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Description  |
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FIELD OF THE INVENTION
The present invention relates to the field of electronic devices and materials. More specifically, the present invention relates to dielectric composite materials having a polymeric matrix comprising dense ceramic beads dispersed therein and
further relating to the methods for making the ceramic beads.
BACKGROUND OF THE INVENTION
In the field of electronic devices, there is a need for materials having a relatively high dielectric constant for passive circuit elements such as capacitors, filters, and the like. It is particularly desirable to have such materials in forms
that comprise a polymer matrix (as a thin, flexible sheet or as a printable composition) in order to incorporate capacitive elements into polymer thick-film circuits and other low-cost electronic assembly methods. The simple incorporation of dielectric
powder, such as barium titanate, into a polymer binder is limited to some degree by the volume fraction of powder that can be incorporated while maintaining adequate; fluidity or workability of the material. This can be alleviated to some degree by
proper control of particle size distribution (J. J. Felten, U.S. Pat. No. 5,744,285). It would be useful to have a material that has a high dielectric constant wherein the material comprises dense, ceramic microspheres that could easily be controlled
and manipulated to solve many of these problems.
OBJECTS OF THE INVENTION
Accordingly, it is an object of the present invention to produce spherical beads of ceramic dielectrics for passive electronic devices.
It is another object of the present invention to produce large numbers of substantially mono-sized dielectric beads for incorporation into a polymer matrix or coating.
It is yet another object of the present invention to produce dielectric beads containing selected dopants to modify the dielectric properties.
It is a further object of the present invention to produce dielectric beads by hydrothermal reaction of titanium oxide (TiO.sub.2) or zirconium oxide (ZrO.sub.2) with selected soluble salts, particularly alkaline earths.
It is still yet a further object of the present invention to produce polymeric films having ceramic dielectric beads dispersed therein.
It is another object of the present invention to produce polymeric films having dispersed ceramic dielectric beads that are capable of being electrically poled.
It is a further object of the present invention to produce capacitors and filters in which the dielectric medium is a polymer composite comprising dense dielectric beads.
Further and other objects of the present invention will become apparent from the description contained herein.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a dielectric medium comprising a polymeric matrix having ceramic spherical beads dispersed therein, wherein the spherical beads have a
substantially high density and wherein the polymeric matrix has a dielectric constant <20 and the spherical beads have a dielectric constant >100.
In accordance with another aspect of the present invention, other objects are achieved by an electrical capacitor comprising a dielectric medium disposed between two conductive electrodes wherein the dielectric medium comprises a polymeric matrix
having sintered ceramic spherical beads dispersed therein. The spherical beads have a substantially high density and the polymeric matrix has a dielectric constant <20 and the spherical beads have a dielectric constant >100.
In accordance with yet another aspect of the present invention, other objects are achieved by a prefired ceramic dielectric for polymer thick-film capacitors comprising a polymeric matrix having sintered ceramic spherical beads dispersed therein. The spherical beads have a substantially high density and the polymeric matrix has a dielectric constant <20 and the spherical beads have a dielectric constant >100.
In accordance with still yet another aspect of the present invention, other objects are achieved by a method for making a dielectric composite material comprising the steps of forming spherical hydrous metal oxide beads by a sol gel process
wherein the beads comprise at least one Group IVb metal species. Then calcining the beads to form sinterable, spherical beads and sintering the spherical beads to a desired density and a desired grain size. Then dispersing the beads in a polymer matrix
to form a dielectric composite material.
In accordance with another aspect of the present invention, other objects are achieved by a method for making an electrical capacitor comprising the steps of forming spherical hydrous metal oxide beads by a sol gel process wherein the beads
comprise at least on Group IVb metal species. Then calcining the beads to form sinterable spherical beads and sintering the spherical beads to a desired density and a desired grain size. Then dispersing the beads in a polymer matrix to form a planar
dielectric composite material and applying electrodes to opposite surfaces of the dielectric composite material.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of an internal gelation process used to make the dielectric beads of the present invention.
FIG. 2 is a schematic diagram of an apparatus suitable for mass producing the dielectric beads of the present invention in a continuous or semi-continuous manner.
FIG. 3 illustrates a device employing the dielectric beads of the present invention as a single layer of substantially mono-sized beads.
