CA2619954A1

CA2619954A1 - Ceramic polymer dielectric fabrication

Info

Publication number: CA2619954A1
Application number: CA 2619954
Authority: CA
Inventors: David A. Kelly
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-01-14
Filing date: 2008-01-14
Publication date: 2009-07-14

Abstract

The manufacture of high performance ceramic polymer capacitors with high dielectric constant involves a number of process steps that differ significantly from current manufacturing methods. The processing is specifically designed to ensure that the ceramic powder is of the highest quality with minimal amount of inert material present. The selection of the type of polymer is specific to other elements of the capacitor's construction, intended range of temperature operation, the ceramic dielectric powder ratio and composition. Finally the specific type of electric processing prior, during and post the polymer cure is specific to each ceramic polymer formulation.

Description

FIELD OF THE INVENTION
The field of the invention relates to the manufacture of high energy density ceramic polymer capacitors. A number of specific design steps and manufacture processes are required to fabricate a ceramic polymer dielectric with the desired characteristics.

BACKGROUND OF THE INVENTION
Traditionally ceramic polymer capacitors have had a very low dielectric constant relative to that of the ceramic powder used in its manufacture. The dielectric constant often is less than 1/501h that of the ceramic powder used in its fabrication. With the cost of the ceramic polymer capacitor being mostly that of the ceramic powder, increasing the final dielectric value will make the capacitors smaller and therefore less expensive to manufacture. Furthermore, ceramic dielectric powder manufactured for ceramic capacitors is not in a form that is best suited for use in a ceramic polymer capacitor.
The ideal ceramic polymer capacitor would have a dielectric constant that is close to that of the ceramic powder being used in its manufacture, reducing greatly the cost of manufacture to that similar or lower than that of conventional all ceramic capacitors. A good example of a high energy density ceramic polymer capacitor design is represented by Canadian patent application 2,562,986.
The purpose of the preferred embodiment is to process ceramic powders using methods that maximize their dielectric properties for use in the manufacture of any type of ceramic polymer capacitor.

