Patent Application
20180342353
The present invention relates generally to polymeric monolithic capacitors and, in particular, to capacitors produced using a non-thermoplastic submicron thick polymer dielectric, chemically configured to maximize self-healing and minimize moisture absorption, along with aluminum metal electrodes that are also configured to maximize capacitor self-healing properties, maximize capacitor energy density, and minimize breakdown failures in high-temperature applications, while at the same time maintaining high pulse and ripple current carrying capacity of the capacitor.
The polymeric monolithic capacitor (PMC) has a polymer dielectric and metallized electrodes and has a structure generally similar to that of a metallized film capacitor (MFC). MFCs are generally produced by utilizing a thermoplastic polymer film dielectric that is metallized to form electrodes, then slit into bobbins which are wound in pairs or stacked to form a capacitor. As a result of the corresponding process of manufacture, however, an MFC does not possess a monolithic structure because it is produced in atmospheric conditions and includes air-filled micro-gaps between the constituent layers.
Self-healing is an advantage of MFC capacitors, when compared to other capacitor technologies such as electrolytic, ceramic and polymer dielectric capacitors with aluminum foil electrodes. For example, such self-healing function is accomplished by combining a relatively low melting point electrode with high conductivity, such as aluminum and zinc, with patterned electrodes, which provide a fuse-like action when a dielectric breakdown occurs (see, for example, US Publication No. 2014/0347780, U.S. Pat. No. 6,040,038A, European Patent No. 0438344A1 and U.S. Pat. No. 7,027,286B2). Thin electrodes that have higher resistance also improve self-healing by reducing the energy dumped into a short and by facilitating the fusing action by reducing the energy required to melt and evaporate the electrode material around the breakdown site (see, for example, US Publication No. 2004/0232465A1). A major limitation of MFC capacitors is that the film must be thick enough for adequate uniformity and strength to be further handled for metallization, slitting, and winding into capacitors.
In stark contradistinction to the MFC, the PMC capacitor is produced in a vacuum by forming thousands of polymer dielectric layers and metallized electrode layers using a single-process step that does not require handling of the metallized dielectrics. The lack of air between the layers, due to the vacuum environment during production, and interlayer bonding turns the resulting structure into a substantially monolithic structure. The high-speed vacuum deposition process of monomer materials to produce thin polymer dielectrics in the vacuum and PMC capacitor technology was originally developed to overcome the thickness limitations of MFC dielectrics (see, for example, U.S. Pat. Nos. 4,954,371 and 5,018,048). The individual polymer dielectric layers in a PMC capacitor structure are pinhole free, and the overall structure is not touched or brought in contact with anything or exposed to air during the process of manufacture until and after a stack of thousands of polymer/metal layers has been already formed (including protective layers on the bottom and top surface of the stack). As a result, the polymer dielectric layers of the PMC structure can be formed to be as much as 100 times thinner that common polymer film dielectrics. This advantage, combined with a wide range of physical and dielectric properties of the PMC structure, results in volumetrically efficient capacitors that can be used in applications that are traditionally served by other capacitor technologies (such as, for example, MFCs, electrolytic, and Ceramic Multilayer Capacitors (MLCs)).
Higher voltage applications that require low-loss dielectrics and self-healing properties are usually served with metallized polypropylene (PP) film capacitors, as long as the capacitor temperature does not exceed about 105° C. As the voltage requirement is reduced, so is the thickness of the PP film. However, the degree to which the thickness of the PP film can be reduced is limited by the film-manufacturing process, and the resulting thickness of PP films is typically no less than about two microns. This limits the voltage at which such a film capacitor can be efficiently used to about 300 Volts DC (VDC). Therefore, lower voltage applications (such as 25V-50V) that require large capacitance and small size cannot use PP film capacitors. Such applications will typically utilize MLC capacitors that are not self-healing or electrolytic capacitors that have other limitations, such as high dissipation factor (DF) and high Equivalent Series Resistance (ESR).
Some applications that are addressed by PMCs include DC-link capacitors, for example, used in inverters of hybrid and electric vehicles. Such capacitors have capacitance of the order of 100s of microfarads and are used to minimize ripple current, voltage fluctuations, and to suppress voltage/current transients. Key characteristics of such DC-link capacitors used in voltage-sourced inverters of electric drive vehicles include self-healing properties to assure a benign failure mode, withstanding high ripple currents, low dissipation factor (DF), high capacitance, and high operating temperature. The list of these requirements effectively excludes the use of electrolytic capacitors and MLCs. Metallized PP capacitors that are almost exclusively used in such DC-link applications have an operating temperature limited to 105° C. with significant derating in voltage, ripple current and capacitor lifetime and, furthermore, PP capacitors are relatively large and costly. Therefore, there remains a need—at least in the automotive industry—to reduce DC-link capacitor size and to extend the capacitor's upper operating temperature to at least 125° C. and preferably as high as 140° C., or even higher.