FIG. 4 illustrates a device employing the dielectric beads of the present invention as a mixture of several diameters in order to maximize volumetric packing density of the high dielectric phase.
FIG. 5 illustrates one process for poling the inventive dielectric composite in order to increase the effective dielectric constant of the material.
FIG. 6 is a schematic diagram of a coprecipitation process used to make the dielectric beads of the present invention.
FIG. 7 is a schematic diagram of an alkoxide process used to make the dielectric beads of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is designed to allow the fabrication of small, dense, generally spherical beads of dielectrics with selected compositions for use in passive electrical devices such as capacitors, filters, and the like. It is contemplated
that the beads described and claimed in the present invention are sintered to as high a density as possible. High density not only makes the beads stronger and less susceptible to breakage during further processing, but it also allows one to maximize
the amount of high-dielectric material incorporated into the composite material. Thus, the sintered density of the beads of the present invention is preferably greater than 80% and more preferably greater than about 95%. The spherical beads are
polycrystalline, both as formed and after sintering. The spherical dielectric beads of the subject invention are preferably formed from porous, generally spherical beads of hydrous titanium or zirconium oxide made by a sol-gel process to form
substantially rigid beads having a generally fine crystallite size and correspondingly finely distributed internal porosity. The resulting gel bead is washed and hydrothermally reacted with a soluble alkaline earth salt such as barium, lead or
strontium, under conditions of elevated temperature and pressure to convert the bead into a mixed hydrous titanium- or zirconium-alkaline earth oxide while retaining the generally spherical shape. This mixed oxide bead is then washed, dried, and
calcined to produce the desired (BaTiO.sub.3 or SrZrO.sub.3) crystal structure. Alternatively, the gel beads may also be made by a coprecipitation route using many of the same process steps but eliminating the hydrothermal conversion step. The calcined
bead is then sintered to form a dense bead of the dielectric phase suitable for incorporation into a polymer matrix for various electronic devices and components. The composite material may be poled after fabrication in order to increase the effective
dielectric constant of the material.
The beads of the present invention may be small to the naked eye (e.g., 10 .mu.m average diameter) but it will be appreciated that they are substantially different from precipitated powders (as described, for instance, by Hirano et al.,
"Preparation of Ba.sub.2 Ti.sub.9 O.sub.20 Ceramics by Hydrolysis of Metal Alkoxides," Adv. In Ceramics 19, 139-46 (1980) even though the individual powder particles might also have a generally spheroidal shape. The points of distinction are: 1. The
"spheroidal powder particles" are much smaller, typically 0.2 .mu.m. The "beads" of the present invention are generally larger than about 10 .mu.m. 2. Spheroidal powders are too fine to conveniently disperse in a polymer matrix and are instead
intended to be consolidated into a dense monolithic body by conventional ceramic processing routes, at which point they will have lost all of their original spheroidal structure. 3. Powder particles are usually small single crystals. The beads of the
present invention are polycrystalline, both as formed and after sintering.
Those skilled in the art will readily appreciate that the inventive dielectric beads, even at the smallest size range contemplated, are clearly different from sinterable "powders" even though the powders may in some cases have a generally
spheroidal shape before consolidation and sintering.
A porous, generally spherical bead of hydrous titanium or zirconium oxide is made by a sol-gel process in which an aqueous droplet containing soluble titanium or zirconium is placed into a surrounding non-aqueous fluid medium, wherein surface
tension causes the droplet to assume a nearly perfect spherical shape. A chemical reaction causes the precipitation of hydrous titanium or zirconium oxide, followed by gelation of this oxide to form a substantially rigid bead having a generally fine
crystallite size and correspondingly finely distributed internal porosity. The resulting gel bead is washed and placed into a hydrothermal reactor with a soluble alkaline earth salt (such as barium, lead or strontium) and reacted under conditions of
elevated temperature and pressure to convert the bead into a mixed hydrous titanium- or zirconium-alkaline earth oxide while retaining the generally spherical shape. This mixed oxide bead is then washed, dried, and calcined to produce a desired (e.g.,
BaTiO.sub.3) crystal structure. The calcined bead is then sintered to form a dense bead, for example of the BaTiO.sub.3 phase, suitable for incorporation into various electrical devices and components. The composite material may be poled after
fabrication in order to increase the effective dielectric constant of the material.
