Patent Application
20170310088
The present disclosure generally relates to insulators for spark plugs and, more particularly, to alumina-based ceramic insulators and methods of making the same.
Methods of manufacturing ceramic spark plug insulators oftentimes include a bisque firing process that heats the ceramic spark plug insulator with radiant heat energy. Bisque firing processes for ceramic spark plug insulators, which includes sintering to a high density (e.g., 97-99% of the theoretical density of the material), may take roughly 30 to 36 hours and may result in high costs due to the amount of energy needed to fire the spark plug insulators. Large tunnel kilns with radiant or convective heat sources, which are typically used to fire spark plug insulators, inefficiently consume energy. Approximately 50% of the energy is used to heat the kiln itself, including losses up the stack. Another 40% of the total energy is used to heat the firing trays that are used to support and transport the insulators through the kiln, leaving only 10% or less of the total energy for actually heating the parts. These inefficiencies can lead to excessive energy costs, longer cycle times, and low throughput.
The spark plug insulator firing process can also structurally impact the quality of the insulator. With bisque firing processes and large tunnel kilns, the firing process takes longer, not only because of the energy inefficiencies, but because of the time required for radiant heat to be transferred uniformly across a firing tray. Insulators on the outside of a firing tray are heated first and insulators toward the center of a tray take longer to heat. Having a more uniform time-temperature profile amongst insulators being fired together is desirable, as a generally uniform time-temperature profile can result in more consistent mechanical and dielectric properties among the fired insulators. Moreover, with conventional heating methods, heat is only applied to the surface of a part and thermal conduction must move that heat from the surface of the part to the interior. With microwave heating, on the other hand, heat is generated at a certain distance into the part, and there is accordingly a much shorter distance for which the heat transfer must rely upon thermal conductivity.
According to one embodiment, there is provided a method of manufacturing a spark plug. The method comprises the step of loading an unfired spark plug insulator into a kiln. The unfired spark plug insulator has a density and at least partially comprises a ceramic material. The method further comprises the step of heating the unfired spark plug insulator with radiant heat energy when the temperature of the unfired spark plug insulator is below a minimum absorption temperature. The ceramic material is at least partially transparent to microwave energy at the minimum absorption temperature. The method further comprises the step of heating the unfired spark plug insulator with microwave energy until the temperature of the unfired spark plug insulator reaches a maximum absorption temperature. The unfired spark plug insulator becomes a fired spark plug insulator upon reaching the maximum absorption temperature, the fired spark plug insulator having a density. The method further comprises the step of unloading the fired spark plug insulator from the kiln. The density of the fired spark plug insulator is higher than the density of the unfired spark plug insulator.
According to another embodiment, there is provided a method of manufacturing a spark plug. The method comprises the step of loading an unfired spark plug insulator into a kiln. The unfired spark plug insulator comprises a ceramic material that is at least partially transparent to microwave energy, and the kiln comprises a pre-heating zone, a sintering zone, and an exit zone. The method further comprises the steps of heating the unfired spark plug insulator with radiant heat energy in the pre-heating zone of the kiln, moving the unfired spark plug insulator to the sintering zone of the kiln, and heating the unfired spark plug insulator with microwave energy in the sintering zone of the kiln until the unfired spark plug insulator becomes a fired spark plug insulator. The unfired spark plug insulator has a density and the fired spark plug insulator has a density, and a ratio of the density of the unfired spark plug insulator to the density of the fired spark plug insulator is in a range of about 1:1.65 to about 1:1.92, inclusive. The method further comprises the steps of cooling the fired spark plug insulator in the exit zone of the kiln, and unloading the fired spark plug insulator from the kiln.
According to another embodiment, there is provided a spark plug insulator firing tray. The spark plug insulator firing tray comprises an exterior housing having a bottom wall and a side wall. The exterior housing is at least partially transparent to microwaves and is comprised of a low mass refractory material that is coated with a layer of ceramic material. The layer of ceramic material is denser than the low mass refractory material. The spark plug insulator firing tray further comprises a spark plug insulator support, wherein the interior spark plug insulator support is configured to maintain at least one spark plug insulator in a vertical position during a spark plug insulator firing process.