Since its inception, PMC capacitor technology has undergone several improvements targeted toward producing mainly low-capacitance, low-voltage (typically less than 100 V) surface-mounted capacitor chips, and operating at temperatures lower than 125° C. Such improvements include methods to segment the electrodes, improved bond strength between the layers to minimize delamination during solder reflow, and various process and equipment improvements (see, for example, U.S. Pat. No. 5,716,532, JP 2003303735A, US Publication No. 2009/0308532 and U.S. Pat. No. 6,195,249B1). Low-capacitance, low-voltage PMC capacitors can utilize thicker dielectrics without the need to maximize energy density and self-healing properties. However, automotive DC-link capacitor applications combine high voltage, high capacitance, high ripple current, low ESR, high operating temperature and small capacitor size and weight, which requires a unique set of PMC capacitor properties unlike those addressed by the prior art.
Yet another group of applications in which capacitors play a critical part in the final product is pulse power systems that deliver a pulse of energy in a short time period. Here, the search for smaller and better-performing capacitors remains ongoing. Examples of such applications include implantable defibrillators and a multitude of defense applications where size and weight are critical capacitor parameters. For example, in an implantable defibrillator, the capacitor occupies about 50% of the defibrillator volume and the aluminum and tantalum electrolytic capacitors that are currently used in this application can weigh more than all the other defibrillator components combined. At another extreme, a metallized PP capacitor bank used to fire a rail gun that could potentially be used in a tank is currently larger than the tank itself. Therefore, there is a need to develop new capacitor technologies to maximize operating temperature, to handle higher ripple and pulse currents, and to reduce the capacitor weight and volume, while improving capacitor lifetime and reliability.
The disclosure presented in the Detailed Description section of the present application will be better understood in conjunction with the following generally not-to-scale Drawings, of which:
This invention is directed at fabrication of polymer monolithic capacitors possessing self-healing properties that prevent catastrophic failure of a capacitor and result only in a benign failure mode, high temperature stability, resistance to degradation in high-temperature and high-humidity environments, and ability to handle high ripple currents, which present a unique set of properties demanded from the polymer monolithic capacitors by, for example, the automotive inverter applications.
The term “polymer monolithic capacitor” (PMC) is used to describe a capacitor that originates as a multilayer composite or a “mother capacitor” material produced around a rotating process drum with thousands of polymer dielectric and electrode layers. The mother capacitor material is removed from the drum and is processed into individual capacitors, via a series of process steps that include pressing the mother capacitor material, segmentation into individual capacitors, etching of the electrodes at the cut edges to prevent flashover and termination steps that include plasma ashing of the polymer at the termination edge to reveal the aluminum electrodes and arc spraying a metal over the exposed aluminum electrodes to connect the thousands of electrode layers and form a capacitor with two sets of electrodes.
A PMC is produced using a non-thermoplastic polymer dielectric, in contrast to wound film polymer capacitors, such as metallized film capacitors (MFCs), which comprise thermoplastic polymer materials (that is, materials capable of repeatedly becoming pliable when heated or reheated, and solid when cooled or recooled). In DC-Link applications, capacitors can be exposed to high ambient temperatures which, when combined with additional heating due to high frequency ripple currents (I), can cause a thermoplastic dielectric to soften. This softening degrades the contact between the nanothick metallized electrode layer and the arc sprayed electrode, which results in a higher resistance (R) that further increases heating (I*I*R). This limits both the performance boundaries of a wound film capacitor that utilizes a low temperature polymer (such as PP), as well as the capacitor life. PMCs, on the other hand, can comprise a highly cross-linked non-thermoplastic polymer material that does not melt, but slowly degrades at, for example, temperatures above 300° C. Furthermore, as discussed in more detail below, it is important that a glass transition temperature of the material of certain embodiments is well above any temperature that results in the above heating effects—this characteristic eliminates the possibility of a phase change which, although not comparable to a softening point, assures that there is absolutely no movement in the polymer dielectric (other than expansion forces) that can compromise the contact resistance between the metallized electrodes and the arc sprayed layer.
The terms “polymer dielectric” and “polymer dielectric layers” may be used interchangeably to describe a layer of polymer, which is produced by flash evaporation of a radiation-curable monomer material that is vaporized and condensed on a rotating drum, and then cured with electron beam or UV radiation.
The terms “cure”, “polymerization” and “cross-linking” may be used interchangeably to refer to the process by which a condensed monomer material is converted into a polymer dielectric material.
The terms “metal electrodes” and “aluminum electrodes” may be used interchangeably to define the polymer monolithic capacitor electrodes, of which aluminum electrodes are but one example.
