g/cm.sup.3, which corresponds 
to 100% of the theoretical density. After carring out the density 
measurements and surface grinding, cylindrical test bodies measuring 
20.phi..times.28 mm and 20.phi..times.1 mm and prismatic small test bars 
2.times.4.times.34 mm for determining the thermal conductivity, the 
specific electric resistance and the bending strength were produced from 
the molded bodies. The thermal conductivity was determined according to 
the comparative rod method up to 927.degree. C. using Armco iron as the 
reference material. The thermal conductivity of the AlN samples in 
function of the testing temperature is given in Table 2. 
TABLE 2 
______________________________________ 
Testing temperature 
Heat conductivity 
(.degree.C.) (K) (W/mK) 
______________________________________ 
27 300 161 
177 450 101 
327 600 76 
477 750 61 
627 900 50 
777 1050 43 
927 1200 38 
______________________________________ 
A value of 10.sup.14 ohm.times.cm was obtained for the specific electric 
resistance which was measured at room temperature (25.degree. C.) with 
direct current according to the three-point measuring method. The bending 
strength of the test body was measured according to the four-point method 
using support distances of 15 mm (upper) and 30 mm (lower). The test bars, 
which break in a transgranular manner, had the following average bending 
strength value at room temperature (average value from 5 measurements): 
621 N/mm.sup.2. A value of 635 N/mm.sup.2 was obtained for the bending 
strength of 1370.degree. C. The microstructure of the samples was, 
according to results of x-ray and microstructural analyses, single phase 
with a maximum grain size of 2 .mu.m. 
EXAMPLES 2 AND 3 (FOR COMISON) 
Example 1 was repeated with the variation that at one time, no carbon was 
admixed with the AlN powder (Example 2) and one time, an excess of carbon 
was admixed with the AlN powder (Example 3). The green bodies in Example 2 
were pressed using 2% by weight camphor in the form of a solution in 
acetone as a temporary binder which was removed without residue during the 
heating operation in the deoxidation annealing step. Table 3 gives a 
characterization of the deoxidized green bodies and the properties of AlN 
bodies made therefrom by hot isostatic pressing in quartz glass casings, 
as indicated in Example 1. 
TABLE 3 
__________________________________________________________________________ 
Analyses results of the deoxidized green bodies and properties of hot 
isostatically pressed molded bodies made therefrom 
hot isostatically pressed molded body 
deoxidized green body .sigma..sub.B.sup.+++ 
Example 
density 
residual O 
residual C 
density 
.LAMBDA..sup.+ 
.rho..sup.++ 
(N/mm.sup.2) 
Rupture 
No. (% TD) 
(% wt) 
(% wt) 
(% TD) 
(W/mK) 
(.OMEGA. .multidot. cm) 
25.degree. C. 
1370.degree. C. 
mode 
__________________________________________________________________________ 
2 59 1.26 0.21 100 98 10.sup.11 
656 459 i 
3 66 0.18 0.62 100 122 10.sup.12 
550 581 t 
__________________________________________________________________________ 
.sup.+ .LAMBDA. . . . thermal conductivity measured at room temperature 
(25.degree. C.) 
i,t . . . Intercrystalline or transcrystalline 
.sup.++ .rho. . . . electrical resistivity measured at 25.degree. C. 
.sup.+++ .sigma..sub.B . . . bending strength measured at 25 and 
1370.degree. C. 
As seen from the data in Table 3, by hot isostatic pressing of encased 
porous AlN green bodies made from the AlN powder without addition of 
carbon for deoxidizing the oxygen impurities, AlN bodies according to the 
invention are not obtained. Although the bodies are compressed to 100% of 
the theoretical density of AlN and possess excellent strength at room 
temperature; as a result of their high content of residual oxygen (see 
Table 3, Example 2), the values of heat conductivity, specific electrical 
resistance and high-temperature strength are clearly inferior to those of 
Example 1. The fractured surfaces of the samples show both at room 
temperature and at 1370.degree. C. an intercrystalline rupture mode, 
which, together with the drastic drop of strength at 1370.degree. C., can 
be ascribed to the presence of an oxygencontaining grain boundary phase. 
It clearly appears from the results set forth in Table 3, for Example 3, 
that when a large excess of carbon is admixed with the AlN powder which in 
this case provided a carbon content of 0.62% by weight in the annealed 
green body, the AlN molded bodies according to the invention were likewise 
not obtained. Even though these bodies have low contents of residual 
oxygen and a transcrystalline rupture mode, the high levels of 
high-temperature and room-temperature strength, thermal conductivity and 
electrical insulating capacity, are not obtained in the dense AlN bodies. 
