Aluminum nitride sintered body with high thermal conductivity and its preparation

An aluminum nitride sintered product with a high thermal conductivity (at least 100 W/m.K) can be prepared at a sintering temperature of less than 1850.degree. C. (often less than 1650.degree. C.) using a sinterable combination of aluminum nitride powder with at least three sintering aids. The sintering aids include a source of a rare earth metal oxide, a source of an alkaline earth metal oxide, a boron source and, optionally, a source of aluminum oxide. The sinterable combinations may also be used to prepare cofired, multilayer substrates.

This application is made under 35 United States Code section 371 of 
international application number PCT/US94/04256, filed Apr. 18, 1994. 
BACKGROUND OF THE INVENTION 
The invention relates to a sintered body of aluminum nitride (AIN) with 
high thermal conductivity and a process for preparing the sintered body at 
a relatively low sintering temperature. The invention relates more 
particularly to ternary sintering aid combinations that enable preparation 
of the sintered body at temperatures of 1850.degree. Centigrade 
(.degree.C.) or less, preferably 1650.degree. C. or less. The sintered 
body is suitable for use in a variety of known applications including 
integrated circuit substrates, integrated circuit heat sinks and packaging 
components or multichip module components. The sintered body may also be 
used in structural applications such as crucibles and components of armor. 
AIN is an excellent material having a high thermal conductivity, insulation 
resistance and a low thermal expansion coefficient among its desirable 
properties. However, since AIN is a covalent bonding compound, it is quite 
difficult to produce a pure AIN sintered product without using sintering 
aids or a hot-press sintering method. 
Sintered AIN bodies are typically prepared by heating an admixture of AIN 
powder and one or more sintering aids to a temperature within a range of 
from 1500.degree. C. to as high as 2100.degree. C. in an atmosphere that 
promotes sintering. The sintering aids typically include one or more 
oxides of alkaline earth metals or oxides of rare earth metals. Kasori et 
al. (U.S. Pat. No. 4,746,637) use a sintering aid combination of yttrium 
oxide (Y.sub.2 O.sub.3) and calcium oxide (CaO) to sinter AIN powder at a 
temperature of 1650.degree. C. or above. Other sintering aids may be used 
in place of, or in addition to, the alkaline earth metal oxides and rare 
earth metal oxides. Okuno et al. (U.S. Pat. No. 4,877,760) use at least 
one boride, carbide or nitride of titanium, zirconium, hafnium, vanadium, 
niobium or tantalum or boride or carbide of chromium, molybdenum or 
tungsten and, optionally, other sintering additives such as alkaline earth 
metal oxides and rare earth metal oxides. The sintering aids, including 
optional sintering aids, should not exceed 5 parts by weight per 100 parts 
by weight of AIN. 
JP H03-146471 discloses mixtures of at least one oxide of yttrium, scandium 
or a lanthanide and at least one of lanthanum hexaboride (LaB.sub.6), 
magnesium hexaboride and calcium hexaboride as sintering aids. JP 
H03-197366 discloses mixtures of calcium oxide and LaB.sub.6 as sintering 
aids. These combinations of sintering aids lead to AIN sintered products 
that show a high thermal conductivity, but prefer a sintering temperature 
of 1900.degree. C. or above. Such temperatures make it necessary to use an 
expensive high temperature sintering furnace and fittings, such as a 
setter, capable of use at those temperatures. In addition, such 
temperatures result in high energy costs. 
JP H04-130064 discloses sintering aids that are mixtures of a boron based 
compound such as boron nitride, boron carbide, boron oxide or boron 
fluoride, at least one oxide, carbide, nitride, or boride of titanium, 
chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, 
molybdenum, cadmium, tin or tungsten and at least one alkaline earth metal 
oxide or rare earth metal oxide. 
SUMMARY OF THE INVENTION 
A first aspect of the invention is a process for producing an aluminum 
nitride (AIN) sintered product with a high thermal conductivity at a 
relatively low sintering temperature. For this aspect, an AIN powder 
having a specific surface area in a range of about 3 to 8 m.sup.2 /g, and 
preferably 4.5 to 7.5 m.sup.2 /g, and an oxygen content between 0.5 and 
1.8 wt %, is used. Optimum amounts of sintering aids are combined with the 
AIN powder. The sintering aids substantially constitute a combination of 
three sintering aids (I), (II) and (III). Sintering aid (I) is at least 
one selected from the group consisting of rare earth oxides and rare earth 
compounds. The rare earth compounds are converted to corresponding rare 
earth oxides during sintering. Sintering aid (I) is incorporated such that 
an equivalent rare earth oxide amount thereof is in a range of 0.5 to 10 
wt %, based on weight of the AIN sintered product. Sintering aid (II) is 
at least one selected from the group consisting of alkaline earth oxides 
and alkaline earth compounds. The alkaline earth compounds are converted 
to corresponding alkaline earth oxides during sintering. Sintering aid 
(II) is incorporated such that an equivalent rare earth oxide amount 
thereof is in a range of 0.1 to 5 wt %, based on weight of the AIN 
sintered product. Sintering aid (III) is at least one selected from the 
group consisting of LaB.sub.6, NbC, and WB. An additive amount of 
LaB.sub.6 is in a range of 0.05 to 3 wt %, based on weight of the AIN 
sintered product. A resulting mixture is compacted to a desired shape, and 
then sintered in a non-oxidative atmosphere at a sintering temperature of 
1650.degree. C. or below to provide a sintered product with a high thermal 
conductivity of 120 W/m.K or more. 
A second aspect of the invention is a sinterable aluminum nitride powder 
composition comprising aluminum nitride powder and a sintering aid 
combination that consists essentially of a rare earth metal oxide source, 
an alkaline earth metal oxide source, a boron source selected from the 
group consisting of aluminum boride, aluminum diboride, calcium boride, 
yttrium boride, strontium boride, barium boride, cerium boride, 
praseodymium boride, samarium boride and neodymium boride, and, 
optionally, a source of aluminum oxide. 
A third aspect of the invention is a sintered aluminum nitride body having 
a high thermal conductivity and comprising, based upon body weight, from 
about 90 to about 99.5 weight percent aluminum nitride as a primary phase, 
from about 0.5 to about 10 weight percent of a secondary phase selected 
from the group consisting of alkaline earth metal aluminates, rare earth 
metal aluminates, alkaline earth metal-rare earth metal aluminates, 
complex alkaline earth metal-rare earth metal oxides and mixtures thereof, 
and boron at a level of from about 50 to about 5000, preferably from about 
50 to about 2000 parts by weight per million parts of body weight, as 
determined by secondary ion mass spectrometry (SIMS). The alkaline earth 
metal is preferably calcium and the rare earth metal is preferably 
yttrium. 
A fourth aspect of the invention is a process for preparing a sintered 
aluminum nitride body having a high thermal conductivity that comprises 
heating the sinterable composition of the second aspect to a temperature 
of from about 1570.degree. C. to about 1850.degree. C., preferably from 
about 1570.degree. C. to about 1650.degree. C. in a nonoxidizing 
atmosphere for a period of time sufficient to attain a density of at least 
95 percent of theoretical density. 