The term “vacuum” as in a “PMC capacitor formed in the vacuum” means a partial atmosphere that is typically in the pressure range of about few torr to about 10−6 torr.
The term “electrode passivation” means that the metal electrodes (and aluminum metal electrodes) are exposed to an oxygen-containing plasma immediately after their deposition to form an oxide layer thereon such an Al2O3 oxide layer, which is the most stable and corrosion resistant aluminum oxide. If not passivated in this manner then, when the multilayer capacitor is exposed to humidity, a hydrated aluminum oxide is formed (Al2O3.(H2O)), which is not as protective as Al2O3, thereby leading to rapid electrode corrosion.
The terms “heavy edge” and “heavy electrode edge” are used to describe a section of the capacitor electrode, located outside the active capacitor area, that is made thicker than an active electrode of the capacitor to facilitate a lower resistance contact with a coating (such as an arc spray coating, for example) that connects the individual electrode layers together.
The terms “self-healing”, “self-healing event” and “clearing” may be used interchangeably, in reference to MFCs, to denote a process occurring during a breakdown of the capacitor's dielectric, as a result of which the electrical short between the two electrodes causes a current flow that melts the thin metal electrodes around the location of the short (like a fuse), thereby isolating the location of the fault, protecting the capacitor component from being destroyed by the dielectric breakdown, and allowing the capacitor to operate with virtually an insignificant amount of capacitance loss.
The terms “plasma ashing” and “plasma etch” refer to a process where a polymer material in the presence of an oxidative plasma is “ashed” or burned away as a result of conversion of the polymer material into low molecular weight gaseous products. This process is used to remove polymer at the edges of the monolithic capacitors to expose the metal electrodes so that the electrodes can be shorted together with an appropriate coating to form a capacitor termination.
The terms “stable capacitor operation”, “stable capacitance”, and “stable dissipation factor” are used to define a capacitor in which no sudden changes of capacitance or dissipation factor occur in the operating temperature range of the capacitor, and especially at higher temperatures. For example, a capacitor designed to operate at 125° C. or higher is considered to have stable capacitance if the capacitance is not reduced by more than 10% and/or not increased by more than 20% at the maximum operating temperature from that at 25° C. Similarly, the dissipation factor of a stable capacitor (a stable dissipation factor) at the maximum operating temperature can be lower than that at 25° C., but it does not increase more than about 20% at the maximum operating temperature.
The idea of the present invention stems from the realization that specific solutions to shortcomings in operational characteristics of existing capacitor structures can be achieved through unique combinations of the chemical structure of the polymer dielectric material, its physical properties, constrains in the thickness and resistivity of the metallized electrodes, treatment of the electrodes and choosing specific capacitor shapes that enhance electrical performance.
In particular, the answer to the problem of widening a temperature range of operation of the PMC and achieving the stable operation at temperatures as least as high as 125° C. or higher (for example, 140° C.) is provided by devising a polymer dielectric material the properties of which ensure that the dissipation factor (DF) of the capacitor is stable throughout the operating temperature range by selecting a polymer dielectric that has a glass transition temperature, Tg, that is higher than the maximum operating temperature. It was discovered that if the Tg of a polymer is between 25° C. and the maximum operating temperature, then at the Tg the dissipation factor of most radiation-cured polymers is increased. Furthermore, above the Tg, the polymer absorbs moisture which has detrimental effects both on the dissipation factor and electrode corrosion. Accordingly, the problem of increasing the range of operating temperatures across which the dissipation factor remains stable is addressed, by the present invention, by formulating a polymer dielectric material with Tg that is significantly higher than the maximum operating temperature.
In addition to satisfying the Tg condition, the capacitor of choice must have the ability to self-heal in the event that a breakdown occurs. A problem of maximizing the self-healing properties of a PMC was solved by devising the chemistry of the polymer dielectric material in which the carbon in the polymer is effectively removed in the form of gases (such as CO, CO2, CH4, and C2H6, for example). Specifically, an effective carbon removal process has been shown possible if the monomer materials used to produce the polymer dielectric have an average hydrogen-to-carbon ratio in excess of 1 and an oxygen-to-carbon ratio in excess of 0.1.
For high temperature and high current applications, another condition that needs to be satisfied is high corrosion resistance of the capacitor electrodes, especially if the electrode comprises aluminum. Generally, an aluminum electrode (when exposed to high temperature and DC voltage in the presence of humidity) undergoes an electrochemical corrosion that reduces the area of the electrode and increases its resistivity. This problem is addressed, at least in part, by passivating the aluminum layers of the PMC structure of the current invention in line with deposition of these layers. It was discovered that, in addition to conventional electrode passivation by exposure to a plasma, post baking of the mother capacitor material to temperatures, in the range of 200° C. to 280° C., also resulted in electrode passivation. The latter form of passivation is possible by creating organo-metallic bonds between the aluminum electrodes and polymer dielectric. Acrylate polymers formed by radiation curing contain certain percent of uncured monomer in the polymer lattice as well a small percent of acrylic acid and monomer that has undergone random scission. It was discovered that, at high temperatures, such functional monomer groups react with the aluminum electrodes, thereby preventing access of the aluminum by moisture.