The dense spherical beads can be used in materials such as a polymer matrix as a thin, flexible sheet or as a printable composition. The dense spherical beads making up the printable composition may have diameters that are about equal to the
final film thickness, allowing them to be dispersed into a mono-layer and easily contacted by electrodes on both sides of the film. These spheres can be easily poled to increase capacitance of the device. Alternatively , three sizes of dense spheres
can be blended in order to get very high solids loading (perhaps as high as 96%) while keeping a substantially fluid mixture for ease of printing or film extrusion. The dielectric spheres can also be dispersed in a fluid medium to create a dielectric
liquid having greatly increased dielectric constant compared to pure fluorocarbon or hydrocarbon dielectric liquids; at the same time, the spheres are large enough to prevent unwanted agglomeration or electrorheological behavior. The beads of the
present invention have an average diameter of about 10 .mu.m and larger. The unique properties of larger diameter and the polycrystalline structure (both as formed and after sintering) of these dielectric beads of the present invention substantially
distinguish the spherical beads of the present invention from spheroidal powder particles described in the literature.
One particularly suitable method for making precursor materials of hydrous titanium oxide gel bead or the hydrous zirconium oxide gel beads for use in the present invention is an internal gelation process generally taught by Collins in U.S. Pat. No. 5,821,186, incorporated herein by reference. Other suitable methods include external gelation (see U.S. Pat. No. 5,420,086 by Brandau et. al., for a description of external gelation processes and chemistries, incorporated herein by reference),
coprecipitation, water extraction (see for example, U.S. Pat. No. 5,062,993 by Arnold et al., describing the method of injecting droplets of an aqueous metal oxide sol into a "forming solution" of 2-ethylhexanol, incorporated herein by reference),
alkoxide processes, and others as are well known in the field of inorganic synthesis of ceramic materials.
In one preferred embodiment of the present invention, the hydrous titanium oxide gel bead or the hydrous zirconium oxide gel bead is made by an internal gelation process in which a chilled broth containing acidified titanium tetrachloride, an
organic base such as hexamethylenetetramine (HMTA), and a complexing agent such as urea is injected into a heated column of silicone oil (Dow Corning Silicone Fluid 200). Heating causes hydrolysis reactions to occur, whereby hydrous titanium oxide
precipitates within the injected droplets as colloidal particles, which gel to form a rigid, porous bead during the residence time in the column. This preferred embodiment for making the hydrous titanium oxide gel beads or hydrous zirconium oxide gel
beads uses optimum formulations and conditions. These optimum formulations and conditions create an optimum process parameter window. The initial concentrations of the listed constituents in the broth and the order of mixing these chemicals are
important. By controlling the parameters of the broth and the reactions, it is possible to affect the final characteristics of the gel, such as size, shape, porosity and density. The key parameters of this method for making these hydrous titanium oxide
or hydrous zirconium oxide gel beads include concentrations of the constituents of the broth, broth stability, reaction temperature, gelation and the structure and chemical composition of gels formed. The constituent concentrations influence the broth
stability, gelation times and types of gels. This bead is then washed and placed into a sealed container where it is hydrothermally reacted with an alkaline earth (typically Ba) to form a mixed hydrous oxide while retaining its spherical shape. This
mixed oxide is calcined and sintered to form a dense spherical bead of the desired phase (generally BaTiO.sub.3 or its analogues).
The four principal reactions involved in preparing hydrous titanium oxide from chilled broths containing acidified titanium tetrachloride, HMTA, and urea is shown as follows:
Complexation/decomplexation,
hydrolysis,
HMTA Protonation,
HMTA Decomposition,
The major constituents for most broths used for making microspheres of hydrous metal oxide are HMTA, urea and a metal salt. Urea serves as a complexing agent for the metal (reaction 1) and at certain concentrations allows for stable broths to be
prepared at .about.0.degree. C. that remain clear and free of gelation or precipitation for reasonable periods of time. As the temperature of the broth droplets rises in the hot organic medium, decomplexation occurs (reaction 1) and thus allows
hydrolysis of the titanium (reaction 2). HMTA, a weak organic base, drives the hydrolysis reaction to completion. At first, the HMTA molecules are singularly protonated (reaction 3). After most of the HMTA molecules (.about.95%) are protonated;
however, they begin to decompose (reaction 4) into ammonium ions, which make the system even more basic. Each protonated HMTA molecule can effectively remove three additional hydrogen ions. The reaction products are ammonium chloride and formaldehyde.