SUMMARY OF THE INVENTION
The present invention provides a manufacturing process that produces a ceramic powder with the highest possible dielectric constant, for a given ceramic material, for use in a ceramic polymer dielectric. The preferred embodiment starts with the synthesis of often sub micron sized ceramic powder, of a preferred chemical composition. After fabrication into individual crystals of the desired size and composition, the ceramic powder is subjected to an annealing process to repair damage to individual crystals and to establish a preferred crystal form. The ceramic powder is next processed to select that portion of the powder that has the highest dielectric constant. This removes any inert material which could be poorly formed crystals or ceramic particles with inappropriate chemical composition. The preferentially selected ceramic powder is then milled, often in an electrically conductive fluid to reduce damage that may occur if the ceramic has piezo electric properties. The Ceramic powder solution is then filtered to remove large particulates, dried then mixed with a solvent. Next a polymer compound is added in a specific ratio to the weight of ceramic powder present. The solvent adjusts the viscosity of the mixture and it is often selected with at least one end polymer group that will catalyze with the polymer during a later cure process. Next a suitable catalyst is added, often with a slow cure that may be accelerated with the application of elevated temperatures. The mixture is then deaired in a vacuum and coated onto a substrate with the required thickness, that is often, but not limited to, less than 25 microns.
The ceramic polymer layer is the main dielectric layer of a capacitor. The substrate is a plastic carrier film with electrical electrodes already present on its surface. The electrical electrodes are of a self healing type, where an electric short circuit between capacitor dielectric layers will vaporize the metal from the carrier film, in the area of the short circuit, disconnecting the short for the remaining portion of the capacitor. After coating the excess solvent is removed by evaporation from the ceramic polymer layer which is then assembled into a capacitor structure. An electrical voltage, DC and AC of a specific frequency and amplitude previously determined for the ceramic polymer dielectric layer, is applied across the capacitor. The DC and AC causes the ceramic particles to aggregate together into solid regions between the capacitor electrodes. After the enhancement process is finished the ceramic polymer compound is cured, often with DC and AC still applied. The capacitor is then subjected to a number of tests to remove devices that fail to meet a set of specifications.
In yet another preferred embodiment the ceramic polymer dielectric layer is fabricated using ceramic powder that is processed to remove ceramic particles with a below average dielectric constant. First the ceramic powder is combined with an electrically insulating fluid and the mixture is passed between two electrically conductive belts, moving in the same direction as the fluid flow, with an AC voltage of a previously predetermined frequency and amplitude, applied between them.
At the output end of the belts the fluid stream closest to the belts is separated from that of the middle portion. The middle portion of the ceramic powder fluid mixture contains ceramic powder with a lower dielectric constant than that removed from the closest to and the conductive belts themselves, which has the highest dielectric constant. The process is often repeated and the rejected ceramic powder is often returned to an earlier stage of the ceramic synthesis.
In yet another preferred embodiment the ceramic polymer dielectric layer is fabricated using ceramic powder that is processed to remove ceramic particles with a below average dielectric constant. In this embodiment the ceramic powder is placed on a flat electrically conductive, vibrated moving belt. AC of a predetermined amplitude and frequency is then applied between the electrically conductive belt and the electrode at its output end. The ceramic particles with the highest dielectric constant will then fall under the influence of gravity with a trajectory different from those with a lower dielectric constant. The two ceramic particles streams are collected separately. The process is often repeated with the rejected ceramic powder if often returned to an earlier stage of the ceramic synthesis.
In other preferred embodiments a multi-layer rectangular capacitor construction is used for ceramic polymer capacitors that use a highly piezo electric ceramic or highly piezo electric polymer as part of the dielectric layer.
In other preferred embodiments the coefficient of thermal expansion of the ceramic polymer dielectric is selected to be the same as the other materials that make up the ceramic polymer capacitor. Often the other elements that the ceramic polymer dielectric coefficient of thermal expansion has to be similar to is the carrier substrate that has on its surface the capacitor electrode structure and the end electrical terminations.
Other embodiments use the preferred method of fabrication of a ceramic polymer dielectric layer but instead of fabricating a capacitor the structure is used as a piezo electric device. In this preferred embodiments either the polymer, ceramic or both have a high quality of mechanical expansion when an electrical voltage is applied across them. In this application the electrical poling of the piezo electric device should not be confused with the dielectric electrical enhancement process as both have a different purpose.
All embodiments may use compound ceramics such as barium titanate crystals that are coated on their outside with an electrically insulating thin aluminium oxide coating.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 Represents a process flow chart of the preferred embodiment;
FIG. 2 Represents a preferred embodiment that preferentially separates out ceramic particles with higher dielectric constant than the bulk material average.;
FIG. 3 Represents another preferred embodiment that preferentially separates out ceramic particles with higher dielectric constant than the bulk material average.;
FIG. 4 Represents the construction of a typical multi-layer rectangular capacitor.