Yet another condition that needs to be satisfied for high temperature operation of the capacitor of choice is the reduction of the heating losses in the capacitor. Current metallized film capacitors used in inverter applications can fail when high-temperature operation is combined with high ripple current, causing heating due to dissipation of power in the resistance of the electrodes. A problem of minimization of the power loss in the PMC is solved by judiciously structuring the capacitor such that a termination edge thereof is defined at the long edge surface of the PMC. In doing so, the mechanical strength of the termination edge of the PMC structured according to an embodiment of the invention is additionally increased by shaping the area of termination of the electrode layers as a heavy edge with the use of an appropriate metallic coating bonded at least to the electrode layers at the terminating surface(s) of the PMC.
According to one embodiment of the invention, the specific chemistries of monomer materials used to produce the polymer dielectric layer(s) are chosen to facilitate the capacitor self-healing process. In implementing the idea of the invention, PMCs were produced using a wide range of radiation-curable materials that included monomers incorporating carbon-to-carbon double bonds that brake with radiation, to cause the polymer dielectric to cross-link. In one example, such chemistries included acrylate monomers that are cured using electron beam and UV radiation. By choosing an acrylate chemistry that has high H:C and O:C atomic ratios, the self-healing properties were improved by aiding the formation of Al2O3, CO, CO2, CH3, C2H6 and other hydrocarbon gases that remove Al and C from the site of a breakdown of the carbon-to-carbon double bonds. The elimination of Al and C from the surface of the polymer layer as the electrodes melt back (or fuse), leads to a rapid, quick fusing action, during which a relatively small amount of current was dumped into the failure site. Such process minimizes the damage caused by the electrical short between the electrodes, thereby allowing the voltage across the capacitor electrodes to recover with minimum or no leakage current flow through the damaged dielectric layer. Indeed, in the alternative—when the surface of the polymer continues to hold a high level of carbon thereon (that is, remains “carbonized”)—the fusing action will be slowed down, which leads to a greater amount energy to be disposed at the failure site in a unit of time, causing additional breakdowns of adjacent dielectric layers. If not interrupted, this process can lead to a thermal runaway failure of the capacitor. While in most cases the capacitor does not necessarily fail as a result of such failure, it nevertheless loses a significant amount of capacitance. Furthermore, if carbon is not removed adequately from the polymer surface, it causes excessive leakage current that compromises the capacitor performance.
In order to compare the self-healing properties of the PMCs made with polymers having different chemical compositions, a test was performed to determine at least a leakage current through the capacitor after a large number of self-healing events. Initially, a mother capacitor material (with an area of 10 square feet) containing 2000 active capacitor layers was produced with different polymer dielectric materials, but keeping the same dielectric thickness (0.65 micrometer) and electrode resistivity (25 Ohms/square). Individual chips (each with an area of 500 mm2) were segmented from the mother capacitor material and processed into capacitors. Depending on the dielectric constant, the capacitors had a capacitance in the range of 30-35 μF. At least 50-80 capacitors from every batch were exposed to a DC voltage of 300 V or greater. Exposure of such thin polymer dielectrics to voltages equal to or exceeding 300 VDC caused clearings of weak spots or defects in the capacitor's dielectric material. After exposure to the DC voltage, various capacitor parameters were measured, including capacitance, dissipation factor, equivalent series resistance (ESR), and leakage current. Capacitors with polymer dielectrics that have acceptable self-healing properties had a leakage current of less than 1 μA. Capacitors with a polymer dielectric that did not have acceptable self-healing properties, had a leakage current exceeding 50 μA. It was empirically determined that the chemistry of the radiation-curable monomers had a significant impact on leakage current generated by the self-healing events and, ultimately, on the maximum voltage that could be applied to the capacitor before the catastrophic failure occurred. For example, a capacitor with a polymer dielectric comprising 2-phenyl-phenyl acrylate (the chain for which is shown schematically in
The 2-phenyl-phenyl acrylate has a hydrogen-to-carbon ratio of H:C=0.8 and an oxygen-to-carbon ratio O:C of 0.13, while the dodecane acrylate is characterized by H:C=1.86 and an O:C=0.14. In general, we found that, in order for a PMC capacitor designed for a high-temperature application to withstand maximum voltage with minimal degradation, it is necessary for the monomer molecules or mixture of monomers used for formation of the polymer dielectric material of such capacitor to have a hydrogen-to-carbon ratio H:C>1 and an oxygen-to-carbon ratio O:C>0.1 and, preferably, H:C>1.2 and O:C>0.1. While it was recognized that the presence of other atoms that can lead to formation of gaseous products capable of removing carbon from the polymer surface (such as nitrogen and fluorine, for example) was also beneficial, not many related embodiments of monomers were evaluated to develop a criterion.