Preferred exemplary embodiments of the invention will hereinafter be described in conjunction with the appended drawings, wherein like designations denote like elements, and wherein:
The method described herein may be used to manufacture a fired ceramic insulator for a spark plug using microwave energy. Fast firing is encouraged through the use of rapid, volumetric heating with microwave energy. Using microwave energy and the particular time-temperature profiles described herein during the spark plug insulator firing or sintering process can be more efficient than normal heating and cooling means, particularly with respect to alumina-based ceramic materials. In some embodiments, the frequencies of the microwaves in the electromagnetic spectrum are optimized based on the particular spark plug insulator design and/or composition. The present method involves loading a ceramic, unfired spark plug insulator into a kiln. The ceramic material, such as alumina, is at least partially transparent to microwave energy when the unfired spark plug insulator is below a minimum absorption temperature. Accordingly, radiant heat energy may be used to supplement the heating process. In one embodiment, a combination of radiant heat energy and microwave energy is applied to bring the unfired spark plug insulator to the minimum absorption temperature. Microwave energy may be used to heat the unfired spark plug insulator once the temperature of the kiln is between the minimum absorption temperature and the maximum absorption temperature. The microwave energy can continue to heat the unfired spark plug insulator until reaching the maximum absorption temperature. At the maximum absorption temperature, the unfired spark plug insulator becomes a fired spark plug insulator, which will be described in further detail below. The fired spark plug insulator may then be cooled and unloaded from the kiln. The present method and time-temperature profiles for firing spark plug insulators can be more efficient than typical firing processes and may be completed in less than 3 hours, and possibly less than 1 hour, when using a microwave-based continuous furnace.
During a microwave-based sintering process, the spark plug insulator may be moved through a kiln with a firing tray. The firing trays discussed herein may withstand extreme heat of at least 1600° C., and up to 1700° C. to properly fire the insulators, have a low thermal mass, be ultra-light weight, and at least in some embodiments, may be composed of a ceramic material with a ceramic coating. In some embodiments, the firing trays or areas of the kiln may include susceptors that absorb microwave energy and convert it to radiant heat to further assist in the spark plug insulator firing process. Although the following description is provided in the context of an automotive spark plug and process for firing, it should be appreciated that the insulator and method described herein may be used with any type of spark plug or ignition device, including glow plugs, industrial plugs, aviation igniters and/or any other device having a fired insulator used to isolate electric current. It should further be appreciated that the terms “firing,” “sintering,” and variations thereof are used interchangeably.
An exemplary spark plug is shown in
Turning now to fired insulator 14, the insulator is an elongated and generally cylindrical component that is made from an electrically insulating material and is designed to isolate the center electrode 12 from the metallic shell 16 so that high-voltage ignition pulses in the center electrode are directed to the spark gap G. The fired insulator 14 includes a nose portion 30, an intermediate portion 32, and a terminal portion 34; however, other configurations or embodiments may be implemented.
The nose portion 30 extends in the axial or longitudinal direction between an external step 36 on the outer surface of the insulator and a distal end 38 located at a tip of the insulator. The nose portion 30 may have a continuous and uniform taper along its axial extent, or it could have sections of differing taper or no taper at all (i.e., straight sections where the outer surfaces are parallel to one another). Moreover, the extent to which the nose portion 30 axially extends or protrudes beyond the end of the metallic shell 16 (sometimes referred to as the “projection”), may be greater or less than that shown in
The intermediate portion 32 of the insulator extends in the axial direction between an external locking feature 50 and the external step 36 described above. In the particular embodiment illustrated in
The terminal portion 34 is at the opposite end of the fired insulator 14 as the nose portion 30 and it extends in the axial direction between the external locking feature 50 and a distal end 60. In the illustrated embodiment, the terminal portion 34 is quite long; however, it may be shorter and/or have any number of other features, like annular ribs. It should be noted that the exemplary embodiment shown in
With reference to
The insulator 14 may be comprised of any operable ceramic-based material, and in one embodiment, includes alumina (Al2O3) ceramic particles. Typically, but not necessarily, ceramic particles are mixed with a liquid medium and a binder, and may include one or more additives, such as zirconia, kaolin, talc, and/or calcium carbonate. Alumina-based ceramics in particular tend to have relatively high mechanical and dielectric strength, as well as high electrical resistivity and low dielectric loss, and are known to retain these properties over a relatively wide temperature range. Preferred ceramic particle compositions for the ceramic material comprise 90 wt % alumina, 96 wt % alumina, and 99 wt % alumina, respectively. These compositions of alumina may be more likely to absorb microwave energy at increasing temperatures for a majority of frequencies. However, other compositions are certainly possible. Preferred ceramic compositions are disclosed in U.S. Pat. No. 4,879,260 to Manning, U.S. Pat. No. 7,169,723 to Walker, Jr., and U.S. Pat. No. 8,434,443 to Lykowski, all of which are incorporated herein by reference in their entirety.
