Note: Descriptions are shown in the official language in which they were submitted.
~".. 1
"METAL OXIDE DIELECTRIC DENSE BODIES, PRECURSOR POWDERS
THEREFOR, AND METHODS FOR PREPARING SAME"
Thia invention relates to metal oxide dielectric dense
bodies, precursor powders and methods therefor, and multilayer
semiconductor substrates made of the same.
Multilayer packaging in the electronics industry involves
the use of ceramic powders which are sintered (or fired). to
form a dense insulating (or dielectric) substrate for
attaching semiconductor chips, connector leads, capacitors,
resistors, and other electronic components. The substrate is
first prepared as unsintered (or "green") sheets, onto which a
desired pattern of electrical conductors (or precursor
compositions therefor) is deposited by spraying, dipping,
screening, etc. Interconnection between the various layers can
be achieved by vias or feedthrough holes punched into them.
The vias are metallized by filling them with a metal paste
which, during the sintering process, is transformed into a
sintered dense metal interconnection of a conductor such as
copper. Thus, by superimposing a plurality of green sheets and
ipterconnecting them with vias and sintering, a multilayer
dielectric structure having a desired conductoc pattern within
it is prepared.
2
"'"'~ A common substrate is alumina (A1203), Which possesses a
number of desirable characteristics, among which are high
resistivity, high thermal conductivity, and good mechanical
properties. However, alumina also possesses some limitations.
Its dielectric constant is undesirably high, as is its thermal
expansion coefficient. The high temperature required to sinter
alumina powder into a dense body, approximately 1500 °C, is
incompatible with the use of a highly conductive and
inexpensive conductor such as copper (mp 1083 °C). Instead,
less conductive and/or more expensive conductors such as
molybdenum or tungsten, having higher melting points, must be
used.
An attractive alternative material to alumina is
cordierite (2Mg0~2A1203~SSi02), due in part to its low
dielectric constant and thermal expansion coefficient. in a
prior art preparation of dense cordierite, a mixture of the
consitituents MgO, A1203, and Si02 is sintered at temperature
in excess of 1300 °C. This method suffers from the same
liditation regarding compatibility with copper conductors as
the alumina method. Cordierite may be sintered to near-
theoretical density by the glass-ceramic method at a lower
temperature, about 1000 °C, which is compatible with the use
of lower melting conductors such as copper, gold, or silver.
In this method, a two-stage process is used. First, an
appropriate composition (e.g., 13.78 wt. % MgO, 34.86 wt. %
A1203, and 51.36 wt. % Si02) is prefired to a high temperature
(about 1500 °C) and then rapidly quenched to form a glass. The
glass may include nucleating agents such as titanium oxide,.
3
zirconium oxide, phosphorus pentoxide, or stannic oxide.
Second, the glass, after forming into a green body of
appropriate shape, is heated to about 1000°C to form the dense
body, with the nucleating agent helping to promote the
crystallization of the glass. The ensuing dense body typically
consists of fine grained crystals dispersed in a glassy
matrix. Because copper conductor is applied after the first,
higher temperature heating cycle this process is compatible
with its use. Discussions of various aspects of glass-ceramic
technology may be found in MacDowell, U.S. Pat. 3,275,493
(1966); Miller, U.S. Pat. 3,926,648 (1975); ~umar et al., U.S.
Pat. 4,301,324 (1981); and Herron et al., U.S. Pat. 4,234,367
(1980). Prunier, Jr., in U.S. Pat. 4,745,092 (1988), discloses
an alternative method of making cordierite having therein
minor amounts of calcia from synthetic raw materials such as
magnesium and aluminum salts and colloidal silica. However,
his sintering temperatures are between 1380 and 1440 °C,
making his process incompatible with copper conductor.
Another alternative material to alumina is mullite
(3A1203~2Si02), due in part to its low dielectric constant and
thermal expansion coefficient. However, conventional
preparation of mullite by sintering a mixture of the
consitituents A1203 and Si02 requires a firing temperature in
excess of 1350 °C and therefore suffers from the same
limitation regarding suitable conductors as alumina. Gardner,
U.S. Pat. 3,826,813 (1974), discloses a process for making
mullite for use as an integrated circuit substrate by a
two-step process, in which the precursor materials are
4
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~ac~aszs
prereacted at 1300-1400 °C and later sintered at 1500-1600 °C.
""
Yet another alternative material to alumina is magnesium
oxide (Mg0), due in part to its high thermal conductivity,
which leads to efficient heat dissipation. However, the
conventional preparation of a magnesium oxide dense body by
sintering magnesium oxide powder requires a firing temperature
in excess of 1350 °C and therefore suffers from the same
limitation regarding suitable conductors as alumina. De Jonghe
et al., in J. Am. Ceram. Soc. 71, C-356 (1988), disclose
another method of making magnesium oxide, by the liquid phase
sintering of a magnesium oxide-bismuth oxide system. Bismuth
oxide is added to an alcoholic suspension of magnesium oxide,
and, after stir drying, the powder mixture is ground up with a
mortar and pestle and sintered at about 1000 °C to produce the
dense body. However, the densification obtained is relatively
low -- only about 70-80% of theoretical.
The present invention provides novel methods of making
dielectric dense bodies comprising cordierite, mullite,
i
magnesium oxide, or other metal oxides, which are compatible
with the use of copper or other low melting conductor because
of their lower preparation temperature. There are also
provided novel metal oxide dielectric compositions made by the
process of this invention.
5
SUMMARY OF THE INVENTION
This invention provides a metal oxide dielectric dense
body, comprising
(I) grains having a predominant crystalline phase comprising
(a) a primary metal oxide selected from the group
consisting of silicon and magnesium oxide and (b)
optionally a secondary metal oxide selected from the
group consisting of aluminum and zinc oxide and
(II) between about 1 and about 20 atom % bismuth, vanadium,
or boron oxide or combinations thereof, discontinuously
located at the boundaries of the crystalline grains or as
inclusions in the crystalline grains, the atom %'s based
on the total atoms of bismuth, vanadium, boron, silicon,
magnesium, aluminum, and zinc;
the dense body having a density which is at least 95% of
theoretical.
This invention also provides a method of making a
dielectric dense body containing silicon and/or magnesium
oxide as a primary oxide and optionally aluminum or zinc oxide
as a secondary oxide, comprising the steps of:
(a) forming an aqueous mixture containing
(i) a source of primary oxide, selected from the group
consisting of colloidal silica; high surface area
silica; mixed oxides of silicon and aluminum;
magnesium salts, oxides and hydroxides; and mixed
6
2QOU~~f
oxides of magnesium and aluminum;
(ii) optionally a source of secondary oxide, selected
from the group consisting of aluminum salts, oxides,
and hydroxides and zinc salts, oxides, and
hydroxides;
(iii) a sintering aid source selected from the group
consisting of bismuth salts, oxides, and hydroxides;
vanadium salts, oxides, and hydroxides; boric acid,
borate salts, and boron oxide; and combinations
thereof; the sintering aid source having a Bi plus V
plus B atom % of between 1.0 and about 20, based on
the total atoms of Mg, Si, A1, Zn, Bi, V, and B; and
(iv) a precipitating agent in an amount sufficient to
precipitate a precursor powder for the dielectric
dense body;
(b) collecting and drying the precursor powder;
(c) forming the dried precursor powder into a green body
having a desired shape; and
(d) si-ntering the green body at a temperature of at least 850
°C to form the dense body.