DETAILED DESCRIPTION OF THE INVENTION
The preferred embodiment is represented by the flow chart in Figure 1. The process starts at 100, where the ceramic dielectric powder is first synthesized 101, which may be performed through sol gel, oxide synthesis, chemical precipitation etc. The powder at this stage is often not fully converted into individual crystals of the desired form. The ceramic powder is compacted and fired at high temperatures represent by 102, the temperature and length of time is specific for each ceramic material to produce the desired particle size. After this stage the ceramic powder is often milled 103 in a liquid solution which is often electrically conductive if the ceramic is piezo electric, which is specifically selected not to leach or chemically react with the ceramic. At this point it may be necessary to repeat step 102, again specific to the type of ceramic powder that is being fabricated.
After the milling 103 the ceramic is fired at a much lower temperature 104, often less than 400 Celsius for 4 hours to heal any minor damage done to the ceramic crystals during the prior milling process.
The next process is called dielectric concentration 105 and is used to separate ceramic material that has a higher than average dielectric constant from the bulk material. The process 105 passes the ceramic powder through a high intensity electric field, see the explanation of Figure 2 and 3 for details. This process is often used for the manufacture of ceramic polymer capacitors with high dielectric constant. Often 20% or more of the ceramic particles are rejected during this process and sent back to the start 101 for reprocessing.
The ceramic powder is often then lightly milled for a final time 106, but it is done in a conductive liquid such as water with a small amount of dilute acid containing hydrochloric acid or ammonium hydroxide to improve the conductivity of the solution. The selection of the additive used to make the water conductive will depend on the type of ceramic that is being processed. The use of chemical additives in water such as hydrochloric acid or ammonium hydroxide ensure no residue will be left after drying as these two chemicals revert to a gas during the final drying. The wet milling of the ceramic in an electrically conductive solution prevents a small amount of damage that occurs when highly piezo electric individual ceramic particles are subjected to intense shock such as that encountered during milling. The highly conductive liquid shorts out the charge that is developed on the ceramic particle surfaces, reducing clumping and damage to the ceramic particle if the intensity of the voltage is sufficient to short across the crystal surface.
Ceramics that are not piezo electric will not necessarily require the use of a conductive liquid for this milling stage. This process is unique to ceramic powder used in ceramic polymer capacitors because there are no further high temperature firing such as that used in the final sintering of a ceramic capacitor. Improvements in the value of the ceramic polymer capacitor dielectric may be as high as 15%, depending on the type of ceramic that is used.
After milling 106 the solution is filtered 107, to remove any large particles, washed and dried at stage 108. Next solvent, often with at least one vinyl end group or another end group that will catalyze, if solvent residue remains, when the ceramic polymer solution is cured at stage 117. The use of a solvent that can be catalyzed ensures that no free solvent residue will be left after the curing process. The amount of solvent used is adjusted to give a solution with the desired viscosity for later step 114. Additional copolymer, the main polymerization component of the polymer mixture, may be required depending on the amount of residue solvent present during the final cure process.
Next the polymer that is used in the final ceramic polymer dielectric is added at 110. The mixture is then thoroughly mixed 111, often using a light mixing process to ensure thorough mixing of the ceramic particles. The catalyst 112 may be added just before the completion of the mixing or immediately afterwards. Often the mixture is deaired 113, by passing it through a chamber that is evacuated to remove trapped air. The mixture is then transferred to 114 where the ceramic polymer is applied to a carrier substrate, that often has the electrode structure deposited on its surface as part of a capacitor or used as a removable carrier for the fabrication of thin sheets of ceramic polymer dielectric. The solvent is then removed 115, often using slight heating.
During this stage electrical enhancement 116 of the orientation of the ceramic particles is performed by applying an intense AC
electric field across the surface of the drying ceramic polymer mixture and is done at this stage only with those ceramic polymer films that are not assembled into a capacitor prior to curing. The enhancement of the electrical properties of the dielectric is made using the behavior of ceramic particles in an insulating fluid under the influence of an external electric field called the "Winslow Effect". For example enhancement at this stage is used for ceramic polymer that is used in the fabrication of embedded capacitors inside a printed circuit board. However, the ceramic polymer is usually formed into the final capacitor structure at this stage 116 and an electric voltage is applied across the capacitor, consisting of a combination of AC and DC, that is empirically determined and specific to each combination and type of ceramic and polymer used.
After the electrical enhancement the ceramic polymer mixture is cured 117 and the capacitor tested at 118. This completes the manufacture of the ceramic polymer dielectric 119, often as the main dielectric in a capacitor.