In an aspect related to the third and fourth aspects, the sintered aluminum 
nitride body can be a cofired, multilayer aluminum nitride substrate 
fabricated by a method comprising: 
a. preparing at least two ceramic green sheets from the sinterable 
composition of second aspect; 
b. depositing a desired pattern of a refractory metal ink on at least one 
major planar surface of at least one ceramic green sheet; 
c. preparing a laminate of a desired number of ceramic green sheets having 
refractory metal ink deposited thereon; 
d. heating the laminate under the conditions of the fourth aspect to effect 
sintering of both the ceramic sheets and the refractory metal ink 
deposited on said sheets. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
In the field of powder metallurgy, it has been already known that as an 
average particle size of a powder used for obtaining its sintered product 
decreases, the sintering temperature can generally be lowered. For 
example, an AIN powder having a specific surface area within a range of 
from 10 to 14 m.sup.2 /g is densely sintered at a temperature of 
1600.degree. C. or below. However, as the AIN powder becomes increasingly 
fine, that is, the specific surface thereof increases, oxygen content of 
the powder also increases. When such an AIN powder having a high oxygen 
content is sintered, the sintered product has a low thermal conductivity. 
For example, when AIN powder made by a conventional carbothermal reduction 
method has a specific surface area of more than 10 m.sup.2 /g, it also has 
an oxygen content of more than 1.8 wt %. Therefore, an AIN powder with a 
low oxygen content should be used for producing the AIN product of the 
first aspect. The AIN powder should also have an optimum surface area in 
order to be sintered at sintering temperatures of about 1650.degree. C. or 
below, and preferably less than 1625.degree. C. These temperatures are 
advantageous because a relatively inexpensive ceramic, such as aluminum 
oxide, can be used for fittings, such as a setter, that are placed in a 
furnace during sintering. A more expensive ceramic for such fittings is 
hexagonal boron nitride. The relatively low temperatures, compared to 
temperatures of 1800.degree. C. or more, lead to energy savings. 
With regard to the first aspect of the invention, it is preferred that the 
AIN powder have an oxygen content between 0.5 and 1.8 wt % and a specific 
surface area in a range of 3 to 8 m.sup.2 /g, more preferably 4.5 to 7.5 
m.sup.2 /g, is used for enhancing low temperature sintering thereof and 
improving thermal conductivity of a resulting AIN sintered product. When 
the oxygen content is more than 1.8 wt %, it becomes difficult to produce 
an AIN sintered product with a high thermal conductivity by sintering at a 
temperature of about 1650.degree. C. or below. On the other hand, it is 
difficult and expensive to produce an AIN powder having an oxygen content 
less than 0.5 wt %. When the specific surface area of the AIN powder is 
less than 3 m.sup.2 /g, the powder is not densified sufficiently at a 
sintering temperature of about 1650.degree. C. or below. An average 
particle size of an AIN powder suitable for use in this aspect of the 
invention is in a range of 0.20 micrometer (.mu.m) to 0.46 .mu.m. It is 
also preferred that the AIN powder be made by a carbothermal reduction 
method because AIN powder made by direct nitridation has an unstable 
aluminum oxide surface layer, so that it is possible to increase the 
oxygen content of the AIN powder during a process for producing the AIN 
sintered product. 
Sinterable compositions of the second aspect of the invention are prepared 
by adding a sintering aid combination to aluminum nitride powder. A 
sinterable composition is heated to a temperature of from 1570.degree. C. 
to about 1850.degree. C., preferably from about 1570.degree. C. to about 
1650.degree. C. in a nonoxidizing atmosphere for a period of time 
sufficient to yield a sintered aluminum nitride body having a density of 
at least 95 percent of theoretical density. The resulting sintered body 
contains boron at a level of from about 50 to about 5000, preferably from 
about 50 to about 2000 parts by weight per million parts of body weight, 
as determined by SIMS. 
AIN powder suitable for purposes of the invention may be of commercial or 
technical grade. It should not contain any impurities that would have a 
significant adverse effect upon desired properties of a resulting sintered 
product. Although some level of impurities is present in commercial 
powders, that level should be less than that which produces the 
aforementioned adverse effect. 
The AIN powder typically has a bound oxygen content of less than 4 wt %. 
The oxygen content is desirably less than 3 wt % and preferably less than 
2 wt %. 
The AIN powder also typically has a surface area, measured by a 
conventional adsorption method such as that taught by S. Brunauer, P. H. 
Emmett and E. Teller in Journal of the American Chemical Society, volume 
60, page 309 (1938) (hereinafter "BET"), of from 1.5 to 10 square meters 
per gram (m.sup.2 /g). The powder surface area is desirably from 2 to 9 
m.sup.2 /g. 
AIN powder meeting these specifications are preferably prepared either by 
carbothermal reduction of alumina (Al.sub.2 O.sub.3) or direct nitridation 
of aluminum metal. AIN powders may also be prepared by other processes 
using aluminum alkyls or aluminum halides. Preferred carbothermal AIN 
powders are available from "THE DOW CHEMICAL COMPANY" under the trade 
designation "XUS 35544" and "XUS 35548" or "TOKUYAMA SODA" under the trade 
designations "GRADE F" and "GRADE H". Mixtures of these and other powders 
may also be used. 
The sintering aid combination includes at least one alkaline earth metal 
oxide source, at least one rare earth metal oxide source, at least one 
boron source. The sintering aid combination may also include, as an 
optional component, at least one source of aluminum oxide (Al.sub.2 
O.sub.3). The alkaline earth metal oxide sources and rare earth metal 
oxide sources include the oxides as well as acetates, carbonates, 
nitrates, hydrides, phosphates, hydroxides, aluminates, formates, oxalates 
and sulfates. The sources of Al.sub.2 O.sub.3 include Al.sub.2 O.sub.3 
itself as well as aluminum acetate, aluminum hydroxide, aluminum butoxide, 
aluminum ethoxide, aluminum propoxide, aluminum oxalate, aluminum nitrate, 
aluminum phosphate and aluminum sulfate. 
Sintering aid (I) is at least one of rare earth metal oxides and rare earth 
metal sources or compounds. The rare earth metal sources are converted to 
their corresponding rare earth metal oxides, rare earth metal aluminates 
or both during sintering. The rare earth metal oxides include yttrium 
oxide (Y.sub.2 O.sub.3), and oxides of elements 57 through 71 of the 
Periodic Table of the Elements. The oxides of said elements are lanthanum 
(La.sub.2 O.sub.3), cerium (Ce.sub.2 O.sub.3), praseodymium (Pr.sub.2 
O.sub.3), neodymium (Nd.sub.2 O.sub.3), promethium (Pm.sub.2 O.sub.3), 
samarium (Sm.sub.2 O.sub.3), europium (Eu.sub.2 O.sub.3), gadolinium 
(Gd.sub.2 O.sub.3), terbium (Tb.sub.2 O.sub.3), dysprosium (Dy.sub.2 
O.sub.3), holmium (Ho.sub.2 O.sub.3), erbium (Er.sub.2 O.sub.3), thulium 
(Tm.sub.2 O.sub.3), ytterbium (Yb.sub.2 O.sub.3), and lutetium (Lu.sub.2 
O.sub.3). The rare earth metal oxide is desirably Y.sub.2 O.sub.3, 
La.sub.2 O.sub.3, Ce.sub.2 O.sub.2, Ce.sub.2 O.sub.3, Dy.sub.2 O.sub.3 or 
Sm.sub.2 O.sub.3, preferably La.sub.2 O.sub.3 or Y.sub.2 O.sub.3. The rare 
earth metal oxide source is desirably present in an amount sufficient to 
provide an equivalent rare earth metal oxide content within a range of 
from 0.25 to 10 wt % (0.5 to 10 wt % for the first aspect and 0.25 to 5 wt 
% for all other aspects of the invention), based upon sinterable 
composition weight. The range is preferably from about 0.7 to about 4, 
more preferably from about 1 to about 3 wt %, based upon sinterable 
composition weight. 