Another aspect of this invention provides a method of
making a precursor powder for a dielectric dense body
containing silicon and/or magnesium oxide as a primary oxide
and optionally aluminum or zinc oxide as a secondary oxide,
comprising the aforementioned steps (a) and (b).
7
In yet another aspect of our invention, there is provided
a multilayer substrate for a semiconductor device, at least
two layers thereof having thereon a conductor pattern and
being electrically connected to each other by metallized vias,
the substrate being made of a dielectric dense body of this
invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
Our invention enables the preparation of dielectric dense
bodies by a sintering process at relatively low temperatures,
so that copper conductors may be used in combination
therewith. However, unlike the glass-ceramic process, the
simpler present process requires only one heating cycle. A
precursor powder containing a combination of primary metal
oxides and optionally secondary metal oxides in the desired
stoichiometry and further containing a sintering aid dispersed
therein is sintered..
The sintering aid is preferably bismuth oxide (Bi203),
vanadium oxide (VZOS), or boron oxide (B203), or combinations
thereof. The corresponding hydroxides are also suitable, as
they are converted to the oxides under the sintering
conditions of this invention or during pre-sintering
processing steps such as drying. Peritectic compositions may
also be used. Exemplary peritectic compositions are
SSBi203~45B203 and 81Bi203~19B203. Boron sintering aid should
be used in conjunction with a bismuth sintering aid,
preferably in the form of a peritectic composition.
8
The sintering aid containing precursor powder is prepared
by a precipitation process. By precipitation, we mean not only
precipitation in the conventional sense, but also related
procesaea such as occlusion, entrainment, metathesis, and the
like.
In one embodiment, an acidic aqueous mixture, which may
be a homogeneous solution or a colloidal or other
heterogeneous mixture and which contains sources of the
primary metal oxide, the secondary metal oxide (if present),
and the sintering aid, in the desired relative amounts, is
prepared. A basic precipitating agent is added, so that
particles of the primary and secondary metal oxide (if
present) having the sintering aid dispersed therein or
thereover are formed. (Alternatively, the sintering aid may be
added to an aqueous mixture already containing the
precipitating agent.) The particles should be of a size which
allows substantial densification to take place upon sintering
and preferably are submicron in average size.
i
In another embodiment, the sintering aid is
preferentially coated on a metal oxide (usually a primary
one), by initially withholding the other metal oxides. The
sintering aid is first precipitated onto the primary oxide and
only thereafter are the other metal oxide sources added, to
complete the preparation of the precursor powder. For example,
in the preparation of a mullite (3A1203~2Si02) dielectric
dense body, the aluminum oxide source is at first withheld
from the aqueous mixture. The sintering aid is precipitated
onto the silicon oxide and then the aluminum oxide source is
9
a
added, to prepare a precursor powder in which the sintering
aid selectively coats the silicon oxide.
After precipitation, the precursor powder is collected,
optionally washed, and dried. Washing is helpful for removing
coprecipitated or entrained extraneous salts formed during the
precipitation process. However, where such salts are heat
volatilizable, as will be discussed hereinbelow, washing can
be omitted.
Where boron oxide or hydroxide is used as a sintering
aid, the boron species (e.g., borate) may be soluble in the
supernatant liquid, in which case the precursor powder is
preferably collected by evaporating off the supernatant, for
example with a rotary evaporator. in such instances, the use
of volatilizable salts is particularly preferred.
Optionally, the precursor powder may be ground to a
desired mesh size. The powder is then formed into a green
sheet or body. Any desired electrical conductor patterns may
beideposited at this point. Also, vias or feedthrough holes
may be formed and filled with metallic paste for their
subsequent metallization. Sintering then produces the dense
body dielectric substrate material containing the desired
conductor pattern, plus any metallized vias.
Additives such as plasticizers, binders, solvents,
dispersants, surfactants and the like normally are used in
making the green body. Typical suitable plasticizers include
glycols (e. g., polypropylene glycol), phthalate esters (e. g.,
dioctyl phthalate and benzyl butyl phthalate), and long chain
to ~~'~fi
carboxylic acids (e. g., oleic and stearic acid), and aaixtures
thereof. Exemplary binders are-cellulose esters, long-chain
thermoplastic polymers such as polyvinyl butyral), polyvinyl
acetate), and poly(methyl methacrylate). Exemplary surfactants
include amine salts of alkyl aryl. suhfonates, alkyl ethers of
polyethylene glycol) (e. g., ethyl ether of polyethylene
glycol)), alkyl aryl polyether alcohols (e.g., ethylphenyl
glycol), polyoxyethylene acetate, and the like. The method of
use and the amounts of such additives are well known in the
art. See, for example, Eggerding et al., U.S. Pat. 4,540,621
and U.S. Pat. 4,235,855. Preferred additives are those which
volatilize during the sintering process. Alternatively, the
additives may be removed by a solvent extraction or leaching
process.
in preparing the green body, a volatile solvent such as
methanol generally is used. The solvent dissolves the binder
(if any) and ensures.that it individually and uniformly coats
the precursor powder particles and helps control the rheology
of the mixture so that it can be conveniently cast,into the
desired shape. The green body is cast into thin sheets by a
conventional technique such as doctor-blading. The sheets are
cut to the desired shape and via holes in the appropriate
configuration are punched into them. A metallizing paste of
gold, silver, or copper is extruded into the via holes by
screen printing. Also, any desired conductor patterns are
screen printed onto each sheet. The sheets are then stacked on
top of each other. making sure that they are properly
11
~r~~~~IG~~.3
~~~~.. registered, and laminated. Sintering then produces a
multilayer dielectric dense body. having conductor patterns on
each layer and the various layers interconnected by metallized
vias.
Additionally, the dielectrics of this invention may be
used as insulators in other applications, and as structural
ceramics.
While we do not wish to be bound by any theory, it is our
belief that the sintering aid, when heated to a temperature
above its melting point, melts and acts as reactive liquid
phase sintering agent. It dissolves the precursor powder
components and reprecipitates them as the crystalline dense
body material at the grain boundaries, causing coalescence of
powder particles. The precipitation method of our invention
distributes the sintering aid homogeneously over the surfaces
of the powder particles, enabling a more effective sintering
process.