In addition to being a complexing agent, urea also functions as a catalytic agent in the decomposition of protonated HMTA molecules.
In another preferred embodiment, a hydrous mixed oxide gel bead is made by an internal gelation process using a chilled broth containing soluble Ti.sup.+4, Zr.sup.+4, or Hf.sup.+4 and one or more additional metals in soluble form, preferably lead
or one of the lanthanide elements. This broth is injected into a heated column of silicone oil. Heating causes reactions to occur analogous to those described above, whereby a hydrous mixed oxide precipitates as colloidal particles, which gel to form a
rigid, porous bead during the residence time in the column. This bead is washed and dried, and then calcined and sintered to form a dense spherical bead of the desired phase (such as PbTiO.sub.3, PbZrO.sub.3 or various rare earth titanates and their
analogues).
In another preferred embodiment, a dielectric phase is produced by coprecipitation in an aqueous droplet while the droplet is suspended in an immiscible medium such as silicone oil. This embodiment is particularly suitable for making compounds
such as lead titanate or rare-earth perovskite compounds by a sol-gel or alkoxide route.
In another preferred embodiment, a hydrous mixed oxide gel bead is made by an alkoxide process. This method is based on the hydrolysis of alkoxides in a droplet of Dowanol.RTM. suspended in mineral oil. A solution of lead acetate trihydrate
and titanium isopropoxide dissolved in Dowanol.RTM. PM (Dow Chemical, Midland Mich.) was partially hydrolyzed by addition of water at a water:OR ligand ratio between 1 and 2. The partially hydrolyzed solution was added dropwise to stirred mineral oil
heated to about 90.degree. C., forming hydrous Pb--Ti--O gel beads.
FIG. 1 illustrates schematically a preferred embodiment of the basic chemical flow chart 10 of the present invention using an internal gelation process. Applicable gelation processes are internal gelation, external gelation, and water
extraction, although internal gelation is the most preferable process.
Illustrated in FIG. 2 and FIG. 2a is one embodiment of an apparatus that is suitable for carrying out the gelation step of the present invention. In this embodiment, the chilled broth is injected through a needle 3 into a column 11 of silicone
oil that is continuously recirculating from a heated reservoir 7. The broth droplets gel during their residence time in a downstream transport line 13 and are collected in a basket 15 for washing.
FIG. 2 illustrates a chilled broth is first formed and then added to the chilled apparatus broth pot 1 and processed through the gel-forming or spherule-forming apparatus. The system also includes a needle 3 that is used in a two-fluid nozzle 5
for placing broth droplets in the hot organic medium where they gel. The apparatus also includes a reservoir 7 for heating the organic medium, a pump 9 for circulating the organic medium, a chilled broth pot 1, a two-fluid nozzle system 5 for
controlling the size of the broth droplets, a glass gelation column (forming column) 11, a downstream transport line 13 to provide a residence time for the gel spherules to hydrolyze and solidify, and a product collector 15 for collecting and aging the
gelled spherules and also for separating the organic medium from the gelled spherules.
The organic medium reservoir 7 may comprise a stainless steel open-top rectangular container. One or more heating blades 17 may be positioned at the rear of the reservoir to heat the organic medium. A thermocouple 19 may be positioned in the
basket at the bottom and near the front of the reservoir and is connected to a temperature controller 21 that is used to control the organic medium temperature. A stirrer 23 with its shaft positioned away from the heating blade or blades 17 (other
blades not shown) and its impeller located near the bottom of the reservoir is used to mix and maintain the organic medium at a desired temperature. Occupying most of the front space in the reservoir 7 is a large removable basket 15 that serves as a
backup to prevent any spilled gelled spherules from being pumped out of the reservoir to the circulating pump.