Sintering aid (II) is at least one of alkaline earth metal oxides and 
alkaline earth metal sources or compounds. The alkaline earth metal 
sources are converted to their corresponding alkaline earth metal oxides, 
alkaline earth metal aluminates or both during sintering. The alkaline 
earth metal oxides are magnesium oxide (MgO), calcium oxide (CaO), barium 
oxide (BaO) and strontium oxide (SrO). Although radium is also an alkaline 
earth metal, its radioactivity removes it from consideration as a suitable 
source of a sintering aid. The alkaline earth metal oxide is preferably 
CaO. Calcium carbonate (CaCO.sub.3) is a preferred source of CaO as it is 
more stable than CaO. The alkaline earth metal oxide source is desirably 
present in an amount sufficient to provide an equivalent alkaline earth 
metal oxide content within a range of from about 0.2 to 5 wt %, based upon 
sinterable composition weight. The range is quite satisfactory when 
sintering is conducted in a graphite furnace. The greater amount should be 
at least 0.5 wt %, based upon sinterable composition weight. The range is 
preferably from about 0.25 to about 3, more preferably from about 0.5 to 
about 2 wt %, based upon sinterable composition weight. If sintering is 
conducted in a refractory metal furnace, such as a tungsten furnace, a 
greater amount of alkaline earth metal oxide, especially calcium oxide, is 
required. 
Sintering aid (III), a source of boron, is suitably: tungsten boride (WB); 
a compound represented as MB.sub.2 where M is a metal selected from 
magnesium (Mg), aluminum (Al) scandium (Sc), yttrium (Y), zirconium (Zr), 
hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), 
molybdenum (Mo), tungsten (W), manganese (Mn), technetium (Tc), rhenium 
(Re), ruthenium (Ru), osmium (Os), uranium (U) and plutonium (Pu); a 
compound represented as M.sub.2 B.sub.5 where M is a metal selected from 
Mo and W; a compound represented as MB.sub.4 where M is a metal selected 
from Ca, Y, Mo and W; MB.sub.6 where M is a metal selected from Ca, 
strontium (Sr), barium (Ba), Y and Lanthanides; a compound represented as 
MB.sub.12 where M is a metal selected from Al, Sc, Y, Zr, an Actinide or a 
Lanthanide; a compound represented as MB.sub.66 where M is Y; or a boron 
rich metal oxide such as calcium borate or aluminum borate. Other boron 
sources should also provide satisfactory results. Sintering aid (III) is 
capable of enhancing low temperature sintering of AIN powder and improving 
thermal conductivity of resulting AIN sintered products. For the first 
aspect of the invention only, sintering aid (III) may be niobium carbide 
(NbC). The source of boron is desirably aluminum boride (AlB.sub.12), 
aluminum diboride (AlB.sub.2), calcium boride (CaB.sub.6), yttrium boride 
(YB.sub.6), lanthanum hexaboride (LaB.sub.6), strontium boride 
(SrB.sub.6), barium hexaboride (BaB.sub.6), cerium tetraboride 
(CeB.sub.4), cerium hexaboride (CeB.sub.6), praseodymium boride 
(PrB.sub.6), samarium boride (SmB.sub.6), WB or neodymium boride 
(NdB.sub.6). The source of boron is preferably AlB.sub.12, AlB.sub.2, WB, 
LaB.sub.6 or CaB.sub.6. The source of boron for all aspects of the 
invention other than the first aspect is desirably present in an amount 
sufficient to provide an equivalent boron content within a range of from 
0.01 to 1 wt %, based upon sinterable composition weight. The range is 
preferably from 0.02 to 0.5, more preferably from 0.04to 0.3 wt %, based 
upon sinterable composition weight. 
For the first aspect of the invention, an optimum amount of LaB.sub.6 is in 
a range of 0.05 to 3 wt %, based on sintered product weight. As the amount 
of LaB.sub.6 is increased within the range, the thermal conductivity of 
the AIN sintered product is remarkably improved. However, an amount of 
LaB.sub.6 in excess of 3 wt % inhibits sintering. Also with respect to the 
first aspect of the invention, an optimum amount of NbC or WB is in a 
range of 0.05 to 5 wt %, based on sintered product weight. As with 
LaB.sub.6, increasing amounts within the range improve thermal 
conductivity of the AIN sintered product. However, an amount of NbC or WB 
in excess of 5 wt % inhibits sintering. In addition, the amount of 
sintering aid (III) used in the first aspect closely relates to the 
specific surface area and oxygen content of the AIN powder. If the AIN 
powder has a relatively small specific surface area and a low oxygen 
content, it is expected that a small amount of sintering aid (III) within 
the range will be sufficient to convert the AIN powder into a sintered 
product with a high thermal conductivity. On the other hand, using an AIN 
powder with a relatively large specific surface area and a high oxygen 
content requires a large amount of sintering aid (III) within the range in 
order to improve thermal conductivity of the sintered product. For the 
first aspect of the invention, sintering aid (III) should have a purity of 
99.9% or more, and an average particle size of less than 10 .mu.m for 
uniformly incorporating the aid into AIN powder. 
Aluminum oxide (Al.sub.2 O.sub.3) may be added as a fourth component of the 
sintering aid combination. Preferred sources of Al.sub.2 O.sub.3 include 
Al.sub.2 O.sub.3 itself and aluminum hydroxide. When added, the source is 
desirably present in an amount sufficient to provide an equivalent 
Al.sub.2 O.sub.3 content within a range of from greater than zero wt % to 
2 wt %, based upon sinterable composition weight. The range is preferably 
from greater than zero wt % to 1 wt %, more preferably from greater than 
zero wt % to 0.5 wt %, based upon sinterable composition weight. 
The sintering aid combination is suitably admixed with AIN powder in an 
amount of from 0.05 wt % to 10 wt %, based upon sinterable composition 
weight. The amount is desirably from 0.5 wt % to 5 wt %, preferably from 
0.5 to 3 wt %, based upon sinterable composition weight. Each component of 
the sintering aid combination suitably has a surface area similar to that 
of the AIN powder. 
An admixture of AIN powder and the sintering aid(s) may be prepared by 
conventional procedures such as attrition milling and wet and dry ball 
milling. Wet ball milling with an appropriate solvent and suitable milling 
media provides satisfactory results. Milling media, usually in the form of 
cylinders or balls, should have no significant adverse effect upon 
admixture components or upon sintered bodies prepared from the admixture. 
A solvent such as ethanol, heptane or another organic liquid may be used. 
A suitable solvent is a blend of ethanol and chlorothene. After milling, 
the organic liquid may be removed by conventional procedures to yield an 
admixture suitable for conversion to ceramic greenware. Oven drying and 
spray drying produce satisfactory results. 
An organic binder may be added during milling of the admixture. Suitable 
binders are well known in the art and typically comprise high molecular 
weight organic materials that are soluble in organic solvents. 