! An advantage of our invention is that sintering or firing
of,the precursor powder can be effected at a relatively low
temperature. The temperature should be at least 925 °C,
preferably is between about 925 and about 1050 °C, and more
preferably is between about 950 and about 1000 °C. Such
temperatures are compatible with the use of relatively low
melting conductors such as copper, gold, or silver. The
sintering time is not particularly critical, provided it is of
sufficient duration. Typically, times of between about 2 and
about 12 hr are sufficient. Longer times of course may be
""'_°-",.... 12
used, but are not required. There may be some variation in the
Nn~ time required, depending on the sintering temperature, the
particle size of the precursor powder, the nature of the
primary and secondary metal oxide sources, the amount of
sintering aid present, etc., as may be readily empirically
determined. As is well known in the art, the sintering process
may be according to a complex heating schedule, in which the
green body is heated initially for A hours at B °C, then for C
hours at D °C, and so forth. Where the green body includes one
or more volatilizable additives, it is desirable that they be
removed before the final densification. In such instances, a
complex heating schedule, with the initial heating stages at a
lower temperature, for example at 200 to 700 °C for 1-60 hr,
is recommended to volatilize the additives. A vacuum may be
applied.
Where the primary metal oxide is silicon oxide (or
silica), the source therefor is selected from the group
consisting of colloidal silica, high surface area silica,
mixed oxides of aluminum and silicon, mixed oxides of
magnesium and silicon, and combinations thereof. Of course a
mixed oxide should be used only if it is desired that the
final composition contain an oxide of the other metal.
Colloidal silica or silicon oxide is available under the
tradename Ludox from Du Pont, with the grade AS being
particularly preferred. Colloidal silica is also available
under the tradename Cab-0-Sil from Cabot. The high surface
area silica should have a surface area at least 10 square
meters per gram. Preferably, the surface area is at least 30
13
square meters per gram. Suitable mixed aluminum-silicon oxides
are clay (A1203~2Si02) and other aluminosilicates, while
suitable mixed magnesium-silicon oxide are talc (3Mg0~4Si02)
and other magnesium silicates.
Suitable sources of the primary oxide magnesium oxide and
the secondary oxides aluminum oxide (or alumina) and zinc
oxide are the respective salts, oxides and hydroxide, and
mixed oxides (e. g. spinel (Mg0~A1203 or other magnesium
aluminates); clay (A1203~2Si02)). and combinations thereof.
Again, a mixed oxide should be used only if it is desired that
the finch composition contain an oxide of the other metal.
Examples of suitable salts include the chlorides, bromides,
oxalates, nitrates, sulfates, and mixtures thereof. The
nitrates are preferred because the nitrate anion can form a
volatilizable salt reaction by-product which can be readily
removed by heating, as for example during the sintering
process. Thia is particularly so if the precipitating agent
has a volatizable catior, e.g., ammonium, so that.a volatile
salt (in this case, ammonium nitrate) is formed during the
precipitation process. Where non-volatilizable salts are
formed, they may still be used, provided that effective
washing is performed to remove them prior to sintering.
The primary and secondary metal oxide sources are used in
proportions which reflect the final desired composition. For
example, if a willemite (2Zn0~Si02) dense body is being made,
the primary metal oxide source and the secondary metal oxide
source should be used in a molar ratio of approximately 1:2.
14
", The sintering aid source is selected from the group
consisting the respective salts, oxides, and hydroxides of
bismuth, vanadium, and boron and combinations thereof.
Examples of suitable salts include the chlorides, bromides,
oxalates, nitrates, sulfates, and mixtures thereof. Boron is
preferably added as boric acid, a borate salt, or boron oxide.
The nitrates are preferred for the aforementioned reasons.
Upon contact with the precipitating agent, the bismuth and/or
vanadium sintering aid is precipitated as the corresponding
oxide or hydroxide.
Preferably, the amount of sintering aid source (and thus
sintering aid) to use is between about 1 and about 20 atom %
bismuth plus vanadium plus boron, more preferably between
about 2 and about ~0 atom %. If vanadium oxide is the sole
sintering aid the amount is preferably at least 5 atom %.
Further, as discussed hereinbelow or as may readily be
determined empirically, within the above preferred range there
may be a particularly preferred range for a particular primary
and secondary oxides.
Throughout this application, the atom % of an element X,
where X is Si, Mg, Al, Zn, Bi, V, or B, is based on the total
atoms of Si, Mg, Al, Zn, Bi, V and B:
atoms (R)
__________ x 100
atom % _ ____________________
atoms ( Sf + Mg + A1 + Zn + Bi + V + B )
Where a mixed oxide is used, it is to be understood that
it serves as a dual source, that is, it supplies more than one
15
element. For example, spinel (Mg0~A1203) supplies two atoms of
A1 for each atom of Mg. Similarly, talc (3Mg0~4Si42) is a dual
source of magnesium (3 atoms) and silicon (4 atoms). For
illustration, a mixture of one mole spinel and one mole talc
would contain 40 atom % Mg, 20 atom % A1, and 40 atom % Si.
Suitable precipitating agents are bases such as sodium,
potassium, and ammonium hydroxide and organic amines (e. g.,
methylamine, ethylamine, ethanolamine). Ammonium hydroxide and
organic amines are preferred for their ability to form
volatilizable salts in combination with anions such as
nitrates. Ammonium hydroxide is particularly preferred. The
amount of precipitating agent is not critical, provided it is
sufficient to at least neutralize the initially acidic mixture
and preferably render it alkaline. where the A1 or Mg source
is a mixed oxide such as spinel, neutralization is sufficient.
Where the Al or Mg source is a salt such as the nitrate, the
mixture is preferably made alkaline, to a minimum pH of about
9-10.
The metal oxide dielectric dense bodies of this invention
comprise grains having a predominant crystalline phase
containing the primary metal oxide and optionally the
secondary metal oxide. Where both primary and the secondary
oxides are present, the two form a complex oxide, such as
cordierite, mullite, or willemite. The dense bodies further
comprise bismuth, vanadium, or boron oxide or combinations
thereof located discontinuously at the boundaries between the
grains or as inclusions within the grains. The grains
preferably have an average particle size between about 0.1 and
16
''about 100 microns, more preferably between about 10 and about
100 microns. The density of the dense body is preferably at
least 95 %, more preferably at least 98 %, of the theoretical
density, which is based on the weighted average of the
densities of pure primary and secondary oxide crystalline
phase and any sintering aid present. For example, a dense body
containing 2 atom % Bi as bismuth oxide (density 8.9 g/cc) and
the balance aluminum, magnesium, and silicon oxides as
cordierfte (density 2.5 g/cc) has 8.206 Weight percent bismuth
oxide and 91.794 weight percent cordierite. The theoretical
density TD for this dense body is given by:
TD ~ _______________1__________ ____ ~ 2.657
(0.08206/8.9) + (0.91794/2.5)
Within the crystalline grains, substantial deviation from
the theoretical stoichiometry for a particular mineral (e. g.,
cordierite, mullite, etc.) is permissible. For example, though
cordierite has the empirical formula 2Mg0~2A1203~5Si02. the
crystalline phase grains need not comprise magnesium,
aluminum, and silicon oxides in the exact molar ratios 2:2:5.