The electrical enhancement process may be used in the fabrication of ceramic polymer dielectrics used in the fabrication of piezo electric transducers. The electrical enhancement should not be confused with the process called `poling' of a piezo electric ceramic polymer dielectric for the purpose of enhancement of its piezo electric properties.
Another aspect of the invention is the formulation of the ceramic polymer to match the coefficient of thermal expansion with that of the other elements of the capacitor. For example one type of ceramic polymer capacitor uses metal film electrodes deposited on a plastic carrier film such as polyester or polypropylene. In these capacitors the ceramic polymer is sandwiched between two electrodes, which are metalized plastic film, with Figure 4 representing a typical cross-section of this construction. The common carrier films are polyester (also known by a common trade name as Mylar) with linear Coefficients of Thermal Expansion or the common short form `CTE' of 60 to 65ppm/ C, polypropylene 100 to 200ppm/ C (parts per million per degree Celsius) and a fiber based carrier such as Kraft paper. A common ceramic used in ceramic polymer capacitors is Barium Titanate with a CTE of 12ppm/ C. Commonly used polymers are silicones with CTE
200 to 400ppm/ C and polyamides 25 to 30ppm/ C and epoxies 40 to 90ppm/ C.
A ceramic polymer capacitor made using a specific type of polypropylene carrier film may require the ceramic polymer dielectric to have a CTE of 200ppm/ C to minimize mechanical stress.
If Barium Titanate is the ceramic then the family of polymers that is often used is silicones preferably with a CTE close to 400ppm/ C. An equal mixture, by volume of Barium Titanate and a silicone polymer would produce a mixture with a CTE of approximately 206ppm/
C, close to the polypropylene carrier material with electrodes deposited on its surface. For a ceramic polymer capacitor using polyester film, with the capacitor electrodes on its surface, having a CTE of 60-65ppm/ C then a polymer with a CTE of approximately 120ppm/ C is required for a 50:50 blend of polymer and Barium Titanate powder. Such polymers are not readily available off the shelf. To make a polymer with a CTE of 120 it is possible to use a silicone with a CTE
of 200ppm/ C or add modified silicone polymers to a polyamide or epoxy base. The silicones are modified to have end groups that will catalyze with the polyamide or epoxy polymer. The higher CTE
of specialty silicone polymers are then blended with a lower CTE polymer to create a polymer with the desired CTE.
A second consideration is the geometry of the ceramic polymer capacitor. A
rectangular multi-layer ceramic polymer capacitor, see Figure 4, is more tolerant of a CTE
mismatch between the ceramic polymer dielectric layers and the electrode carrier layer. The rectangular multi-layer capacitor design allows the ceramic polymer dielectric to expand vertically without restriction. The capacitor's ability to expand vertically accommodates the piezo expansion that ceramics such as Barium Titanate undergo when subjected to strong electric fields. Of course this assumes that the polymer has sufficient flexibility to tolerate the magnitude of the piezo electric mechanical expansion. Further it is assumed that the capacitor end terminations are sufficiently flexible to accommodate the CTE of the stacked capacitor and often a conductive flexible polymer is used for that purpose.
Figure 2 represents a dielectric concentration process that involves dispersing the ceramic powder in an electrically insulating fluid such as silicone oil and passing the fluid between two closely positioned, moving electrically conductive belts, connected to AC of opposite polarity. The ceramic particles with the highest dielectric constant will be preferentially drawn towards the belts and those of lower dielectric constant will remain in the center of the fluid stream. Figure 2 is a side view, where 200 represents a source of AC, the frequency and amplitude is best to be imperatively determined and is related to such variables as the ceramic particle size, fluid viscosity, velocity of flow, spacing and length of the conductive belts etc.. 201 represents the direction of the flow of the ceramic powder fluid mixture, 202 and 203 represent electrically conductive belts that move in the direction of the fluid. At the output 204 and 206 represent a ceramic fluid flow that has ceramic particles with a higher dielectric constant than the flow exiting at 205. The output flows 204 and 206 often include a belt scrapper to prevent the ceramic particles from building up on the belts.
Figure 3 represents another dielectric concentration process that involves placing the ceramic powder 302 on a moving belt. The moving belt 301 is often vibrated to ensure the ceramic powder forms an uniform layer. The moving belt is electrically conductive and connected to one polarity of AC source 300. An AC electric field is set up between moving belt 301 and electrical conductor 303. The ceramic particles exit the belt and continue to travel in the direction of the belt as they fall under the influence of gravity. The particles with the highest dielectric constant preferentially stick longer to the belt and partially follow the belt as it turns around its pulley. This means the ceramic particles with the higher dielectric constant will fall in a path as that shown by 304 and those with lower dielectric constant follow a path further from the belt 305. The ceramic particles are collected in a divided bin 306. For both Figure 2 and 3 it may be necessary to repeat the process a number of times to achieve the desired degree of enhancement.
Figure 4 represents a cross section of a rectangular multi-layer ceramic polymer capacitor where 400 and 404 represent the capacitor end terminations. The ceramic polymer dielectric is 401, 403 is one capacitor electrode polarity and is electrically isolated from 402 the opposite electrode.