The pump 9 is used to pump the hot organic medium from the reservoir 7 through a line to the vertically positioned glass gelation column 11. The flow from the pump 9 is divided into two streams that are controlled by manual valves. The flow of
one of the streams may be routed to a position above the center of the top of the gelation column 11. Vertically attached to this line may be a tube whose outlet end is inserted into the entrance of the gelation column. The tube is part of the
two-fluid nozzle 5 system that is used to control the size of the droplets. The other hot organic medium stream from the pump 9 is routed to a fitting at the bottom of the gelation column 11 and flows up through a shell 25 that surrounds the gelation
column 11. The hot organic medium over-flows at the top of the column, first, into the gelation column 11 and, then, into an overflow cup 27. A large tube 29 is connected to a fitting from the overflow cup 27 to route any overflow back to the hot
organic medium reservoir 7. During operation, the flow of organic medium from the heating shell 25 is normally adjusted to provide only a slight overflow.
As shown in FIG. 2a, the two-fluid nozzle system 5 is very simple. It comprises a needle 3 that is perpendicularly inserted through the wall of the tube to the midpoint of the hot organic medium carrier stream and is positioned approximately 5
inches above the entrance to the gelation column 11. The chilled broth is jetted into the laminar flowing oil by air pressurizing the broth pot 1, forcing the broth out a tube at the bottom of the broth pot through a short plastic line that is connected
to the needle 3. The size of the droplets formed is dependent upon the gauge of the needle used and the flow rates of the hot organic medium and of the broth.
The hot organic medium carrying the droplets from the two-fluid nozzle tube 5 flows directly into the central concurrent flow tube of the jacketed gelation column 11 where it is desirable for the droplets to begin to gel. On exiting the gelation
column 11, the gelling spheres flow into a serpentine transport line 13. This line is long enough (about 8-ft.) to allow the gelling spherules to have a total residence time of 25 to 35 seconds to the collection basket 15. The transport time also
includes the time the spherules are passing through the gelation column. The gelation column and serpentine transport system are designed to be a siphoning system with a gravity head of about 60-cm for oil temperatures in the range of about 45.degree.
to 100.degree. C.
The collection basket 15 is positioned above the hot organic medium reservoir 7, and is used to collect and separate the gel spherules from the hot organic medium as they exit the serpentine transport line 13. The collected gel spherules are
aged by lowering the collection basket 15 into the reservoir 7 for between 15 and 30 minutes, preferably 20 minutes. After aging, the bulk of the organic medium is drained from the gel spherules and the residual organic medium is removed by a series of
washing steps to remove the reaction impurities.
Another basket of similar design may also be positioned above the hot oil reservoir to filter the return organic medium from a tube, which is connected to the overflow drain line at the top of the gelation column.
Conversion of the hydrous titanium oxide gel beads is carried out in a Parr:reactor by first allowing the wet microspheres to equilibrate overnight in a solution of the hydroxide at the same concentration that will be used for hydrothermnal
treatment. After equilibration, the beads are placed in a stainless steel mesh basket and suspended in the reactor vessel with enough excess hydroxide solution to provide about a 50% volume head. The vessel is sealed and then heated to a desired
temperature and held for a desired time, typically several hours.
As will be illustrated in the following examples, the aforedescribed method can be carried out in a wide range of modifications to selectively synthesize beads having selected compositions and therefore selected dielectric properties.
EXAMPLE 1
The apparatus shown in FIG. 2 and FIG. 2a was used to make hydrous titanium oxide gel beads by the internal gelation process in accordance with U.S. Pat. No. 5,821,186, incorporated herein by reference. A broth containing containing acidified
titanium tetrachloride (1.64 M Ti in 2.25 M HCl), 3.09 M HMTA, and 3.09 M urea was made up to achieve the following targets: Mole ratio of HMTA/Ti=2.32 Mole ratio of urea/Ti=2.32 Ti concentration=78.56 g/L
A batch consisting of 80 mL of Ti solution, 41.25 mL deionized water, 92.3 mL of HMTA/urea solution was chilled and placed into the reservoir. The broth was injected into the silicone oil (Dow Corning 200) through an 18 gauge needle, forming
droplets about 500 .mu.m in diameter. Residence time of the droplets in the column was about 0.5 minutes, after which the resulting gel spheres had sufficient rigidity to collect them in the basket. The gel spheres were allowed to age for about 15
minutes in the basket, after which they were washed in trichloroethylene and dilute ammonium hydroxide and then stored in deionized water.