Table I provides exemplary crystalline metal oxide dense
bodies which may be prepared according to this invention and
the preferred and more preferred molar ratios of primary and
secondary oxides therein. Table I further provides the
preferred and more preferred amounts of bismuth, vanadium, or
boron oxide or combinations thereof, to be used in the
preparation of the respective compositions. While there may be
some loss of the bismuth, vanadium or boron oxide sintering
..._._---
17
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~~u~,. aid during the sintering process, the final amount of these
materials in the dense bodies is not substantially different
from the pre-sintering amounts.
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The crystalline phase in the grains preferably
predominantly comprises one of the crystalline forms listed in
Table I, namely cordierite, magnesium oxide, mullite,
mullite-silica, willemite, or silica. More preferably, the
sole detectable crystalline phase (as determined by X-ray
crystallography) is one of the aforementioned ones.
While the dielectric dense bodies of this invention may
include additional substances, it is preferred that they
consist essentially of the aforementioned components, namely
primary metal oxide, secondary metal oxide (if any), and
sintering aid. Further, those skilled in the art will-
appreciate that some unavoidable impurities will necessarily
be present, for example adventitious amounts of transition
metals. The presence of alkali metals is preferably avoided.
The practice of our invention can be further understood
from the following examples, which are provided by way of
illustration and not of limitation.
Ex m le 1
This example illustrates the preparation of a cordierite
dense body by the method of our invention, with a bismuth
sintering afd, with spinel as a mixed oxide source for Mg and
A1, and with the bismuth oxide sintering aid selectively
coated onto the silicon oxide.
A bismuth stock solution was prepared by dissolving
bismuth nitrate pentahydrate (Bi(N03)3~5H20, 582 g) in
concentrated nitric acid (384 mL) and then diluting with water
20
to a final volume of 4 L.
Into a vessel were placed colloidal silica (Ludox (TM),
375.5 g, 44.5 atom % Si), 2000 mL deionized water, and 93 mL
concentrated ammonium hydroxide. The resultant mixture was
homogenized 5 min. using a Janke & Kunkel Ultra-Turrax mixing
apparatus. To this mixture was added 373.7 mL of the above
bismuth stock solution (2.0 atom % Bi), an action which
resulted in the precipitation of the bismuth species onto the
silica. The suspension was homogenized for 5 min. Finally,
spinet (Baikowski S50CR, 142.3 g, 35.6 atom % A1 and 17.8 atom
% Mg) waa added and the mixture homogenized for 20 min.
The thus obtained precursor material was collected by
suction filtration, washed with water, and dried at 140 °C.
The dried powder was subsequently calcined to remove residual
ammonium nitrate by heating according to the following
schedule: 4.5 hr at 30-300°C, then 1 hr at 300°C.
In order to more easily form green shapes from the
calcined powder, polymeric binders were employed. Thus. 25 g
of calcined precursor powder Were combined with 150 mL
isopropyl alcohol and 150 mL deionized water. The resultant
was homogenized 20 min. To this were added 37.6 g of an
aqueous solution containing 1.128 g Elvanol 51-05 PVA (DuPont)
and 0.188 g Carbowax 8000 PEG (Union Carbide). The obtained
slurry was evaporated to dryness, ground, sieved (106 micron
mesh) and uniaxially pressed at about 6000 psi into a two inch
diameter disc which was fired as follows: 43.5 hr at 50-700
°C, 0.5 hr at 700-1000 °C, and 12 hr at 1000 °C.
21
2~~fl~~
Analysis of the X-ray diffraction pattern of the fired
disc showed the presence of cordierite as the sole crystalline
species. Density was determined to be 2.7 gm/cc, or 100
percent of theoretical.
Example 2
This example illustrates the preparation of a cordierite
dense body by the method of our invention, with a bismuth
sintering aid, with Al and Mg salts as the sources of Al and
Mg, and with the bismuth oxide sintering aid preferentially
coated onto the silicon oxide by a sequential precipitation
process.
A bismuth stock solution was prepared by dissolving
bismuth nitrate pentahydrate (582 g) in concentrated nitric
acid (384 mL) and then diluting with water to a final volume
of 4L.
Aluminum nitrate nonahydrate (112.54 g, 35.3 atom % A1)
and magnesium nitrate hexahydrate (38.4 g, 17.6 atom % Mg)
were dissolved in 150 mL deionized water.
In a separate container were placed colloidal silica
(Ludox (TM), 56.3 g, 44.1 atom % Si), 150 mL deionized water,
and 143.6 mL concentrated ammonium hydroxide. The resultant
was homogenized 5 min. using a Janke & Runkel Ultra-Turrax
mixing apparatus. To this solution were added 85.0 mL of the
above bismuth stock solution (3.0 atom % Bi), an action which
resulted in the precipitation of the bismuth species. The
suspension was homogenized for 5 min. Finally, the aqueous
22
2~~~~~
solution of aluminum nitrate and magnesium nitrate described
above was added, precipitating the aluminum and magnesium from
solution, and the mixture homogenized 5 min.
The thus obtained precursor material was collected by
suction filtration, washed with water, and dried at 140 °C.
The dried powder was subsequently calcined to remove residual
ammonium nitrate as follows: 4.5 hr at 30-300 °C, then 1 hr at
300 °C.
The calcined powder was ground, sieved (106 micron mesh)
and uniaxially pressed at 25,000 psi into a pellet which was
fired as follows: 1.5 hr at 30-1000 °C, then 12 hr at 1000 °C.
Analysis of the X-ray diffraction pattern of the fired
pellet showed the presence of cordierite as the sole
crystalline species. Linear shrinkage was measured as 23.7 %.
Density was determined to be 2.7 gm/cc, or 100 percent of
theoretical.
Exam le 3
This is a comparative example not according to the method
of our invention, in which the preparation of a cordierite
dense body is attempted without using any sintering aid.
Aluminum nitrate nonahydrate (112.54 g, 36.4 atom % A1)
and magnesium nitrate hexahydrate (38.4 g, 18.2 atom % Mg)
were dissolved in 150 mL deionized water.
In a separate container were placed colloidal silica
(LudoX (TM), 56.3 g, 45.4 atom % Si), 150 mL deionized water,
23
and 122.4 mL concentrated ammonium hydroxide. The resultant
was homogenized 5 min. using a Janke & Kunkel Ultra-Turrax
mixing apparatus. The aqueous solution of aluminum nitrate and
magnesium nitrate described above was added, precipitating the
aluminum and magnesium from solution, and the mixture
homogenized 5 min.
The thus obtained precursor material was collected by
suction filtration, washed with water, and dried at 140 °C.
The dried powder was subsequently calcined to remove residual
ammonium nitrate as follows: 4.5 hr at 30-300 °C, then 1 hr at
300 °C.