Illustrative binders include polyethyloxazoline, industrial waxes such as 
paraffin, highly viscous polyglycols, polymethylmethacrylate and polyvinyl 
butyral. A blend of polyethyloxazoline in an amount of from 20 to 80, 
preferably from 35 to 65, wt % and polyethylene glycol in an amount of 
from 80 to 20, preferably from 65 to 35, wt %, based upon blend weight 
wherein the amounts total 100 percent, is particularly suitable. The 
binder is suitably added to admixture components prior to milling. 
Any well known dispersing aid or dispersant may also be added during 
milling of the admixture. Fish oil is a particularly suitable dispersant. 
Ceramic greenware may be prepared by any one of several conventional 
procedures such as extrusion, injection molding, die pressing, isostatic 
pressing, slip casting, roll compaction or forming or tape casting to 
produce a desired shape. Particularly satisfactory results are obtained by 
dry pressing an admixture (preferably spray dried) or tape casting a 
slurry. 
The ceramic greenware is desirably subjected to conditions sufficient to 
remove the organic binder prior to sintering. Binder removal, also known 
as binder burn out, typically occurs by heating the greenware to a 
temperature that ranges from 50.degree. C. to 1000.degree. C. to pyrolyze, 
or thermally decompose, the binder. A suitable time and temperature 
combination for removing the blend polyethyloxazoline and polyethylene 
glycol is from 1 to 7 hours at a temperature of from 400.degree. to 
800.degree. Centigrade (.degree.C.), with four hours at a temperature of 
575.degree. C. being particularly suitable. The temperature and hold time 
vary depending upon the binder. and dimensions of the greenware. Thermal 
decomposition may be carried out at or near ambient pressure or in a 
vacuum. It may be carried out in the presence of atmospheric air or in a 
neutral atmosphere. The neutral atmosphere is desirably established with 
at least one gas selected from nitrogen and a noble gases such as argon. 
The gas is preferably nitrogen. As a general rule, binder burn out in the 
presence of an inert gas such as nitrogen yields a higher residual carbon 
level than binder burn out in the presence of atmospheric air. Binder 
burnout in the presence of nitrogen is preferred for purposes of the 
present invention. 
Conventional procedures may be used to prepare a cofired multilayer 
aluminum nitride substrate. The procedures include using greenware in the 
form of sheets, depositing a conventional refractory metal ink or paste on 
at least one major planar surface of at least one greenware sheet, forming 
a laminate of a desired number of ceramic green sheets having refractory 
metal ink deposited thereon, and sintering the laminate to effect 
sintering of both the ceramic sheets and the refractory metal ink 
deposited on said sheets. Prior to sintering, the laminate may be 
subjected to a binder burnout step. A suitable refractory metal ink is a 
tungsten ink ("CRYSTALEKO, #2003 ink"). 
Sintering of the greenware, after binder burnout, occurs in a nonoxidizing 
atmosphere established by gaseous nitrogen or a source of gaseous nitrogen 
and is followed by cooling in a vacuum or in a neutral atmosphere like 
that used for thermal debindering. The source of gaseous nitrogen may be 
gaseous nitrogen, gaseous ammonia, gaseous mixtures of nitrogen and 
ammonia, gaseous mixtures of nitrogen, ammonia or both with an inert or 
noble gas such as argon, or gaseous mixtures of nitrogen, ammonia or both 
with hydrogen and, optionally, an inert or noble gas. A favorable 
sintering atmosphere may be established by placing the greenware into a 
crucible fabricated from a refractory material, such as boron nitride, 
aluminum nitride, molybdenum metal or tungsten metal, prior to sintering 
and cooling. The greenware may also be placed on a setter within the 
crucible. The setter is preferably fabricated from the same material as 
the crucible. The refractory material will vary depending upon which type 
of furnace is used for sintering. Boron nitride and aluminum nitride are 
preferred refractory materials for a graphite furnace, whereas molybdenum 
metal or tungsten metal is preferred for a tungsten furnace. 
Sintering desirably occurs at a temperature of from about 1570.degree. to 
about 1850.degree. C. (1650.degree. C. or below for the first aspect of 
the invention). In accordance with the first aspect, when using 
intermediate surface area (4.5to 7.5 m.sup.2 /g) AIN powders and LaB.sub.6 
as sintering aid (III), the temperature is preferably in a range of 
1570.degree. C. to 1640.degree. C., more preferably 1570.degree. C. to 
1625.degree. C. For all aspects of the invention and any other sintering 
aid (III), including WB or NbC suitable for use in the first aspect of the 
invention, the temperature is preferably from about 1570.degree. to about 
1650.degree. C., more preferably from about 1600.degree. to about 
1650.degree. C. The sintering temperature is maintained for a period of 
time sufficient to attain a density of at least 95, preferably at least 
97, percent of theoretical density. The period of time is desirably from 
0.5 hour to 24 hours, preferably from 2 to 10 hours, and more preferably 6 
hours. If the sintering time is less than 0.5 hour, the density will be 
less than 95 percent of theoretical density unless the sintering 
temperature is raised to 1700.degree. C. or higher. Although this may be 
done, it disregards any economic or physical property advantage resulting 
from sintering at lower temperatures. 
During sintering, sintering aids (I), (II) and (III) react with a surface 
layer of aluminum oxide on the AIN powder to generate a complex oxide with 
a relatively low melting point. The complex oxide is presumably capable of 
removing oxygen atoms from grain boundaries of the sintered AIN bodies. 
The removal of oxygen atoms improves thermal conductivity of the sintered 
AIN bodies. 
Sintered AIN bodies of the invention have a thermal conductivity in excess 
of 100 watts per meter-Kelvin (W/m.K). The thermal conductivity is 
desirably greater than 120 W/m.K, preferably greater than 140 W/m.K and 
more preferably greater than 150 W/m K. A theoretical maximum thermal 
conductivity for single crystal AIN is 319 W/m.K. An upper limit for 
thermal conductivity is therefore 319 W/m.K. An acceptable upper thermal 
conductivity limit for many practical applications is 230 W/m.K. 
Sintered AIN bodies of the invention also have a density of at least 95 
percent of theoretical density display color/translucency combinations 
that range from light cream and translucent to dark gray or even black and 
opaque. Skilled artisans can attain a desired combination of color and 
thermal conductivity without undue experimentation. 
Sintered AIN bodies of the invention comprise, based upon body weight, from 
90 to 99.5 weight percent aluminum nitride as a primary phase; from 0.5 to 
10 weight percent of a secondary phase, and boron at a level of from 50 to 
5000, preferably from 50 to 2000 parts by weight per million parts of body 
weight, as determined by secondary ion mass spectrometry. The boron is 
present as a boron derivative that is a secondary phase or is dispersed 
throughout the secondary phase(s), or is dispersed on an atomic level 
within aluminum nitride's crystal lattice or a combination thereof. The 
secondary phase is at least one material selected from yttrium aluminates, 
calcium-yttrium aluminates, complex calcium-yttrium oxides and mixtures 
thereof. The secondary phase may also include an amount of calcium 
aluminate. Specific examples of secondary phase materials include Al.sub.2 
Y.sub.4 O.sub.9, AlYO.sub.3, Al.sub.5 Y.sub.3 O.sub.12, CaYAlO.sub.4, 
CaY.sub.2 O .sub.4 and mixtures thereof. 
The following examples are solely for purposes of illustration and are not 
to be construed, by implication or otherwise, as limiting the scope of the 
present invention.