It will be clear to those skilled in the art that the size of the gel beads can be controlled by the diameter of the nozzle used to inject the broth, as well as by other commonly known techniques such as ultrasonic agitation (P. A. Haas,
"Formation of Uniform Liquid Drops by the Application of Vibration to Laminar Jets," Ind. Eng. Chem. Res. 31(3), 959-67 (1992), incorporated herein by reference.). Alternatively, the application of electric fields (W. G. Sisson et al., U.S. Pat.
No. 5,759,228 Nozzle for Electric Dispersion Reactor (1998), incorporated herein by reference) may also be used to control droplet size. It will further be clear that the density of the gel beads (i.e., the volume fraction solids) can be controlled by
the concentration of metal species in the feed broth and that the volume fraction solids will control the amount of shrinkage that the beads undergo during sintering. It can thus be appreciated that beads of virtually any desired size (from tens to
thousands of .mu.m) can be made by the process described.
EXAMPLE 2
The hydrous titanium oxide gel beads from the previous Example were about 500 .mu.m. These beads were suspended in a stainless steel mesh basket and placed into a Parr reactor in a solution of 0.1 M Ba(OH).sub.2 after overnight equilibration.
Additional solution was used that included Ba(NO.sub.3).sub.2 so that there was about 27 g of excess Ba(NO.sub.3).sub.2, corresponding to a 3:1 ratio of Ba:Ti by weight. The reactor was heated to 150.degree. C. while maintaining sufficient pressure to
prevent boiling of the solution and the sample was held for 6 hours. Upon removal, the beads were intact and had changed color (white to tan). The beads were dried and fired in air by heating at 10.degree. C./min to 1200.degree. C. and held for 1 min
at 1200.degree. C. The beads, some of which were cracked and some of which were still intact, were then crushed and examined by X-ray diffraction, which determined that they were substantially single-phase BaTi.sub.4 O.sub.9.
EXAMPLE 3
A second conversion experiment was done using conditions similar to those in Example 2. Again, excess barium nitrate was added to achieve a Ba:Ti ratio of 3:1. Conversion was carried out for 8.5 h at 150.degree. C. Upon removal, the beads were
intact and had changed color (to medium brown). The beads were dried and fired in air by heating at 10.degree. C./min to 1200.degree. C. and held for 1 min at 1200.degree. C. The beads, about 80% of that were intact, were about 100 to 150 .mu.m in
diameter.
EXAMPLE 4
Another conversion experiment was done using conditions similar to those in Example 3, except that the gel beads were suspended in a Pyrex container. In this case, excess barium nitrate was added to achieve a Ba:Ti ratio of 4: 1. Conversion was
carried out for 9 h at 215.degree. C. Upon removal, the beads were intact and had changed color (to medium brown). The beads were dried and fired in air by heating at 10.degree. C./min to 1200.degree. C. and held for 1 min at 1200.degree. C. The
beads, about 70% of that were intact, were about 100 to 150 .mu.m in diameter. Some beads were then crushed and examined by X-ray diffraction, which determined that they were substantially single-phase BaTi.sub.5 O.sub.11.
EXAMPLE 5
Another conversion experiment was done using conditions similar to those in Example 3. In this case, 0.5M Ba(OH).sub.2 was used as the hydrothermal solution. Conversion was carried out for 4 h at 200.degree. C. The beads were dried and fired
in air by heating at 2.5.degree. C./min to 1200.degree. C. and held for 1 min at 1200.degree. C. Some beads were then crushed and examined by X-ray diffraction, which determined that they were substantially single-phase BaTiO.sub.3.