The calcined powder was ground, sieved (106 micron mesh)
and uniaxially pressed at 25.000 psi into a pellet which waa
fired as follows: 1.5 hr at 30-1000 °C, 100 hr at 1000 °C.
X-ray analysis of the fired pellet revealed no evidence
of cordierite formation.
Example 4
i
This example illustrates the preparation of a cordierite
dense body by the method of our invention, with a bismuth
sintering aid, in which the aluminum, magnesium and silicon
oxides are precipitated simultaneously.
A bismuth stock solution was prepared by dissolving
bismuth nitrate pentahydrate (196 g) in concentrated nitric
acid (128 mL) and then diluting with water to a final volume
of 4 L.
24
2~~~~~~
Aluminum nitrate nonahydrate (35.3 g, 35 atom % A1) and
magnesium nitrate hexahydrate (17.6 g, 18 atom % Mg) were
dissolved in 0.16 N nitric acid (120 mL). To this solution
were added colloidal silica (Ludox (TM), AS grade, 9 g, 44
atom % Si) and bismuth stock solution (40.8 mL, 3 atom % Bi).
The precursor material was precipitated by the addition of
concentrated aqueous ammonium hydroxide (20 mL). The resulting
material was collected by suction filtration, washed
thoroughly with water, and dried at 140 °C. The dried powder
waa ground, sieved (106 micron mesh), and uniaxially pressed
into a pellet which was fired at 1000 °C for 2 hr.
X-ray analysis of the diffraction pattern of the fired
pellet showed the presence of cordierite as the sole
crystalline species. Linear shrinkage as measured by
dilatometry was 22 %.
Exam le 5
This example illustrates the preparation of a cordierite
deAse body by the method of our invention, with a bismuth
sintering aid and with spinal as the raw material, and with
the aluminum, magnesium, and silicon oxides precipitated
simultaneously.
Spinal (Baikowski S50CR, 3.4 g, 35 atom % A1 and 18 atom
% Mg) and colloidal silica (Ludox (TM), 9.0 g, 44 atom % Si)
were added to 0.16 N nitric acid (120 mL). To this was added
bismuth stock solution (40.8 mL, 3 atom % Bi) described in
Example 4. The precursor material was precipitated by the
addition of concentrated aqueous ammonium hydroxide (20 mL).
25
'"- The resulting material was collected by suction filtration,
washed thoroughly with water, and dried at 140 °C. The dried
powder was ground, sieved (106 micron mesh), and uniaxially
pressed into a pellet which was fired at 1000 °C for 2 hr.
X-ray analysis of the diffraction pattern of the fired
pellet showed the presence of cordierite as the sole
crystalline species. Linear shrinkage as measured by
dilatometry was 22 %.
Exam le 6
This example illustrates the preparation of a cordierite
dense body by the method of our invention, with a vanadium
sintering aid, in which the aluminum, magnesium, and silicon
oxides are precipitated simultaneously.
Magnesium nitrate hexahydrate (4.43 g, 16.5 atom % Mg).
aluminum nitrate nonahydrate (12.9 g, 32.9 atom % A1), and
colloidal silica (LUdox AS, 6.45 g, 41.0 atom % Si) were added
to 0.17 N nitric acid (400 mL). To this solution was added
i
vanadium oxide (VZ05, 0.91 g, 9.5 atom % V). Concentrated
ammonium hydroxide (15 mL) was then added to precipitate the
precursor material. The resulting material was collected by
suction filtration and calcined at 300 °C for 1 hr. The
resulting powder was ground and uniaxially pressed into a
pellet which was sintered at 1000 °C for 12 hr.
X-ray diffraction analysis of the sintered pellet showed
the presence of cordierite and vanadium oxide as the sole
crystalline species.
26
~.
Exam le 7
This example illustrates the preparation of a mullite
dense body by the method of our invention, with a bismuth
sintering aid, and in which the sintering aid is
preferentially depositied onto silica by sequential
precipitation.
A bismuth stock solution was prepared by dissolving
bismuth nitrate pentahydrate (Bi(N03)3~5H20, 582 g) in
concentrated nitric acid (384 mL) and then diluting with
deionized water to a final volume of 4 L.
Aluminum nitrate nonahydrate (A1(N03)3~9H20. 56.27 g,
71.25 atom % A1) was dissolved in 100 mL deionized water.
In a separate container were placed colloidal silica
(Ludox (TM), 7.51 g, 23.75 atom % Si), 100 mL deionized water,
and 54.64 mL concentrated ammonium hydroxide. The resultant
mixture was homogenized 5 min. using a Janke & lCunkel Ultra-
Turrax mixing apparatus. To this were added 35.09 mL of the
above bismuth stock solution (5.0 atom % Bi), an action which
resulted in the precipitation of the bismuth species onto the
silica. The suspension was homogenized for 5 min. Next, the
aqueous solution of aluminum nitrate described above was
added, precipitating the aluminum from solution, and the
mixture homogenized for another 5 min.
The thus obtained~precursor material was collected by
suction filtration, washed with water, and dried at 140 °C.
The dried powder was subsequently calcined to remove residual
27
2~~~~~.'~
ammonium nitrate according to the following schedule: 4.5 hr
w~..
at 30-300 °C and then 1 hr at 300 °C.
The calcined powder was ground, sieved (106 micron mesh)
and uniaxially pressed at 25,000 psi into a pellet which was
fired as follows: 1.5 hr at 30-1000 °C and then 12 hr at 1000
°C.
x-ray diffraction analysis of the fired pellet showed the
presence of mullite as the sole crystalline species. Linear
shrinkage was 22 %. Density was determined to be 2.8 gm/cc, or
78 percent of theoretical.
Exam le 8
This is a comparative example not according to the method
of our invention, in which the preparation of a mullite dense
body is attempted without using any sintering aid.
Aluminum nitrate nonahydrate (A1(N03)3~9H20, 56.27 g.
75.0 atom % Al) was dissolved in 100 mL deionized water.
In a separate container were placed colloidal silica
(Ludox (TM), 7.51 g, 25.0 atom % Si), 100 mL deionized water,
and 45.92 mL concentrated ammonium hydroxide. The resultant
was homogenized 5 min. using a Janke & Runkel Ultra-Turrax
mixing apparatus. The aqueous solution of aluminum nitrate
described above was added, precipitating the aluminum from
solution, and the mixture homogenized for another 5 min.
The thus obtained precursor material was collected by
suction filtration, washed with water, and dried at 140 °C.
28
The dried powder was subsequently calcined to remove residual
ammonium nitrate as follows: 4.5 hr at 30-300 °C and then 1 hr
at 300 °C.
The calcined powder was ground, sieved (106 micron mesh)
and uniaxially pressed at 25,000 psi into a pellet which was
fired as follows: 1.5 hr at 30-1000 °C and 12 hr at 1000 °C.