EXAMPLE 1 
Ball mill an AIN powder (1.25 wt % oxygen, specific surface area of 5.0 
m.sup.2 /g), sintering aids and isopropyl alcohol as a solvent to prepare 
a mixed powder. The sintering aids are Y.sub.2 O.sub.3 as (I), CaCO.sub.3 
as (II) and LaB.sub.6 as (III). The amounts of Y.sub.2 O.sub.3, CaCO.sub.3 
and LaB.sub.6 are 2.0 wt %, 0.89 wt % and 0.1 wt %, respectively. An 
equivalent CaO amount of CaCO.sub.3 is 0.5 wt %. The mixed powder is 
compacted under a pressure of 1.5 ton/cm.sup.2 (1361 kilogram 
(kg)/cm.sup.2) with a rubber press to a disc having a diameter of 20 mm 
and a height of 10 mm. The disc is set in a boron nitride setter and 
sintered for 4 hours at a temperature of 1600.degree. C. in a 
non-oxidizing atmosphere including nitrogen gas to obtain a sintered 
product. 
EXAMPLES 2-25 
Repeat Example 1, but use AIN powders, sintering temperatures and sintering 
aid amounts as shown in Table I to obtain sintered products. Each sintered 
product is ground and polished to provide an AIN sintered disc having a 
diameter of 10 mm and a thickness of 3 mm. Each disc is then measured for 
relative density (% of theoretical density) and thermal conductivity 
(laser flash method). Results of the measurements are also shown in Table 
I. 
TABLE I 
__________________________________________________________________________ 
AlN 
Properties 
Specific Density 
Thermal 
Exam- 
Surface 
Oxygen 
Sinter 
Sintering Aids (wt %) 
(% Conduc- 
ple Area 
Content 
Temp. CaCO.sub.3 
Theor- 
tivity 
No. (m.sup.2 /g) 
(wt %) 
(.degree.C.) 
Y.sub.2 O.sub.3 
(CaO)** 
LaB.sub.6 
etical) 
(W/m .multidot. K) 
__________________________________________________________________________ 
1 5.0 1.25 
1600 0.89 (0.5) 
0.1 
99.5 
135 
2 5.0 1.25 
1600 
2.0 
0.89 (0.5) 
0.2 
99.0 
137 
3 5.0 1.25 
1600 
2.0 
0.89 (0.5) 
0.3 
99.0 
140 
4 5.0 1.25 
1600 
2.0 
0.89 (0.5) 
0.4 
98.5 
145 
5 5.0 1.25 
1620 
2.0 
0.89 (0.5) 
0.5 
98.3 
160 
6 6.5 1.40 
1580 
2.0 
0.71 (0.4) 
0.6 
98.3 
130 
7 8.0 1.75 
1570 
3.0 
0.89 (0.5) 
0.5 
98.5 
120 
8 5.5 1.32 
1600 
5.0 
1.79 (1.0) 
0.8 
98.0 
135 
9 7.5 1.60 
1600 
7.0 
1.79 (1.0) 
1.0 
99.1 
130 
10 4.6 1.15 
1600 
3.0 
3.57 (2.0) 
0.5 
98.0 
140 
11 6.0 1.3 1620 
3.0# 
0.89 (0.5) 
2.0 
99.5 
140 
12 3.5 1.0 1600 
2.0 
1.79 (1.0) 
1.0 
93.5 
106 
13 3.5 1.0 1650 
2.0 
1.79 (1.0) 
1.0 
98.0 
125 
14 3.6 1.0 1600 
1.4 
2.5 (1.4) 
0.5 
92.0 
100 
15 3.6 1.0 1650 
1.4 
2.5 (1.4) 
0.5 
98.0 
127 
16 3.6 1.0 1700 
1.4 
2.5 (1.4) 
0.5 
98.5 
140 
17* 
4.5 1.10 
1570 
2.0 
0.89 (0.5) 
0.5 
89.0 
100 
18 4.5 1.10 
1600 
2.0 
0.89 (0.5) 
0.5 
98.5 
145 
19 4.5 1.10 
1650 
2.0 
0.89 (0.5) 
0.5 
99.5 
150 
20 4.5 1.10 
1700 
2.0 
0.89 (0.5) 
0.5 
99.5 
155 
21* 
7.5 1.60 
1550 
2.0 
0.89 (0.5) 
0.5 
86.0 
75 
22 7.5 1.60 
1570 
2.0 
0.89 (0.5) 
0.5 
99.5 
120 
23 7.5 1.60 
1600 
2.0 
0.89 (0.5) 
0.5 
99.5 
135 
24 7.5 1.60 
1650 
2.0 
0.89 (0.5) 
0.5 
99.5 
140 
25 7.5 1.60 
1700 
2.0 
0.89 (0.5) 
0.5 
99.5 
145 
__________________________________________________________________________ 
*means not an example of the invention 
**Equivalent CaO amount of CaCO.sub.3 
# means 2 wt % Y.sub.2 O.sub.3 and 1 wt % of La.sub.2 O.sub.3 
The results in Table I show that a combination of Y.sub.2 O.sub.3, CaO and 
LaB.sub.6 provides acceptable AIN sintered bodies at temperatures as low 
as 1570.degree. C. with AIN powders having a variety of oxygen contents 
and specific surface areas. Acceptable bodies have a relative density of 
at least 90% and a thermal conductivity of at least 100 W/m.K. The results 
also show that while a given sinterable composition may provide 
unacceptable results in terms of relative density, thermal conductivity or 
both at a particular temperature, a small increase in temperature produces 
acceptable results. Examples 17* and 18 as well as 21* and 22 illustrate 
this point. 
EXAMPLES 26-35 
Repeat Example 1, but change at least one of the AIN powder, the sintering 
temperature and the sintering aids as shown in Table II. Table II also 
shows thermal conductivity measurements for resulting sintered bodies. All 
sintered bodies have a relative density of greater than 98% except example 
37 which has a relative density of 94.0. 
TABLE II 
__________________________________________________________________________ 
AlN 
Properites 
Specific 
Surface Oxygen 
Sinter 
Sintering Aids (wt %) 
Thermal 
Example 
Area 
Content 
Temp. CaCO.sub.3 Conductivity 
No. (m.sup.2 /g) 
(wt %) 
(.degree.C.) 
Y.sub.2 O.sub.3 
(CaO)** 
NbC 
WB (W/m .multidot. K) 
__________________________________________________________________________ 
26 5.3 1.29 
1600 
2.0 
1.79 (1.0) 
0.5 
0.0 
137 
27 4.0 1.00 
1625 
2.0 
1.79 (1.0) 
0.1 
0.0 
140 
28 7.5 1.60 
1580 
3.0 
0.89 (0.5) 
3.5 
0.0 
121 
29 6.0 1.35 
1600 
2.0 
0.89 (0.5) 
0.5 
0.0 
133 
30 4.5 1.10 
1600 
1.0 
0.89 (0.5) 
0.5 
0.0 
147 
31 9.0 2.10 
1600 
2.0 
1.79 (1.0) 
3.5 
0.0 
111 
32 5.3 1.29 
1600 
2.0 
1.79 (1.0) 
0.0 
0.5 
135 
33 4.0 1.00 
1640 
2.0 
1.79 (1.0) 
0.0 
0.1 
142 
34 7.5 1.60 
1580 
3.0 
0.89 (0.5) 
0.0 
3.5 
120 
35 6.0 1.35 
1600 
2.0 
0.89 (0.5) 
0.0 
0.5 
130 
36 4.5 1.10 
1600 
1.0 
0.89 (0.5) 
0.0 
0.5 
141 
37 3.3 0.80 
1625 
2.0 
1.79 (1.0) 
0.0 
0.5 
104 
38 9.0 2.10 
1600 
2.0 
1.79 (1.0) 
0.0 
3.5 
109 
__________________________________________________________________________ 
**Equivalent CaO amount of CaCO.sub.3 
The data in Table II show that NbC and WB are acceptable substitutes for 
LAB.sub.6. 