EXAMPLE 6
The inventive process can be further modified to produce selected dielectric phases such as lead titanate by coprecipitation, thereby eliminating the hydrothermal treatment step. A solution was prepared using 40 mL of deionized water and 42.68 g
of lead acetate and heated to fully dissolve the lead acetate. This solution was added to a broth containing 80 mL of chilled Ti stock solution, and 96.5 mL HMTA/urea stock solution as described previously. Upon mixing, a precipitate was formed (owing
to the low solubility of the lead acetate). This mixture was added dropwise into the heated silicone oil (92.degree. C.) to form Pb-Ti-O gel beads. These were allowed to age as described in Example 1, and then washed and dried as before. This
material was dried and calcined at 800.degree. C. and XRD showed that the PbTi.sub.3 O.sub.7 phase was obtained.
EXAMPLE 7
A similar coprecipitation test was done as described in Example 6 but using Pb(NO.sub.3).sub.2 with a target Pb:Ti ratio of 1:1. This material was determined to be the PbTi.sub.3 O.sub.7 phase after calcining at 800.degree. C. These results
suggest that the material was not the desired 1:1 stoichiometry, perhaps because the limited solubility of Pb caused nonuniform precipitation when the lead salt was added to the chilled broth.
EXAMPLE 8
Many dielectric compositions have been developed for various purposes. For example, compositions based on rare earth titanates (e.g., NdTiO.sub.3 and modifications thereof) do not have as high a dielectric constant as barium titanate, but are
used when a very low temperature coefficient is needed. These formulations may be prepared in a manner analogous to the process described in Example 1 by substituting Nd(NO.sub.3).sub.3.6H.sub.2 O for some of the TiCl.sub.4 in the appropriate molar
ratio.
EXAMPLE 9
We also conducted preliminary tests of an alternative technique to make dielectrics such as lead titanate. This method is based on the hydrolysis of alkoxides in a droplet of Dowanol.RTM. suspended in mineral oil. A solution of lead acetate
trihydrate and titanium isopropoxide dissolved in Dowanol.RTM. PM was partially hydrolyzed by addition of water at a water:OR ligand ratio between 1 and 2. The partially hydrolyzed solution was added dropwise to stirred mineral oil heated to about
90.degree. C. This process initially yielded well-formed gel spheres of mixed lead-titanium oxide but as the spheres aged, they agglomerated resulting in a thick mass. After drying the mass and calcining at 700 .degree. C. we obtained lead titanate,
which as shown by x-ray diffraction was predominately in the Macedonite structure. It is clear that with a larger process vessel and longer aging of the beads, this process yields well-formed gel spheres of mixed lead-titanium oxide suitable for further
processing.
The importance of this observation is that it demonstrates that the inventive dielectric composites (dense ceramic beads suspended in a polymer matrix) can be made by a variety of routes with dielectric ceramics of many useful compositions.
All of the exemplary results are summarized in the following table:
TABLE 1 Summary of Dielectric Material Preparations.sup.a Gel Composition Conversion Sintering Phases/Comments HTiO.sup.b None 10.degree./min to 1000.degree. C., hold 1 min Rutile; cracked beads 125-175 .mu.m HTiO.sup.b None 5.degree.
C./min to 100, hold 1 hr, Anatase 10.degree. C./min to 700.degree. C. HTiO.sup.b 0.1M Ba(OH).sub.2 + added 10.degree./min to 1200.degree. C., hold 1 min BaTi.sub.4 O.sub.9 ; about 50% Ba(NO.sub.3).sub.2 for Ba:Ti = 3:1 cracked 6 h at 150.degree.
C. HTiO.sup.b 0.1M Ba(OH).sub.2 + added 10.degree./min to 1000.degree. C., hold 1 min BaTi.sub.5 O.sub.11 ; about 50% Ba(NO.sub.3).sub.2 for Ba:Ti = 4:1 cracked 9 h at 215.degree. C. HTiO.sup.b 0.1M Ba(OH).sub.2 + added 10.degree./min to
1200.degree. C., hold 1 min About 20% cracked; Ba(NO.sub.3).sub.2 for Ba:Ti = 3:1 beads .about.100-250 .mu.m 8.5 h at 150.degree. C. HTiO.sup.b 0.1M Ba(OH).sub.2 + added 10.degree./min to 1200.degree. C., hold 1 min BaTi.sub.5 O.sub.11 ; about 30%
Ba(NO.sub.3).sub.2 for Ba:Ti = 4:1 cracked 9 h at 215.degree. C. HTiO.sup.b 0.5M Ba(OH).sub.2 4 hr at 2.5.degree./min to 1200.degree. C., hold 1 BaTiO.sub.3 + minor 200.degree. C. min unknown phase Pb-Ti-O.sup.c gelled None Calcined 800.degree. C. PbTi.sub.3 O.sub.7 as spheres but agglomerated during aging in the oil Pb-Ti-O.sup.d some None Calcined 800.degree. C. PbTi.sub.3 O.sub.7 spheres, a lot of agglomeration Pb-Ti-O.sup.e better None Calcined 700.degree. C. PbTiO.sub.3 formation
of beads .sup.a Summary of about 30 separate drying, calcining, and sintering experiments .sup.b Hydrous titanium oxide microspheres were made as described in [1]. .sup.c Lead acetate trihydrate and titanium isopropoxide dissolved in Dowanol .RTM.