A firing tray 70 is shown in
The thermal and dielectric properties of the firing tray 70 can also be optimized for compatibility with microwaves. For example, the exterior housing 72 may be composed of a low mass refractory material which may include any heat resistant material capable of withstanding maximum absorption temperatures. In a preferred embodiment, the firing tray 70 is made from a low mass fibrous refractory material which is coated with a layer of denser ceramic to provide strength and durability. One particular example is a firing tray made from low mass mullite and coated with mullite. Using ultra-light weight mullite firing trays can enable fast heating rates. Preferably, the firing trays have minimal interaction with the microwaves and are optimally microwave transparent. If the firing trays do not absorb microwaves, they will not be directly heated by the microwaves so that the microwave energy can be more efficiently directed to the spark plug insulators. Further, low thermal mass firing trays can be thermally insulating and help prevent the spark plug insulators from losing heat to the surroundings. In this respect, the energy efficiency of the furnace is increased because the energy is directed to primarily heating the insulators.
In
In
Kiln 100 provides a combination of microwave energy, via waveguide ports 110 for example, and radiant heat energy, from either susceptors 80, electric heating elements 112, and/or another radiant heat source. While the microwave power level may not be as finite and stepwise as if a batch type kiln were used, the microwave field intensity can be controlled via the number and baffling of the waveguide ports 110. The microwave waveguide ports 110 can penetrate the metal shell of the chamber 102 to input the microwave energy. However, it is desirable to seal the waveguide inputs 110 from the rest of the atmosphere or effluent from inside the kiln 100. In a preferred embodiment, microwave energy is applied through the roof of the kiln through two ports 110, one in the preheating zone 104 and one in the sintering zone 106. In some embodiments, various microwave choking systems and susceptors 80, or more particularly, silicon carbide absorbers, can be used in the preheating zone 104 and/or the exit zone 108 of the kiln 100 to help mitigate microwave leakage. Exhaust ports (not shown) can use an array of tube chokes where large diameter openings are desired. The particular time-temperature profiles and their relation to various microwave power levels will be discussed in greater detail with respect to the process 200.
Turning now to
Step 202 of the method 200 involves loading an unfired spark plug insulator 14′ into a kiln, such as the kiln 100 shown in
Step 204 of the method involves heating the unfired spark plug insulator 14′ with radiant heat energy when the temperature of the unfired spark plug insulator is below a minimum absorption temperature. Below the minimum absorption temperature, the ceramic material is at least partially transparent to microwaves. Radiant heating allows the insulators to be heated to a temperature where they can more efficiently absorb microwave energy and be heated by the microwaves. Radiant heat energy may be provided in the preheating zone 104 of the kiln 100 through any operable source, such as through electric heating elements, microwave susceptors that are heated by microwaves and transmit heat to the insulators, or a combination of both, to cite a few examples. Accordingly, a combination of radiant heat energy and microwave energy is applied in the preheating zone 104, which helps sinter the insulators to a high density in a short amount of time. In a preferred embodiment, the heating rate is about 50° C. per minute. This step may also include a de-binding substep. In some embodiments, it may be desirable to heat the unfired spark plug insulator 14′ at a slower rate up to about 500° C. to remove organic processing aids such as organic binders which are used during the manufacture of the unfired insulators.
As addressed above, during step 204, radiant heat energy is preferably applied when the temperature of the unfired spark plug insulator is below the minimum absorption temperature where the ceramic material is at least partially transparent to microwaves. Skilled artisans will appreciate that the minimum absorption temperature may depend on a number of factors, including but not limited to the ceramic material composition, the frequency of the microwave energy being used, the desired firing time, and the dimensions of the kiln. For example, if the microwave frequency is 2.45 GHz, the minimum absorption temperature may be about 650° C. for a ceramic particle composition of 90 wt % alumina, about 850° C. for a ceramic particle composition of 96 wt % alumina, and about 900° C. for a ceramic particle composition of about 99 wt % alumina. In another example, if the microwave frequency is 915 MHz, the minimum absorption temperature may be about 560° C. for a ceramic particle composition of 90 wt % alumina, about 760° C. for a ceramic particle composition of 96 wt % alumina, and about 830° C. for a ceramic particle composition of about 99 wt % alumina. Accordingly, the minimum absorption temperature may be between about 560° C. and 900° C. Preferably, the minimum absorption temperature is 900° C. to ensure that the ceramic material can efficiently absorb microwave energy. Moreover, while 900° C. may be preferred in some embodiments, in other embodiments, the temperature can be adjusted based on the composition to have maximal heating accomplished by the microwave energy.