EXAMPLE 39 
Ball mill 100 grams (g) AIN powder ("THE DOW CHEMICAL COMPANY" as "XUS 
35544", oxygen content of 1.1.+-.0.1 wt %, carbon content of less than 
0.08 wt %, both percentages being based on powder weight, and a surface 
area of 3.2.+-.0.2 m.sup.2 /g), 2 g Y.sub.2 O.sub.3 powder ("UNOCAL 
MOLYCORP" 99.99% purity), 0.9 g CaCO.sub.3 powder ("FISHER SCIENTIFIC"), 
0.25 g AlB.sub.2 ("ALDRICH CHEMICAL CO. INC.") and 3.1 g of a binder 
composition in 60.5 g of a solvent blend. The binder composition is a 
35/65 weight ratio blend of polyethyloxazoline and polyethylene glycol 
3350 ("THE DOW CHEMICAL COMPANY"). The solvent blend is a 50/50 (by 
volume) blend of ethanol and chlorothene. The binder is dissolved in the 
solvent blend before the AIN, Y.sub.2 O.sub.3, AlB.sub.2 and CaCO.sub.3 
powders are added. Ball milling continues for a period of 4 hours to 
provide a milled slip. Solids contained in the milled slip are separated 
from most of the solvent blend using a rotary evaporator. Remaining 
solvent blend removal occurs via drying under vacuum at a temperature of 
60.degree. C. for a period of 15 hours. After drying is complete, the 
solids are crushed and screened through a 60 mesh (250 .mu.m sieve 
opening) sieve to provide a dried powder. 
The dried powder is dry pressed into greenware using a 7/8 inch (2.2 cm) 
round die under uniaxial pressure of 15,000 pounds per square inch (psi) 
(about 103 megapascals (MPa)). The binder composition is removed from the 
greenware in the presence of flowing nitrogen (N.sub.2 BBO). Binder 
removal employs a heating rate of 90.degree. C./hour up to 575.degree. C., 
a four hour hold at that temperature and a cooling rate of 3.degree. C./min 
down to room temperature (25.degree. C.). 
After binder removal is complete, the greenware is enclosed in a boron 
nitride setter that is placed in a boron nitride crucible to establish a 
neutral environment. The crucible is placed in a graphite furnace (one 
cubic foot (0.028 cubic meter) capacity, "THERMAL TECHNOLOGY MODEL" 
121212G). The crucible and its contents are heated, in the presence of 
nitrogen flowing at a rate of 2 standard cubic feet per hour (scfh) (about 
0.057 standard cubic meters per hour (scmh)) using a heating schedule that 
starts with heating to a temperature of 1200.degree. C. at a rate of 
25.degree. C./min, held at 1200.degree. C. for 30 minutes to ensure 
conversion of CaCO.sub.3 to CaO), heated to 1625.degree. C. at a rate of 
10.degree. C./min, held at 1625.degree. C. for 6 hours and then cooled at 
a rate of 25.degree. C./min down to 1000.degree. C. The crucible contents, 
now sintered parts, are opaque, gray in color, with a smooth surface 
finish. The sintered parts have a density of 3.20 g/cm.sup.3 (greater than 
97% of theoretical density) and a thermal conductivity (laser flash method) 
of 157 W/m.K. X-ray diffraction (XRD) analysis of the sintered parts 
reveals yttrium aluminate (Al.sub.2 Y.sub.4 O.sub.9) and calcium-yttrium 
aluminate (CaYAlO.sub.4) as secondary phases. Boron K.sub..alpha. X-ray 
mapping using a microprobe shows a generally uniform distribution of boron 
containing phases. Analytical transmission electron microscopy (ATEM) shows 
that these phases are boron nitride. 
Repeating this example, but without the AlB.sub.2, leads to a lower 
density, a lower thermal conductivity, and a Secondary phase chemistry 
that is predominantly Al.sub.2 Y.sub.4 O.sub.9. 
EXAMPLES 40-56 
Repeat Example 39 using the formulations shown in Table III. The CaO 
amounts are equivalents resulting from higher amounts of CaCO.sub.3. For 
example, about 0.9 wt % CaCO.sub.3 results in 0.5 wt % CaO. 
TABLE III 
______________________________________ 
Exam- Density Thermal 
Ple Y.sub.2 O.sub.3 
CaO AlB.sub.2 
Density 
(% Conductivity 
No. (g) (g) (g) (g/cm.sup.3) 
Theoretical) 
(W/m .multidot. K) 
______________________________________ 
40* 2.475 0.275 0 2.750 83.3 127 
41* 2 0.5 0 2.834 86.1 133 
42* 1.375 1.375 0 3.025 92.1 133 
43* 0.55 2.2 0 2.953 90.4 122 
44* 0.275 2.475 0 2.922 89.6 115 
45 2.25 0.25 0.25 3.159 95.9 151 
46 2 0.5 0.05 3.189 96.8 148 
47 2 0.5 0.25 3.205 97.4 152 
48 1.75 0.75 0.25 3.159 96.1 150 
49 1.25 1.25 0.25 3.112 94.9 140 
50 0.5 2 0.25 3.004 92.0 126 
51 1.375 0.275 1.1 3.081 94.1 138 
52 0.825 0.825 1.1 3.120 95.4 141 
53 0.275 1.375 1.1 3.003 92.2 124 
54 0.275 0.275 2.2 3.060 94.2 114 
55 1.875 0.625 0.25 3.204 97.3 148 
56* 1.875 0.625 0 2.829 86.0 -- 
______________________________________ 
*means not an example of the invention 
-- means not measured 
The data in Table III, especially Example 55 and 56* and 41*, 46 and 47, 
demonstrate that a ternary sintering aid composition (Y.sub.2 O.sub.3, CaO 
and AlB.sub.2) leads to a combination of density and thermal conductivity 
that exceeds the combination attainable with similar amounts of a binary 
sintering aid composition (Y.sub.2 O.sub.3 and CaO). The data also 
demonstrate that a low rare earth metal oxide content, as in Examples 53 
and 54, leads to a lower thermal conductivity than a greater rare earth 
metal oxide content as in Example 51. 
EXAMPLES 57-73 
Repeat Examples 40-56 at a sintering temperature of 1650.degree. C. using a 
tungsten furnace and enclosing the greenware in molybdenum-tungsten setters 
rather than boron nitride setters. Data similar to that of Table III are 
shown in Table IV. 