PM; hydrolyzed as droplets in stirred mineral oil at 90.degree. C. .sup.d Coprecipitation from titanium chloride and lead nitrate with HMTA/urea by injecting solution into silicone oil at about 92.degree. C. .sup.e Coprecipitation from titanium
chloride and lead acetate with HMTA/urea by injecting solution into silicone oil at about 92.degree. C.
It will be understood that the inventive process may be further modified by the addition of various dopants and modifiers as are well known in the art. As illustrated by the foregoing examples, compositional modifications can be incorporated
either into the gelation process by coprecipitation (in the case of additives such as Pb, Nb, rare earths, and transition metals that will form insoluble hydrous oxides) or into the hydrothermal step (in the case of soluble species such as Mg, Ca, Sr,
etc.).
It will be appreciated that Applicants' material and method may be employed for a very wide variety of applications. These include primarily capacitors and filters, but the inventive materials can be used for any application where it is desired
to have a polymeric material with a high dielectric constant.
Shown in FIG. 3 is one useful embodiment of the inventive material to form a capacitor 30 whereby a dielectric composite material 31 is a dielectric medium comprising a polymeric matrix 32 having a monolayer of ceramic beads 33 arranged between
two electrodes 34. The composite material 31 may be made in bulk as a flexible polymer tape by well-known tape-casting or "doctor blade" techniques and diced or slit to convenient sizes. The electrodes may be applied to this tape either before or after
dicing. Alternatively, if a large capacitance is needed, a long length of the material 31 may be electroded and then rolled into a cylindrical geometry with an interlayer of insulating paper or the like to prevent contact of the two electrodes. The
matrix 32 may alternatively be formulated as a printable vehicle containing polymer and solvent, whereby the dielectric composite material 31 and electrodes 34 may be printed onto a substrate to form a capacitor. This capacitor would not require firing
in order to achieve high capacitance because the beads 33 within the material's matrix have already been fired before printing. The dielectric composite material in this case is a prefired ceramic dielectric material. The general principles of
printable thick-film formulations are well known in the art, and many suitable polymer/solvent combinations are available. The particular combination chosen for a specific situation will depend on the substrate material, feature size to be printed, and
other relevant factors as are well understood by those skilled in the art.
The inventive composite material may also be formulated as a paintlike material for coating selected surfaces to modify their dielectric properties. For these applications, conventional water-based or solvent-based paint vehicles may be used.
Alternatively, a more durable coating may be formulated by dispersing the dielectric beads in a thermosetting system such as epoxy or the like, and curing the coating after application. It will be understood that the conformal coating described here may
be applied as a single layer or as a plurality of layers, whereby the dielectric properties of the coating may be graded. Successive layers might contain differing bead size, composition, volume fraction, etc. For example, the first (inner) coating
might contain barium titanate beads synthesized generally in accordance with EXAMPLE 5 and a second (outer) coating might contain lead titanate beads synthesized generally in accordance with EXAMPLE 7 and having a lower dielectric constant, whereby a
graded structure is produced.
It will be further understood that conformal coatings as contemplated herein may additionally contain other functional or inert components such as plasticizers, colorants, light-absorbent pigments, etc. as are well known in the art.
Skilled artisans will appreciate that dielectric spheres as illustrated in FIG. 3 will exhibit the phenomenon of dielectric resonance at predictable frequencies t | | |