The electrodes on the left side, 403 are connected together to the end terminations 400 and 404 using a flexible electrical conductive polymer 405. Further the electrodes on the right side, 402 are connected together to the end terminations 407 and 408 using a flexible electrical conductive polymer 406. The end terminations 400, 404, 407 and 408 are often a substrate with electrically isolated terminations on their opposite sides, in this example left 400 & 404 and right 407 & 408.
The metalized electrodes 402 and 403 are often metalized plastic films such as polypropylene, polyester or deposited on a fiber based material such as Kraft paper.
In variations of the preferred embodiment it is possible to form layers of ceramic polymer on a removable carrier film, spray on or print the electrically conducting electrodes 402 and 403 on each layer as they are stacked. This embodiment is similar to that currently used to manufacture MLC ceramic capacitor structures. Often the assembly is then isotropically hot pressed using high pressure with elevated temperature to fuse the layers together into a solid void free matrix.
Alternately, the carrier film with self-healing electrode structure deposited on its surface is isotropically hot pressed into the capacitor structure along with the ceramic polymer dielectric layers, using high pressure and heat to form a void free structure. These two preferred embodiments are often used for ceramic polymer capacitors where the ceramic content exceeds by volume greater than 70%.
The ceramic fabrication process is compatible with all types of ceramic dielectric powder, even those with a compound layered structure, for example where the inner part is Barium Titanate and outer layer is thin Aluminum Oxide, used to reduce the electrical leakage of the ceramic powder.
It should be noted that anyone skilled in the art can apply the dielectric concentration process and the electrical enhancement process to the fabrication of any nano-material that has dielectric properties differing from the fluid it is suspended in.
Although the invention has been described in connection with a preferred embodiment, it should be understood that various modifications, additions and alterations may be made to the invention by one skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A method of separating ceramic particles, with dielectric constant below the bulk average from ceramic particles with dielectric constant above the bulk average, from a quantity of ceramic powder; and a) the ceramic powder to under go separation is first mixed with an electrically insulating fluid;
and b) the fluid is passed between two electrically conducting belts; and c) the belts are moving in the same direction as the fluid; and d) where the electrically conducting belts are connected to opposite polarities of a source of AC
voltage; and e) the frequency and amplitude of the applied AC is specific to the arrangement being used; and f) the ceramic fluid mixture is separated into three or more streams; and g) where the fluid ceramic mixture streams closest to the belts and the ceramic removed from the belts are combined together into a first fluid ceramic mixture; and h) the fluid ceramic stream from the flow equal distant from the belts is collected separately as a second fluid ceramic mixture; and i) the ceramic powder from first fluid ceramic mixture has dielectric constant average different from the second fluid ceramic mixture; and j) the ceramic powder is removed from the fluid ceramic mixture that has the desired dielectric constant; and k) the ceramic from the rejected fluid ceramic stream is rejected and sent to be reprocessed; and l) the process is often repeated a number of times until the degree of dielectric constant concentration is achieved with the preferred ceramic.

2. A method of separating ceramic particles, with dielectric constant below the bulk average from ceramic particles with dielectric constant above the bulk average, from a quantity of ceramic powder; and a) the ceramic powder to under go separation is first placed on an electrically conductive, moving belt; and b) at the output end of the belt, where ceramic powder falls off the end a second electrically conductive electrode is placed; and c) and the belt and electrode are connected to opposite polarities of an AC
voltage; and d) the frequency and amplitude of the applied AC is specific to the arrangement being used; and e) the ceramic powder is separated into two columns as it falls from the end of the belt; and f) where the ceramic powder that falls closest to the belt is collected into a first ceramic powder bin; and g) where the ceramic powder that falls furthest from the belt is collected into a second ceramic powder bin; and h) the ceramic powder from first ceramic powder bin has dielectric constant average different from the second ceramic powder bin; and i) the ceramic powder that has the desired dielectric constant is kept for use in an application;
and j) the rejected ceramic powder is sent to be reprocessed; and m) the process is often repeated a number of times until the degree of dielectric constant concentration is achieved.

3. A ceramic polymer capacitor where the ceramic powder component is annealed at lower temperature to heal damage and often to set the powder into a preferred crystal form prior to mixing with a polymer compound for use at a later stage of manufacture.

4. A ceramic polymer capacitor where the ceramic powder component last milling process is performed using an electrically conductive fluid.

5. A ceramic polymer capacitor where the ceramic powder component is processed such that it has a higher dielectric component than the bulk ceramic powder average at a process stage prior to mixing with a polymer compound for use at a later stage of manufacture.

6. A piezo electric transducer made up of a piezo ceramic polymer material that has used the application of electric fields across the ceramic polymer mixture prior to curing to enhance the piezo electric properties prior to the curing of the ceramic polymer compound.

7. A multiplayer ceramic polymer capacitor structure that has the electrode structure deposited on the carrier substrate and the carrier substrate along with the ceramic polymer dielectric is isotropically hot pressed into a void free matrix.