TABLE IV 
______________________________________ 
Exam- Density Thermal 
Ple Y.sub.2 O.sub.3 
CaO AlB.sub.2 
Density 
(% Conductivity 
No. (g) (g) (g) (g/cm.sup.3) 
Theoretical) 
(W/m .multidot. K) 
______________________________________ 
57* 2.475 0.275 0 2.886 87.4 113 
58* 2 0.5 0 2.974 90.3 122 
59* 1.375 1.375 0 3.074 93.6 119 
60* 0.55 2.2 0 2.866 87.7 106 
61* 0.275 2.475 0 2.840 87.0 98 
62* 2.25 0.25 0.25 2.714 82.3 122 
63 2 0.5 0.05 3.139 95.3 142 
64 2 0.5 0.25 3.190 96.9 149 
65 1.75 0.75 0.24 3.181 96.8 149 
66 1.25 1.25 0.25 3.135 95.6 138 
67 0.5 2 0.25 3.019 92.4 122 
68* 1.375 0.275 1.1 2.665 81.4 117 
69 0.825 0.825 1.1 3.127 95.8 148 
70 0.275 1.375 1.1 3.006 92.3 121 
71* 0.275 0.275 2.2 2.466 75.9 80 
72 1.875 0.625 0.25 3.190 96.9 152 
73* 1.875 0.625 0 2.466 86.8 115 
______________________________________ 
*means not an example of the invention 
The data in Table IV, especially Example 72 in comparison with Example 73* 
and Example 58* in comparison with Examples 63 and 64, verify the 
observation made following Table III in that the ternary sintering aid 
composition including a boron source provides better results than a binary 
sintering aid composition lacking a boron source. A comparison of Examples 
45, 62*, 63, 68* and 71* shows that a metal refractory furnace may require 
an added amount of alkaline earth metal oxide in order to obtain 
performance equivalent to a graphite furnace using a boron nitride 
crucible. Example 71 shows that, at the constant total sintering aid level 
used in these examples, an excessive amount of AlB.sub.2 relative to other 
components of the sintering aid composition leads to unacceptably low 
levels of density and thermal conductivity. 
EXAMPLES 74-76 
Repeat Example 55, but change the AIN powder and vary the temperature as 
shown in Table V. Examples 74-76 therefore contain 1.875 wt % Y.sub.2 
O.sub.3, 0.625 wt % CaO and 0.25 wt % AlB.sub.2. The AIN powder has a 
surface area of 3.8 m.sup.2 /g, an oxygen content of 1.35 wt % and a 
carbon content of 0.09 wt % (experimental powder designated 6419R, "THE 
DOW CHEMCIAL COMPANY"). The density and thermal conductivity data are 
shown in Table V. 
TABLE V 
______________________________________ 
Exam- Density Thermal 
Ple Temperature 
Density (% Conductivity 
No. (.degree.C.) 
(g/cm.sup.3) 
Theoretical) 
(W/m .multidot. K) 
______________________________________ 
74 1625 3.203 97.3 135 
75 1600 3.205 97.4 128 
76 1575 3.149 95.7 119 
______________________________________ 
EXAMPLES 77-78 
Repeat Example 72, but vary the temperature as shown in Table VI and use 
the AIN powder of Examples 74-76. The density and thermal conductivity 
data are shown in Table VI. 
TABLE VI 
______________________________________ 
Exam- Density Thermal 
Ple Temperature 
Density (% Conductivity 
No. (.degree.C.) 
(g/cm.sup.3) 
Theoretical) 
(W/m .multidot. K) 
______________________________________ 
77 1650 3.215 97.7 136 
78 1625 3.194 97.0 132 
______________________________________ 
The data in Tables V and VI show that an increase in AIN surface area 
improves sinterability as determined by maintaining acceptable density at 
lower temperatures than those used in Examples 55 and 72. The data also 
show that an increase in oxygen content over that used in Examples 55 and 
72 causes a minor decrease in thermal conductivity, but may also 
contribute to enhanced densification. 
EXAMPLES 79-95 
Repeat Example 39, but use a different lot of the AIN powder used in 
Example 39, change the furnace to the tungsten used in Examples 57-73, 
increase the sintering temperature to 1650.degree. C. and, for some 
examples, add an amount of Al.sub.2 O.sub.3. The AIN powder has a surface 
area of 3.43 m.sup.2 /g, an oxygen content of 1.16 wt % and a carbon 
content of 0.07 wt %. The component amounts, density and thermal 
conductivity data are shown in Table VII. 
TABLE VII 
______________________________________ 
Ex- Density 
Thermal 
am- (% Conduc- 
Ple Y.sub.2 O.sub.3 
CaO AlB.sub.2 
Al.sub.2 O.sub.3 
Density 
Theor- 
tivity 
No. (g) (g) (g) (g) (g/cm.sup.3) 
etical) 
(W/m .multidot. K) 
______________________________________ 
79 1.875 0.625 0.150 
0 3.180 96.6 155 
80 2.344 0.781 0.188 
0 3.183 96.5 150 
81 2.813 0.938 0.225 
0 3.166 95.7 146 
82 1.875 0.625 0.225 
0 3.166 97.2 148 
83 10875 0.625 0.100 
0 3.200 97.2 148 
84 1.406 0.563 0.113 
0 3.165 96.4 151 
85 2.344 0.781 0.188 
0.15 3.203 97.0 151 
86 1.875 0.625 0.150 
0.15 3.199 97.1 151 
87 1.875 0.625 0.150 
0.15 3.200 97.2 149 
88 1.000 1.000 0.150 
0.15 3.176 96.9 139 
89 2.344 0.781 0.188 
0.25 3.197 96.8 149 
90 1.875 0.625 0.150 
0.25 3.198 97.1 151 
91 1.875 0.625 0.100 
0.25 3.198 97.1 151 
92 1.875 0.625 0.250 
0.24 3.194 97.0 146 
93 2.344 0.781 0.188 
0.40 3.191 96.6 150 
94 1.875 0.625 0.150 
0.40 3.205 97.3 145 
95 1.875 0.625 0.100 
0.40 3.207 97.3 148 
______________________________________ 
The data in Table VII demonstrate that the sintering aid combination of 
Y.sub.2 O.sub.3, CaO and AlB.sub.2 tolerates higher levels of oxygen 
without significant adverse effects as measured by either density or 
thermal conductivity. Substantially higher levels of oxygen that accompany 
much higher levels of Al.sub.2 O.sub.3 should lead to improvements in 
density at the expense of decreases in thermal conductivity. 
EXAMPLES 96-97 
Repeat Examples 41 and 56 as, respectively Examples 96 and 97, but sinter 
in the presence of a green body containing 2 wt % AlB.sub.2. Table VIII 
shows comparative data for Examples 41,56, 96 and 97. 
TABLE VIII 
______________________________________ 
Exam- Density Thermal 
Ple Y.sub.2 O.sub.3 
CaO AlB.sub.2 
Density 
(% Conductivity 
No. (g) (g) (g) (g/cm.sup.3) 
Theoretical) 
(W/m .multidot. K) 
______________________________________ 
41* 2 0.5 0 2.834 86.1 133 
96 2 0.5 0 3.140 95.4 140 
56* 1.875 0.625 0 2.829 86.0 -- 
97 1.875 0.625 0 3.137 95.3 139 
______________________________________ 
*means not an example of the invention 
-- means not measured 
The data in Table VIII show that a volatile species such as boron enhances 
sintering of aluminum nitride compositions even when it is not part of the 
composition, so long as it is present, for example, in a crucible used to 
sinter the compositions. 