A parameter that is equally critical for maximization of self-healing properties is the thickness of the metallized capacitor electrodes (which for many applications comprise thin metallized aluminum layers). Since thickness measurements are extremely difficult to conduct on nanometer-size metal layers, the thickness of the aluminum electrodes is quantified by their electrical conductivity or surface resistivity, measured in ohm per square (Ohm/square). This correlation works well for electrodes that include highly conductive low-melting point metals (such as aluminum, aluminum alloys), two-layer electrodes (such as aluminum/zinc, aluminum/copper, zinc/silver, and others). It was empirically found that, in addition to selecting polymers that have the proper H:C and O:C ratios, the resistivity of the metallized electrodes needs to be within a judiciously defined range in order to maximize the self-healing properties. Such choice of resistivity translates, in practice, into a lower electrode thickness with a heavy electrode edge (the aluminum layer is thicker outside the active capacitor area), to assure good electrical contact without impacting the capacitor self-healing properties.
It should also be noted that the better the self-healing properties, the higher the voltage (V) that can be applied to the capacitor without causing a catastrophic failure. The capacitor energy is determined by the relationship of ½*C*V2. Therefore the greater the applied voltage V, for a given capacitance C, the higher the energy that one can store in a capacitor. Experimentation indicated that, in conventional wound capacitors, air-filled micro-gaps contribute to the formation of the Al2O3 as well as to the removal of carbon in the form of CO and CO2, which aids the self-healing process. PMCs have no air gaps; therefore, in order to maximize the self-healing properties, in addition to oxygen supplied by the polymer (as described above), the electrode resistance value needs to be greater than resistance values of the conventional wound film capacitors. However, with increase of the electrode resistance, the capacitor Equivalent Series Resistance (ESR) increases, which increases the capacitor loss in the form of heat dissipated in the electrodes (I2R where R=ESR). It was determined that, for a high ripple current application where the continuous current is high, the maximum value of sheet resistance should be limited to less than 100 Ohm/square to minimize losses and to maintain a dissipation factor smaller than about 0.02. This range, however, is not the case for low repetition rate, high pulse voltage/current applications that have lower average continuous current. For these applications, according to an embodiment of the invention, we found that PMCs can have electrodes with sheet resistance values as high as 500 Ohm/square and dissipation factors greater than 0.02. In order to avoid high ESR that can result in high capacitance losses, the high electrode resistivity is counteracted by using thousands of electrodes in parallel and choosing a capacitor shape that limits the distance that the current has to travel to charge the capacitor. In this manner, the higher electrode resistivity maximizes the self-healing process, which in turn increases the breakdown voltage and energy density of the PMC capacitor, thereby resulting in a smaller capacitor volume and weight.
PMC capacitors designed for a high current DC-link application (with 60,000 layers having a polymer dielectric chemical structure characterized by a hydrogen-to-carbon ratio of greater than 1.0 and an oxygen-to-carbon ratio greater than 0.1, glass transition temperature Tg>100° C., dielectric thickness 0.6 μm, and aluminum-based electrodes with a sheet resistance of 30-40 Ohm/square in the active capacitor area and less than 5 Ohm/square in the area of the heavy edge, yielded parts with dissipation factor smaller than 0.02 and a breakdown strength corresponding to breakdown voltage of less than 250 V. Three of such capacitors connected in series (internally) were used to produce a 500 μF/600 V capacitor system which was approximately three times smaller than state of the art PP capacitors with lower ESR (that is, lower I2R heating losses). At the same time, the operating temperature of the so-produced PMC capacitor system was 140° C. versus 105° C. for conventionally-used PP capacitors. This evidenced a dramatic improvement in energy density and reliability of the capacitors configured according to an embodiment of the invention for demanding automotive applications, which, in turn, also translates to and results in smaller and more efficient and lower cost inverters.
In another example, PMC capacitors with the same dielectric material and thickness as those discussed above and including the same number of layers, were designed for high energy density pulse voltage applications and produced with electrodes that had a sheet resistance of approximately 200 Ohm/square in the active capacitor area and less than 10 Ohm/square in the area of the heavy edge. Such design yielded parts with dissipation factor>0.02 at 25° C. and a breakdown strength exceeding 550 V. Considering that the energy density of the capacitor varies as the square of the voltage, PMC capacitors with the higher resistivity electrodes, as discussed, are characterized with energy density that is about four times higher than PMC capacitors with the same dielectric thickness but lower resistivity electrodes. In fact, the energy density of the active capacitor area in these samples had a record value for a full-size polymer capacitor of 9.5 J/cc (assuming a maximum voltage of 500 V). A person of skill in the art will readily recognize that such operational parameters are not expected, are truly remarkable and attributed to the combination of a polymer dielectric (with chemical composition favorable to perpetuate self-sealing properties), a metallized electrode designed to maximize the capacitor self-healing properties, and a heavy edge electrode that allows for a reliable high current contact.