X-ray diffraction analysis of the fired pellet revealed
no evidence of mullite formation.
Example 9
This example illustrates the preparation of a mullite
dense body by the method of our invention, with a bismuth
sintering aid and simultaneous precipitation process.
A bismuth stock solution was prepared by dissolving
bismuth nitrate pentahydrate (196 g) in concentrated nftri.c
acidt(128 mL) and then diluting with water to a final volume
of 4 L.
3
Aluminus nitrate nonahydrate (11.04 g, 67.5 atom % Al)
was dissolved in 0.2 N nitric acid (100 mL). To this solution
were added colloidal silica (Ludox (TM), 1.47 g, 22.5 atom %
Si) and 43.6 mL of the above bismuth stock solution (10 atom %
Bi). Concentrated aqueous ammonium hydroxide (200 mL) was
added to precipitate the precursor material, which was
collected by suction filtration, washed thoroughly with water,
and dried at 140 °C. The dried powder was ground, sieved (106
micron mesh), and uniaxially pressed at 25,000 psi into a
29
2(~Q(~b~~
pellet which was fired at 1000 °C for 2 hr.
X-ray analysis of the diffraction pattern of the fired
pellet showed the presence of mullite as the sole crystalline
species. Linear shrinkage as measured by dilatometry was 17%.
Example 10
This example illustrates the preparation of a mullite
dense body by the method of our invention, with a vanadium
sintering aid and a simultaneous precipitation process.
Aluminum nitrate nonahydrate (25.5 g, 67.5 atom % Al) and
colloidal silica (Ludox AS, 3.3 g, 22.5 atom % Si) were added
to 0.17N nitric acid (400 mL). To this solution was added
vanadium oxide (V205, 0.91 g, 10 atom % V). Concentrated
ammonium hydroxide was then added to precipitate the precursor
material. The resulting material was collected by suction
filtration and calcined at 300 °C for 1 hr. The resulting
powder was ground and uniaxially pressed at 25,000 psi into a
pe~let which was sintered at 1000 °C for 12 hr.
X-ray diffraction analysis of the fired pellet showed the
presence of mullite and vanadium oxide as the sole crystalline
species.
Example 11
This example illustrates the preparation of a magnesium
oxide dense body by the method of our invention, with a
bismuth sintering aid, and in which the sintering aid is
deposited onto the magnesium oxide precursor by sequential
30
precipitation.
A bismuth stock solution was prepared by dissolving
bismuth nitrate pentahydrate (Bi(N03)3~5H20, 582 g) in
concentrated nitric acid (384 mL) and then diluting with
deionized water to a final volume of 4 L.
Magnesium nitrate hexahydrate (Mg(N03)2~6H20, 25.1 g. 98
atom % Mg) was dissolved in 300 mL of 0.15N nitric acid
solution. To this solution was added 13.4 mL of concentrated
ammonium hydroxide, an action which resulted in the
precipitation of the magnesium species. To this were added 6.6
mL of the above bismuth stock solution (2.0 atom % Bi), an
action which resulted in the precipitation of the bismuth
species onto the magnesium species.
The thus obtained precursor material was collected by
suction filtration and calcined at 300 °C for 1 hr. The dried
powder was ground and uniaxially pressed at 25,000 psi into a
pellet which was sintered at 1000 °C for 12 hr.
X-ray diffraction analysis of the fired pellet showed the
presence of magnesiua oxide as the sole crystalline species.
Linear shrinkage was 22 %. Densities were determined to be at
least 99 % of theoretical (3.82 g/cc).
Example 12
This example illustrates the preparation of a magnesium
oxide dense body by the method of our invention with a
vanadium sintering aid and simultaneous precipitation process.
31
Magnesium nitrate hexahydrate (23.0 g, 90 atom % Mg) was
dissolved in 0.17N nitric acid solution (400 mL). To this
solution waa added vanadium oxide (V205, 0.91 g, 10 atom % V).
Concentrated ammonium hydroxide was then added to precipitate
the precursor material. The resulting material was collected
by suction filtration and calcined at 300 °C for 1 hr. The
resulting powder was ground and uniaxially pressed at 25,000
psi into a pellet which was sintered at 1000 °C for 12 hr.
Linear shrinkage was 14 %.
Example 13
This example illustrates the preparation of a cordierite
dense body according to this invention, using a peritectic
composition of bismuth oxide-boron oxide (81:19 mole ratio of
bismuth oxide:boron oxide) as a sintering aid, with spinal as
a mixed oxide source for Mg and A1 ~and with the bismuth oxide
portion of the sintering aid selectively coated onto the
silicon oxide.
A bismuth stock solution was prepared by dissolving
bismuth nitrate pentahydrate (Bi(N03)3~5H20, 582 g) in
concentrated nitric acid (384 mL) and then diluting with water
to a final volume of 4 L.
A boron stock solution was prepared by dissolving boric
acid (H3B03, 30.9 g) in 500 mL water and then diluting with
water to a final volume of 1 L.
into a beaker were placed colloidal silica (Ludox (TM),
37.55 g, 44.34 atom % Si), 100 mL deionized water, and 40 mL
32
-- concentrated ammonium hydroxide. The resultant mixture was
homogenized 5 min. using a Janke & Kunkel Ultra-Turrax mixing
apparatus. To this mixture was added 5.30 mL of the above
boron stock solution (0.47 atom % B) and 37.4 mL of the above
bismuth stock solution (1.99 atom % Bi), an action which
resulted in the precipitation of the bismuth species onto the
silica. The suspension was homogenized for 5 min. Finally,
spinel (Baikowski S50C14.23 g, 35.46 atom % A1 and 17.74 atom
% Mg) was added and the mixture homogenized for 10 min.
The thus obtained precursor material was stripped of free
water using a Buchi Rotavapor-R rotary evaporating apparatus,
and then dried at 140 °C. The dried powder was subsequently
calcined to remove ammonium nitrate by heating according to
the following schedule: 4.5 hr at 30-300 °C, then 1 hr at 300
°C.
The calcined powder was ground, sieved (<106 micron mesh)
and uniaxially pressed at 10,000 psi into a pellet which was
fired according to the following schedule: 1.5 hr at 30-1000
°C, then 12 hr at 1000 °C.
Analysis of the X-ray diffraction pattern of the fired
pellet showed the presence of cordierite as the sole
crystalline species. Density was determined to be 2.6 g/cc, or
96 % of theoretical.
Exa_ mple 14
This example illustrates the preparation of a cordierite
dense body according to this invention using a peritectic
' 33
. , ~ , 2~0~~~~
""~ composition of bismuth oxide-boron oxide (55:45 mole ratio of
bismuth oxide:boron oxide) as a sintering aid, with spinal as
a mixed oxide source for Mg and A1 and with the bismuth oxide
portion of the sintering aid selectively coated onto the
silicon oxide.