EXAMPLES 98-102 
Repeat Example 47, but vary the boron source to provide the data in Table 
IX. Examples 47 and 98-102 all contain 2 wt % Y.sub.2 O.sub.3 and 0.5 wt % 
CaO. Although the weight percent of the various boron sources differs, each 
source is present in an equivalent molar percentage. 
TABLE IX 
______________________________________ 
Exam- Boron Density Thermal 
Ple Source Density (% Conductivity 
No. Type g (g/cm.sup.3) 
Theoretical) 
(W/m .multidot. K) 
______________________________________ 
47 AlB.sub.2 
0.25 3.194 97.0 151 
98 YB.sub.6 0.264 3.184 96.7 152 
99 SrB.sub.6 
0.260 3.203 97.3 149 
100 CaB.sub.6 
0.180 3.210 97.5 150 
101 AlB.sub.12 
0.134 3.193 97.0 150 
102 LaB.sub.6 
0.350 3.213 97.6 152 
______________________________________ 
The data in Table IX show that a variety of boron sources provide 
satisfactory results in terms of density and thermal conductivity at 
1625.degree. C. in a graphite furnace. 
EXAMPLES 103-108 
Repeat Examples 47, 98-102, but change the sintering temperature to 
1600.degree. C. to provide the data in Table X. 
TABLE X 
______________________________________ 
Exam- Boron Density Thermal 
Ple Source Density (% Conductivity 
No. Type g (g/cm.sup.3) 
Theoretical) 
(W/m .multidot. K) 
______________________________________ 
103 AlB.sub.2 
0.25 3.097 94.1 137 
104 YB.sub.6 0.264 2.984 90.6 129 
105 SrB.sub.6 
0.260 3.105 94.3 136 
106 CaB.sub.6 
0.180 3.064 93.1 134 
107 AlB.sub.12 
0.134 3.115 94.6 136 
108 LaB.sub.6 
0.350 3.133 95.2 138 
______________________________________ 
The data in Table X show the same trends as in Table IX. The data for 
Example 105, when compared to that of Examples 103, 104 and 106-108 simply 
show that some boron sources provide enhanced performance over other boron 
sources. Grinding YB.sub.6 may provide better dispersion of the boron 
source and lead to improved results comparable to those of the other 
examples. 
EXAMPLES 109-123 
Repeat Example 39, but change the heating schedule to: heat to a 
temperature of 1400.degree. C. at a rate of 10.degree. C./minute, hold at 
1400.degree. C. for one hour, heat to 1850.degree. C. at a rate of 
2.5.degree. C./minute, hold at 1850.degree. C. for two hours, and then 
cool to 1000.degree. C. at a rate of 10.degree. C./minute. The 
formulations and resulting thermal conductivity are shown in Table XI. 
TABLE XI 
______________________________________ 
Exam- Thermal 
Ple Y.sub.2 O.sub.3 
CaO AlB.sub.2 
Al.sub.2 O.sub.3 
Conductivity 
No. (g) (g) (g) (g) (W/m .multidot. K) 
______________________________________ 
109 3.0 0.0 0.0 0.0 186 
110 2.34 0.78 0.18 0.0 181 
111 2.81 0.94 0.23 0.0 177 
112 2.34 0.78 0.19 0.25 179 
113 1.88 0.63 0.15 0.25 181 
114 1.88 0.63 0.10 0.25 181 
115 1.88 0.63 0.25 0.25 179 
116 1.88 0.63 0.15 0.15 183 
117 1.88 0.63 0.15 0.40 182 
118 2.34 0.78 0.19 0.40 178 
119 1.88 0.63 0.10 0.15 177 
120 1.88 0.63 0.10 0.40 181 
121 1.41 0.56 0.1l 0.0 182 
122 1.00 1.00 0.15 0.15 185 
123 1.75 0.75 0.25 0.0 184 
______________________________________ 
*means not an example of the invention 
The data in Table XI show that the three component sintering combination of 
Y.sub.2 O.sub.3 --CaO--AlB.sub.2 (Examples 110, 111, 121 and 123) and four 
component sintering aid combination of Y.sub.2 O.sub.3 --CaO--AlB.sub.2 
--Al.sub.2 O.sub.3 (Examples 112-120 and 122), when used in sintering 
sinterable AIN compositions at temperatures as high as 1850.degree. C., 
yield thermal conductivities that are statistically equivalent to that 
provided by a standard sinterable AIN composition using only 3 g Y.sub.2 
O.sub.3 at the same temperature (Example 109*). When sintered under 
equivalent conditions, authors writing in open literature would predict 
that addition of CaO, Al.sub.2 O.sub.3 or both to Y.sub.2 O.sub.3 would 
yield thermal conductivities below that provided by Y2O.sub.3 as a sole 
sintering aid. The data in Table XI show that the presence of a small 
amount of a boron source as a sintering aid counters this prediction. 
Similar results are expected with other alkaline earth metal sources, rare 
earth metal sources, boron sources and aluminum oxide sources. 
EXAMPLE 124 
Prepare a milled slip using the procedure of Example 39 and 300 g AIN 
powder, 3.35 g CaCO.sub.3 powder 5.62 g Y.sub.2 O.sub.3 powder, 0.45 g 
AlB.sub.2 powder, 93 g of toluene as the solvent, 33 g of a 
binder/dispersant ("ROHM AND HAAS, CO. ACRYLOID B-72") and 9 g of a 
plasticizer ("ARISTECH CHEMICAL COMPANY, PX-316"). The powders are the 
same as used in Example 39. The milled slip is converted to a cast green 
tape using conventional doctor blade techniques. The green tape is screen 
printed with a test ink pattern using a tungsten ink (CRYSTALERO, #2003 
ink). Multilayer test pieces are fabricated by stacking and thermally 
laminating screen printed green tape pieces under an isostatic pressure of 
2000 pounds per square inch (psi) (13.8 megapascals) at a temperature of 
70.degree. C. Binder burnout occurs in a flowing nitrogen atmosphere using 
a heating rate of 90.degree. C./hour up to 750.degree. C., holding at 
750.degree. C for four hours and cooling at a rate of 180.degree. C./hour 
to room temperature (25.degree. C.). 
The multilayer test pieces are sintered in the tungsten furnace used in 
Examples 57-73 using the same sintering conditions as in Examples 57-73 to 
produce cofired (also known as cosintered) substrates. The cofired 
substrates have a density of 3.175 g/cm.sup.3 (96.4% of theoretical) and a 
thermal conductivity of 134 W/m.K. Test circuit patterns resulting from the 
ink have an electrical resistance that varies from 13 to 26 
milliohms/square. 
EXAMPLE 125 
Repeat Example 124, but reduce the amount of AlB.sub.2 powder to 0.30 g. 
The cofired substrates have a density of 3.174 g/cm.sup.3 and a thermal 
conductivity of 124 W/m.K. Test circuit patterns resulting from the ink 
have an electrical resistance that varies from 13.1 to 13.6 
milliohms/square. 
Examples 124 and 125 demonstrate the suitability of sinterable compositions 
of the invention for use in preparing multilayer, cofired substrate 
materials. The narrow variability of electrical resistance in Example 125 
relative to Example 124 suggests that boron source levels should be kept 
relatively low. By way of illustration, when using AlB.sub.2 as the boron 
source, the level should be at or below 0.15 wt %, based on sintered body 
weight, for more consistent results. Similar results are expected with 
other sinterable compositions and AIN powders, both of which are disclosed 
herein.