In another example, an 18,000 layer capacitor was produced using the same polymer dielectric chemistry as that disclosed in Examples 2 and 3, but with a polymeric layer thickness of 0.2 microns, designed for lower voltage applications. We found that at lower voltages, typically in the range from about 12 VDC to about 48 VDC, the electrode resistance in the active capacitor area becomes an even more critical parameter. This is because when a breakdown occurs, a significant amount of energy is required to melt and vaporize the electrode material in order for the capacitor to self-heal. This required energy is supplied by the capacitor itself as it discharges into the short, and is proportional to the square of the voltage at which the capacitor is charged. At lower voltages, a condition can be found where the capacitor cannot efficiently self-heal if forced to undergo multiple breakdown events, leading to a high resistance short. While an obvious solution to this situation would be to have a dielectric that is thick enough at a given voltage level to avoid multiple self-healing events (and thereby avoiding a short), such solution leads to a larger capacitor with lower energy density. Contrary to such otherwise obvious solution, and in order to maximize the breakdown voltage and energy density, the 18,000 layer capacitor was produced according to an embodiment of the invention to have sheet resistance in the active capacitor area of about 60 to about 80 ohm/square, instead of 30-40 ohm/square that would have been used with a 0.6 μm dielectric for a higher voltage application. This solution resulted in a capacitor that could undergo multiple breakdowns without leading to a high resistance short.
In the above Examples 2, 3, and 4, the polymer dielectric had Tg>100° C. For lower temperature applications, the high Tg may be not be necessary as long as the polymer H:C ratio, O:C ratio and electrode resistance criteria are met. Lowering the Tg opens the possibility of varying the polymer chemistry, increasing the number of available polymers (many of which have higher dielectric constants k—for example as high as k=9) that meet the self-healing requirements. The use of such polymers in the capacitor structure(s) at hand, in the related embodiments, can further increase the capacitor energy density. The combination of using higher dielectric constant polymers and meeting the H:C and O:C criteria with electrode sheet resistance values in the range from about 10 Ohm/square to about 100 Ohm/square (and more so in the range of 100 Ohm/square to 500 Ohm/square) was demonstrated to yield PMCs with maximized energy density, as such combination led to the realization of higher energy storage in the dielectric material with maximum self-healing properties of the capacitor structure.
In order to address the production of a polymer monolithic capacitor capable of operating reliably at elevated temperatures (up to 125° C. and higher, for example 140 degrees), a behavior of the dielectric loss of the polymer dielectric material was considered. For a stable operation of the PMC, it is important that the dissipation factor is stable throughout the desired operating temperature range. The curves 210, 220, and 230 of
In addition to the DF stability, the value of Tg imposes another limitation on the selection of the polymer dielectric for a PMC. Specifically, it was determined that the use of polymers with lower Tg values leads to absorption of moisture by the capacitor. That is, if the capacitor is operated in ambient conditions of humidity and temperature that is lower than the Tg, the dielectric layers will absorb ambient moisture which, in turn, impacts the stability of capacitance of the PMC. While polymer films such as polypropylene have Tg that is lower than room temperature, they do not absorb water due to the low polarity of the polypropylene chemistry. In comparison, acrylate and other radiation-curable chemistries incorporate higher polarity bonds (such as CO, CN, CF, for example), which may be advantageous because they result in higher polarity chemistry that leads to a higher dielectric constant and, therefore, an increase in energy storage by the resulting PMC. However, the same high polarity chemistry also leads to an increase of moisture absorption. The moisture absorption is minimal until the Tg is reached. Beyond the Tg temperature, the molecular relaxation that occurs at Tg can dramatically increase moisture absorption.