A bismuth stock solution was prepared by dissolving
bismuth nitrate pentahydrate (8i(N03)3~5H20, 582 g) in
concentrated nitric acid (384 mL) and then diluting with water
to a final volume of 4 L.
A boron stock solution was prepared by dissolving boric
acid (H3B03, 30.9 g) in 500 mL water and then diluting with
water to a final volume of 1 L.
into a beaker were placed colloidal silica (Ludox (TM),
37.55 g, 43.83 atom % Si), 200 mL defonized water, and 30 mL
concentrated ammonium hydroxide. The resultant mixture was
homogenized 5 min. using a Janke & Runkel Ultra-Turrax mixing
apparatus. To this mixture was added 18.37 mL of the above
bo;on stock solution X1.61 atom % B) and 37.4 mL of the above
bismuth stock solution (1.97 atom % 8i), an action which
resulted in the precipitation of the bismuth species onto the
silica. The suspension was homogenized for 5 min. Finally,
spinal (Haikowski S50CR, 14.23 g, 35.06 atom % A1 and 17.53
atom % Mg) was added and the mixture homogenized for 10 min.
The thus obtained precursor material was stripped of free
water using a Buchi Rotavapor-R rotary evaporating apparatus,
and then dried at 140 °C. The dried powder was subsequently
calcined to remove ammonium nitrate by heating according to
34
2~~~~~~
~.., the following schedule: 4.5 hr at 30-300 °C, then 1 hr at 300
°C.
The calcined powder was ground, sieved (<106 micron mesh)
and uniaxially pressed at 10,000 psi into a pellet which was
fired according to the following schedule: 1.5 hr at 30-940
°C, then 6 hr at 940 °C.
Analysis of the X-ray diffraction pattern of the fired
pellet showed the presence of cordierite as the sole
crystalline species. Density was determined td be 2.6 g/cc, or
98 % of theoretical.
Example 15
This example illustrates the preparation of a
mullite-silica dense body by the method of our invention with
a bismuth sintering aid, with clay (kaolinite, A1203~2Si02) as
the mixed oxide source, and with the bismuth oxide sintering
aid precipitated onto the clay.
! A bismuth stock solution was prepared by dissolving
bismuth nitrate pentahydrate (Bi(ND3)3~5H2o, 582 g) in
concentrated nitric acid (384 mL) and then diluting with water
to a final volume of 4 L.
A mill jar (U.S. Stoneware Roalox burundum-fortified, 0.3
gallon) was charged with 30 burundum cylinders (U. S.
stoneware, 13/16 x 13/16), clay (R. T. Vanderbilt Peerless (TM)
Clay 3, 125 g,. 46.8 atom % Si and 48.2 atom % A1), and 300 mL
deionized water. The mixture was ball-milled for 72 hr, after
which the clay-water slurry was transferred and diluted with
35
water to a volume of 1 L, giving a slurry composition of 0.125
g clay/mL slurry.
into a vessel were placed 1 L of the above clay slurry, 1
L deionized water, and 200 mL concentrated ammonium hydroxide.
The mixture was homogenized 15 min. using a Janke & Kunkel
Ultra-Turrax mixing apparatus. Finally, 332.2 mL of the above
bismuth stock solution (5.0 atom % Bi) were added to the
mixture, which resulted in the precipitation of the bismuth
species onto the clay. The resultant was homogenized for 10
min.
The thus obtained precursor material was collected by
suction filtration and dried at 140 °C. The dried powder waa
subsequently calcined to remove residual ammonium nitrate by
heating according to the following schedule: 4.5 hr at 30- 300
°C, then 1 hr at 300 °C.
The calcined powder was ground, sieved (<106 micron mesh)
and uniaxially pressed at 10,000 psi into a pellet which was
fited according to the following schedule: 1.5 hr at 30-1100
°C, then 12 hr at 1100 °C.
Analysis of the X-ray diffraction pattern of the fired
pellet showed the presence of mullite and silica as the sole
crystalline species. Density was determined to be 3.05 g/cc,
or 97 percent of theoretical.
36
2~~~~~~
~.. Exam le 16
This is a comparative example not according to the method
of our invention, in which the preparation of a mullite-silica
dense body is attempted using clay as a mixed oxide source,
without any sintering aid.
Clay (R.T. Vanderbilt Peerless Clay 3, 51.1 atom % A1,
and 48.9 atom % Si) as the mixed oxide source was uniaxially
pressed at 10,000 psi into a pellet which was fired according
to the following schedule: 1.5 hr at 30-1100 °C, then 100 hr
at 1100 °C.
Analysis of the X-ray diffraction pattern of the fired
pellet showed the presence of mullite and silica as the
crystalline species. Density was determined to be 2.1 g/cc, or
77 % of theoretical.
Exam le 17
This example illustrates the preparation of a willemite
dense body by the method of our invention, with a bismuth
sintering aid selectively coated onto the silicon oxide.
A bismuth stock solution was prepared by dissolving
bismuth nitrate pentahydrate (Bi(N03)3~5H20, 582 g) in
concentrated nitric acid (384 mL) and then diluting with water
to a final volume of 4 L.
Into a vessel were placed colloidal silica (Ludox (TM),
15.02 g, 32.3 atom % Si), 200 mL deionized water, and 7.7 mL
concentrated ammonium hydroxide. The resultant mixture was
37
homogenized 5 min. using a Janke & Kunkel Ultra-Turrax mixing
apparatus. To this mixture was added 30.9 mL of the above
bismuth stock solution (3.0 atom % Bi), an action which
resulted in the precipitation of the bismuth species onto the
silica. The mixture was homogenized for 5 min. Finally, zinc
oxide (New Jersey Zinc Radox (TM) 930, 16.27 g. 64.7 atom %
Zn) was added and the mixture homogenized for 15 min.
The thus obtained precursor material was collected by
suction filtration and dried at 140 °C. The dried powder was
subsequently calcined to remove residual ammonium nitrate by
heating according to the following schedule: 4.5 hr at 30- 300
°C, then 1 hr at 300 °C.
The calcined powder was ground, sieved (<106 micron mesh)
and uniaxially pressed at 10,000 psi into a pellet which was
fired according to the following schedule: 1.5 hr at 30-1000
°C, then 12 hr at 1000 °C.
Analysis of the X-ray diffraction pattern showed the
presence of willemite and a bismuth silicate (Bi2Si05) as the
only crystalline species. Density was determined to be 4.25
g/cc, or 98 percent of theoretical.
Exam le 18
This example illustrates the preparation of a willemite
dense body according to our invention using a peritectic
composition of bismuth oxide-boron oxide (81:19 mole ratio of
bismuth oxide:boron oxide) as a sintering aid, with the
bismuth oxide portion of the sintering aid selectively coated
38
'~' onto the silicon oxide.
A bismuth stock solution was prepared by dissolving
bismuth nitrate pentahydrate (Bi(N03)3~5H20, 582 g) in
concentrated nitric acid (384 mL) and then diluting with water
to a final volume of 4 L.