EXAMPLES 4-5 
Example 1 was repeated with the essential changes that follow: 
(1) a different AlN powder with regard to purity and particle size was 
used; 
(2) the carbon was in the form of elemental carbon; 
(3) the deoxidizing annealing was carried out under a nitrogen atmosphere 
with a pressure of 5000 Pa and a final temperature of 1400.degree. C. 
(Example 4) and 1700.degree. C. (Example 5), 
(4) the deoxidized green bodies were encased in vacuum sealed molybdenum 
capsules and finally 
(5) 2000.degree. C. was selected as the final temperature for the hot 
isostatic pressing. 
The chemical analysis of the AlN sintering powder, which had a specific 
surface of 5.6 m.sup.2 /g and a maximum particle size of 3 .mu.m, is shown 
in Table 4. Carbon black with a specific surface of 150 m.sup.2 /g in an 
amount corresponding to 0.63 g calculated on 100 g AlN powder was the 
elemental carbon used. To improve the pressability, in analogy to Example 
2, the AlN-C mixture was further processed with a camphor solution to form 
a powder for pressing. Table 4 discloses the analyses of the AlN sintering 
powder and the deoxidized green bodies annealed at 1400.degree. C. and 
1700.degree. C. The data in Table 4 shows that deoxidation annealing at 
1400.degree. C. (Example 4), contrary to Example 5, does not provide 
sufficient deoxidation. The deoxidation reaction stopped under the 
selected conditions with respective contents of residual oxygen and 
residual carbon above the required limit of 0.35% by weight. 
TABLE 4 
______________________________________ 
Analysis of the AlN sintering powder and of the green 
bodies deoxidized at 1400.degree. C. and at 1700.degree. C. 
Green bodies deox. at 
AlN sintering powder 
1400.degree. C. 
1700.degree. C. 
Elements 
(% weight) (% weight) 
______________________________________ 
N 33.1 n.d.* 34.0 
O 1.67 0.41 0.15 
C 0.11 0.39 0.12 
Fe 0.002 n.d. 0.075 
Si 0.010 n.d. 0.020 
Ca 0.002 n.d. 0.003 
Mg 0.001 n.d. 0.002 
______________________________________ 
n.d. = not determined 
The green bodies annealed at 1400.degree. and 1700.degree. C. were encased 
in vacuum-sealed molybedenum capsules by welding and after the hot 
isostatic pressing cycle which was carried out similarly as indicated in 
Example 1 but at a final temperature of 2000.degree. C., opened with the 
help of an electron welding apparatus. 
The hot isostatically pressed AlN bodies, after removing the bodies from 
the casing, were analysed with regard to their density, their 
microstructure and values for thermal conductivity and specific electric 
resistance, as already described in Example 1. The results of these 
measurements are set forth in Table 5. 
TABLE 5 
______________________________________ 
Heat con- 
spec. electr. 
ductivity 
resistance 
Grain size of 
Example 
Density at 25.degree. C. 
at 25.degree. C. 
microstructure 
No. (% TD) (W/mK) (ohm .multidot. cm) 
.mu.m 
______________________________________ 
4 100 90 10.sup.13 
&lt;3 
5 100 206 10.sup.14 
&lt;4 
______________________________________ 
Thus, according to Example 5, after practically complete deoxidation that 
is, by hot isostatic pressing of green bodies deoxidized down to contents 
of residual oxygen and residual carbon of .ltoreq.0.15% by weight, the 
thermal conductivity of non-porous polycrystalline AlN can be increased to 
more than 200 W/mK. The higher thermal conductivity obtained in the 
comparison of Example 1 can be attributed not only to the extremely low 
contents of non-metallic impurities (oxygen and carbon), but also to the 
degree of purity of the deoxidized green bodies of about 99.9% based on 
the metallic impurities. 
The influence of the content of residual oxygen and residual carbon of more 
than 0.35% on the heat conductivity and electric resistance is clearly 
shown from a comparison of Examples 4 and 5. Despite the use of a very 
pure AlN sintering powder having a purity of more than 99.9% based on the 
metal impurities, an AlN body of the invention was no longer obtained in 
Example 4. 
It is observed that pure AlN sintering powder can be compressed without 
using the hot isostatic pressing technique and without other additives 
which promote sintering according to the conventional (axial) hot pressing 
process with graphite dies which form non-porous AlN molded bodies having 
a density of 3.26 g/cm.sup.3, but with thermal conductivities below 100 
W/mK.