The absorption of moisture causes both short-term and long-term reliability issues with the performance of the capacitors performance. Short term, the capacitance value increases due to the high dielectric constant of water and, longer term, the water corrodes the thin capacitor electrodes leading to permanent capacitance loss. This is demonstrated in
As part of the test, capacitors fabricated with the first and second polymer dielectrics were exposed to the same conditions of temperature and humidity but with the addition of DC voltage bias. The illustration of
Very few radiation-curable monomers were found to result in polymers that combine both low DF and Tg that was higher than 125° C. (which is a target temperature for DC-link capacitors for automotive applications). Most such high Tg monomers have either cycloaliphatic chemistry or they have more than two double bonds. Trifunctional monomers such as tris (2-hydroxy ethyl) isocyanurate triacrylate and triallyl isocyanurate (the behavior of which is represented by curve 230 of
At the same time, it was discovered that certain monomers that result in highly cross-linked polymer dielectrics (at least two acrylate groups), such as for example hexane diol diacrylate with Tg=41° C., trimethyl propane triacrylate with Tg=62° C., and dimethyl propane tetracrylate with Tg=96° C., also have a stable DF at temperatures exceeding 125° C. due to the molecular weight and crosslinking nature of the monomer. When producing PMC capacitors with such polymer materials we found that when such capacitors were life tested at 125° C. with the application of DC voltage, there was high moisture absorption which rapidly degraded the capacitor properties. Therefore, the measurement of the temperature-dependent behavior of the DF alone is not adequate to determine if a polymer has a Tg that will make it usable for this invention. To determine the Tg of a polymer dielectric material, 2000 layers of polymer-only stacks were produced using the same radiation curing conditions as those used to produce a PMC. The complex modulus of the polymer dielectric was then measured as a function of temperature using a Dynamic Mechanical Analysis (DMA) methodology to determine the Tg. Therefore, the process of selecting a monomer formulation that leads to a high performance high temperature polymer dielectric is elaborate and not obvious to someone skilled in the art.
According to the teachings of the present disclosure, the geometry of a polymer monolithic capacitor is devised such as to reliably service a DC-link application that combines high ripple currents and high ambient temperatures. The related art teaches that, in order to increase the capacitor energy density of polymer monolithic capacitors, it is important to have capacitor electrodes with resistivity corresponding to the sheet resistance values in the range of 10 Ohms/square to 500 Ohms/square). The equivalent series resistance (ESR) of the capacitor, which depends on the sheet resistance, contributes to capacitor losses. While lower values of the ESR can be easily attained by increasing the conductivity of the electrodes (which can be done by increasing electrode thickness, for example), the thicker electrodes result in poor self-healing properties. The approach chosen for the purposes of the present disclosure is rooted, at least in part, in a judicious choice of the capacitor's geometry and is devoid of an overall increase of the thickness of the electrodes across the capacitor.
If one increases the electrode resistivity, for example, to maximize the self-healing properties, heating losses via I2R (where R denotes the equivalent series resistance (ESR) and I denotes the AC current) will increase the capacitor temperature. Such increased temperature, e.g., when combined with high ambient temperatures, can lead to the capacitor failure, mainly by accelerating the rate of electrode corrosion. We determined that to minimize the thermal effects of the current while also maintaining the electrodes with higher levels of sheet resistance, which aids the self-healing process, the geometry of the capacitor must be chosen to reduce the capacitor's ESR as much as it is practical. This goal can be achieved by shaping the capacitor's body to be elongated, extended on one direction to a greater degree than in another, transverse direction, and establishing the termination structure at the long side of so-shaped capacitor. The higher the target value of the sheet resistance of the electrodes, the higher the ratio of the extent of the termination edge to that of the other edge has to be. For example, a 2000 layer capacitor shaped as a square (when viewed perpendicularly to the capacitor's layers) and that has electrodes with the sheet resistance of 50 Ohms/square has an ESR at a resonance frequency of approximately 25—mohm. If the capacitor is five times as long along the termination side as it is wide, however, the ESR value drops to approximately five times to 5 mohm, and so do the heating losses.
It has been known that decreasing the thickness of PMC capacitor dielectrics increases breakdown strength (see U.S. Pat. No. 6,092,269). However, a high voltage capacitor with a rating in the range of 500 VDC to 1000 VDC requires a thick dielectric, typically in the range of 2.5 μm to 5.0 μm. Therefore, in order maximize the energy density of a PMC capacitor, multiple very thin dielectric capacitors connected in series can be utilized rather than a single capacitor with a thicker dielectric. One approach is to produce multiple lower voltage PMC capacitors, as shown in
In order to practice the invention and produce a PMC that can reliably function at temperatures as high as at least 125° C., the capacitor electrodes must be passivated. A mechanism leading to potential failure of all metallized film capacitors includes excessive capacitance loss during the lifetime of the capacitor in an environment with high temperature and high humidity. Typically, in most applications, if the loss of capacitance exceeds about 5% to 10%, the capacitor is considered to have failed.