Into a beaker were placed colloidal silica (Ludox (TM),
22.53 g, 32.3 atom % Si), 200 mL deionized water, and 15 mL
concentrated ammonium hydroxide. The resultant mixture was
homogenized 5 min. using a Janke & Runkel Ultra-Turrax mixing
apparatus. To this mixture was added 0.172 g of boric acid
(H B03, 0.6 atom % B), and 39.54 mL of the above bismuth stock
3
solution (2.55 atom % Bi), an action which resulted in the
precipitation of the bismuth species onto the silica. The
suspension was homogenized for 5 min. finally, zinc oxide (New
Jersey Zinc Radox (TM) 930, 24.41 g, 64.57 atom % Zn) was
added and the mixture homogenized for 10 min.
The thus obtained precursor material was stripped of free
wafer using a Buchi Rotavapor-R rotary evaporating apparatus,
then dried at 140 °C. The dried powder was subsequently
calcined to remove ammonium nitrate by heating according to
the following schedule: 4.5 hr at 30-300 °C, then 1 hr at 300
°C.
The calcined powder was ground, sieved (<106 micron mesh)
and uniaxially pressed at 10,000 psi into a pellet which was
fired according to the following schedule: 1.5 hr at 30-1000
°C, then 12 hr at 1000 °C.
"~,---
39
""~ Analysis of the X-ray diffraction pattern of the fired
pellet showed the presence of willemite as the sole
crystalline species. Density was determined to be 4.28 g/cc,
or 100 % of theoretical.
Exam le 19
This example illustrates the preparation of a willemite
dense body according to our invention, using a peritectic
composition of bismuth oxide-boron oxide (55:45 mole ratio of
bismuth oxide:boron oxide) as a sintering'aid, with the
bismuth oxide portion of the sintering aid selectively coated
onto the silicon oxide.
A bismuth stock solution was prepared by dissolving
bismuth nitrate pentahydrate (Bi(N03)3~5H20, 582 g) in
concentrated nitric acid (384 mL) and then diluting with water
to a final volume of 4 L.
into a beaker were placed colloidal silica (Ludox (TM),
22.53 g, 32.2 atom % Si), 200 mL deionized water, and 15 mL
concentrated ammonium hydroxide. The resultant mixture was
homogenized 5 min. using a Janke & Kunkel Ultra-Turrax mixing
apparatus. To this mixture was added 0.44 g of boric acid
(H3B03, 1.53 atom % B), and 28.96 mL of the above bismuth
stock solution (1.87 atom % Bi), an action which resulted in
the precipitation of the bismuth species onto the silica. The
suspension was homogenized for 5 min. Finally, zinc oxide (New
Jersey Zinc Radox (TM) 930, 24.41 g, 64.4 atom % Zn) was added
and the mixture homogenized for 10 min.
40
The thus obtained precursor material was stripped of free
water using a Buchi Rotavapor-R rotary evaporating apparatus,
then dried at 140 °C. The dried powder was subsequently
calcined to remove ammonium nitrate by heating according to
the following schedule: 4.5 hr at 30-300 °C, then 1 hr at 300
°C.
The calcined powder was ground, sieved (<106 micron mesh)
and uniaxially pressed at 10,000 psi into a pellet which was
fired according to the following schedule: 1.5 hr at 30-900
°C, then 12 hr at 900 °C.
Analysis of the X-ray diffraction pattern of the fired
pellet showed the presence of willemite as the sole
crystalline species. Density was determined to be 4.2 g/cc, or
100 % of theoretical.
Exam le 20
This is a comparative example not according to the method
of our invention, in which the preparation of a willemite
t
dense body is attempted without the use of a sintering aid.
Into a vessel were placed colloidal silica (Ludox (TM),
15.02 g; 33.3 atom % Si), 200 mL deionized water, and zinc
oxide (New Jersey Zinc Radox (TM) 930, 16.27 g, 66.7 atom %
Zn). The resultant mixture was homogenized 15 min. using a
Janke & Runkel Ultra-Turrax mixing apparatus. The thus
obtained precursor powder was collected by suction filtration
and dried at 140 °C. The dried powder was subsequently
calcined according to the following schedule: 4.5 hr at 30-300
---
41
°C, then 1 hr at 300 °C.
The calcined powder was ground, sieved (<106 micron mesh)
and uniaxially pressed at 10,000 psi into a pellet which Was
fired according to the following schedule: 1.5 hr at 30-1000
°C, then 12 hr at 1000 °C.
Analysis of the X-ray diffraction pattern of the fired
pellet showed the presence of zinc oxide and willemite as the
crystalline species. Density was determined to be 3.3 g/cc, or
80 % of theoretical.
Exam le 21
This example illustrates the preparation of a silica
dense body by the method of our invention, with a bismuth
sintering aid.
A bismuth stock solution was prepared by dissolving
bismuth nitrate pentahydrate (Bi(N03)3~5H20~ 582 g) in
concentrated nitric acid (384 mL) and then diluting with water
tova final volume of 4 L.
Into a vessel were placed colloidal silica (Ludox (TM),
75.21 g, 95.7 atom % Si), 200 mL deionized water, and 50 mL
concentrated ammonium hydroxide. The resultant mixture was
homogenized 5 min. using a Janke & Kunkel Ultra-Turrax mixing
apparatus. To this mixture were added 75.0 mL of the above
bismuth stock solution (4.3 atom % Bi), an action which
resulted in the precipitation of the bismuth species onto the
silica. The mixture was homogenized for 10 min.
42
2Q~~~~~
The thus obtained precursor material was collected by
~~~~.-
suction filtration and dried at 140 °C. The dried powder was
subsequently calcined to remove residual ammonium nitrate by
heating according to the following schedule: 4.5 hr at 30-300
°C, then 1 hr at 300 °C.
The calcined powder was ground, sieved (<106 micron
mesh), and uniaxially pressed at 10,000 psi into a pellet
which was fired according to the following schedule: 1.5 hr at
30-1100 °C, then 12 hr at 1100 °C.
Analysis of the X-ray diffraction pattern of the fired
pellet showed tridymite (a crystalline silica) and a bismuth
silicate (Bi2Si05) as the only crystalline species. Density
was determined to be 2.51 g/cc, or 98 percent of theoretical.
Exam le 22
This is a comparative example not according to the method
of our invention, in which the preparation of a silica dense
body is attempted without the use of a sintering aid.
Colloidal silica (Ludox (TM)) was placed in a beaker and
dried at 140 °C. The resulting powder was ground, sieved (<
106 micron mesh) and uniaxially pressed at 10,000 psi into a
pellet which was fired according to the following schedule:
1.5 hr at 30-1100 °C, then 12 hr at 1100 °C.
Analysis of the X-ray diffraction pattern of the fired
pellet showed the presence of cristobalite as the sole
crystalline species. Density was determined to be 1.87 g/cc,
or 80 percent of theoretical.