Two different methods for minimizing such corrosion mechanism have been devised according to embodiments of the invention. One such method stems from discovery that, when a PMC is heated above a certain temperature (and, specifically, temperatures in the range of 200° C. to 280° C.), organometallic bonds are formed between the surface of the metal electrodes and the polymer dielectrics, resulting in blocking access of water to the metal of the electrodes. Because the metal electrode is formed in the vacuum in the absence of an oxidizing gas, the highly reactive surface of an electrode (made of aluminum, zinc or other metal) can be forced to react with the polymer or residual monomer in the already-formed polymer dielectric material of the PMC. The organometallic bonds on the electrode surface block molecules of water from reaching the surface and thus increase the corrosion resistance of the electrodes. For example, a PMC capacitor produced using two thousand layers of hexane diol diacrylate radiation-cured monomer, when placed in an accelerated corrosion test chamber at 125° C. and 30 psi of steam for one hour, exhibits full corrosion of the electrodes, as a result of which most of the originally metal-electrode surface turns into the transparent aluminum oxide.
Advantageously, when the electrodes of the PMC fabricated according to an embodiment of the invention were baked in the same chamber at 260° C. for six hours, the capacitor electrodes exhibited no apparent degradation. For capacitors employing different dielectric materials, the required passivation temperature may vary with polymer chemistry and time of exposure of the capacitor to the elevated temperature. It was empirically shown that for most radiation-cured polymer dielectric materials that can be used in an embodiment of the invention, passivation at 160° C. requires at least a six-to-seven day bake, while passivation at 200° C. reduces the duration of this process to 24 hours, and passivation at 240° C. requires only a 12-hour bake to achieve the desired results.
According to another implementation of the invention, the capacitor electrodes are passivated by exposing them to an oxidative plasma during the deposition process. The plasma may contain oxygen or other gasses that react with the electrode surface. When a metal electrode (such as an aluminum electrode, for example) deposited in the vacuum chamber is exposed to an oxygen-containing gas in the chamber, a high-quality A2O3 oxide is formed on its surface, which is superior to the hydrated oxide (Al2O3.(H2O)) that is formed upon exposure of the metal surface to air (containing both oxygen and moisture). In related embodiments, different plasma gases have been successfully used for this process including O2, Ar, CO2, N2 and N2O and mixtures thereof.
Yet another method for enhancing the electrode passivation was discovered by exposing the polymer dielectric layer of the capacitor, prior to the deposition of the electrode layer thereon, to a plasma containing an oxidizing gas. The oxidizing gas was shown to create functional groups on the surface of the polymer (such as carboxyl and hydroxyl groups, for example) that react with the metal of the electrode layer and aid the passivation process.
Notably, one of the questions accompanying the fabrication of a reliable PMC is how to establish a reliable and mechanically strong electrical contact with the individual electrode layers of the PMC structure (such as the electrodes 512 of the embodiments of
In a related embodiment, an alternative termination method has been developed, according to which plasma-ashing was performed to expose a controlled length 618 of a heavy edge 530 as shown in the SEM-captured cross-section in
Additional processing steps may be optionally performed in producing the capacitors according to an embodiment of the invention, including creating demetallization zones (such as the ones used for the internal series connections) using shadow masking or oil-vapor masking; cutting the mother capacitor material; etching the exposed edge of the individual capacitor to prevent flashover at higher voltages; including protective layers (that may include metallized and polymer-only layers, for example) on the bottom and top of the capacitor stack, lead attach, and packaging of the resulting structure, to name just a few.
The present invention addresses a PMC containing a polymer dielectric material that possesses self-healing properties and life expectancy superior to those of conventional PMCs. This is founded primarily on:
The fabrication of a single PMC with the use of the solutions outlined above yields a capacitor having superior performance and durability, in comparison with a metallized polypropylene capacitor of the related art, for example, when used under conditions typical for under-the-hood automotive applications.
It will be readily apparent to those skilled in this art that various changes and modifications of an obvious nature may be made, and all such changes and modifications are considered to fall within the scope of the present invention. Herein, the term “about” means a range of plus or minus 20% with respect to the specified value, more preferably plus or minus 10%, even more preferably plus or minus 5%, most preferably plus or minus 2%. In the alternative, as known in the art and recognized by a skilled artisan, the term “about” indicates a deviation, from the specified value, that is equal to half of a minimum increment of a measure available during the process of measurement of such value with a given measurement tool.
This application is a continuation-in-part of U.S. patent application Ser. No. 15/625,282, filed on Jun. 16, 2017 and published as US 2017/0301465, which is a continuation-in-part of U.S. patent application Ser. No. 15/483,780, filed on Apr. 10, 2017 and published as US 2017/0213646, which is a continuation of U.S. patent application Ser. No. 14/668,787 filed on Mar. 25, 2015 and issued as U.S. Pat. No. 9,711,286. The disclosure of each of the above-mentioned applications is incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14668787 | Mar 2015 | US |
| Child | 15483780 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15625282 | Jun 2017 | US |
| Child | 16035475 | US | |
| Parent | 15483780 | Apr 2017 | US |
| Child | 15625282 | US |