Step 206 of the method 200 involves heating the unfired spark plug insulator 14′ with microwave energy when the temperature of the unfired spark plug insulator is between the minimum absorption temperature and a maximum absorption temperature. As described above, microwave energy may be used in combination with radiant heat energy to heat the unfired spark plug insulator to the minimum absorption temperature, but it is preferable to use microwave energy for sintering once the minimum absorption temperature is reached and the unfired spark plug insulator is less transparent to microwaves. In some implementations, a combination of conventional radiant heat sources and microwave heating may be used when the temperature is between the minimum absorption temperature and the maximum absorption temperature. The maximum absorption temperature may be the temperature at which the slope of the loss tangent (tan δ) curve sharply increases (e.g., the slope increases by 50% or more). In one embodiment, the maximum absorption temperature is greater than about 1300° C. In one particular embodiment, the maximum absorption temperature is about 1450° C. At about 1450° C., a minimum half power depth of 2.8 cm was observed. The half power depth will also be discussed in detail below with regard to the dielectric property testing, but in brief, the half power depth of 2.8 cm at about 1450° C. indicates that a cross-section thickness of about 5.6 cm could be uniformly heated by microwave energy. More preferably, the maximum absorption temperature is about 1600° C., or it may be as high as about 1700° C. Once the unfired spark plug insulator 14′ reaches a temperature of about 1600° C. in the preheating zone 104, the sintering zone 106 may be maintained at a temperature of about 1600° C. until the spark plug insulator 14 is fully fired to a desired density. In one embodiment, the ramp time, or the time when the spark plug insulator is in the preheating zone 104, is about 32 minutes, and the dwell time, or the time when the spark plug insulator is in the sintering zone 106, is about 26 minutes.
The microwave power and frequency used in the method 200 depends upon a number of factors, such as dielectric property indicia including loss tangent (tan δ) and half power depth, which will be described in further detail below. In one embodiment, a power requirement of about 1136 W/kg or less is preferred. The total microwave power is about 33.8 kW when implementing a system such as kiln 100 and the mass of the spark plug insulators is approximately 39.8 kg in total being fired at one given time. In this implementation, it may be preferred to employ about 18.5 kW in the preheating zone 104 and about 15.3 kW in the sintering zone 106. The microwave frequency may be 915 MHz or 2.45 GHz, which are the most common industrial frequencies allowed for use by the Industrial Scientific and Medical (ISM) standard. Microwave equipment that operates at these frequencies is readily available and considerably more economical compared to equipment operating at other frequencies. Preferably, the microwave frequency is 2.45 GHz, however, other frequencies are possible, such as 5.8 GHz, 22 GHz, or 28 GHz, to cite a few examples. It has been estimated that a 30 kW, 2.45 GHz microwave generator will provide sufficient power. It is also possible to use two 15 kW, 2.45 microwave generators. Other power and frequency combinations are certainly possible, and as mentioned, will depend on factors such as the insulator composition, dielectric property indicia, and/or firing process parameters.
Step 208 involves obtaining a fired spark plug insulator 14 when the unfired spark plug insulator 14′ reaches the maximum absorption temperature. As described above, microwave energy is typically still applied once the spark plug insulator reaches the maximum absorption temperature in order to maintain the spark plug insulator at the maximum absorption temperature for a certain amount of time. The amount of time that the microwave energy is applied will typically depend upon the desired final density and other structural characteristics of the fired spark plug insulator 14. The ratio of the density of the unfired spark plug insulator to the density of fired spark plug insulator may range from about 1:1.65 to about 1:1.92. For 90 wt % alumina, the density of the fired spark plug insulator is preferably about 3.55 g/cc. For 96 wt % alumina, the density of the fired spark plug insulator is preferably about 3.75 g/cc. For 99 wt % alumina, the density of the fired spark plug insulator is preferably about 3.90 g/cc. Typically, the unfired spark plug insulator 14′ is off-white in color, whereas the fired spark plug insulator 14 is white in color.
Step 210 of the method 200 involves cooling the fired spark plug insulator 14 to a thermal shock avoidance temperature. Thermal shock can occur if an insulator is quenched in air from the maximum absorption temperature. Accordingly, it may be desirable to cool the fired spark plug insulator to the thermal shock avoidance temperature in the exit zone 108 of the kiln 100 before unloading the fired spark plug insulator from the kiln (step 212). In one particular embodiment, the thermal shock avoidance temperature is about 1000° C. Cooling the insulators to about 1000° C. over a period of a few minutes helps to avoid thermal shock. It has been shown that if the insulators are cooled too quickly, they can develop microcracks which reduce the mechanical and dielectric strength of the insulator. In one embodiment, the fired spark plugs 14 cool in the exit zone 108 of the kiln 100 for about 19 minutes. As described above, with a ramp time of about 32 minutes (heating at about 50° C. per minute), a dwell time of about 26 minutes, and a cool time of about 19 minutes, the total cycle time is about 77 minutes using kiln 100, which is about 14 feet in length. Using a firing tray 70 that holds 121 spark plug insulators, 30 insulators per minute may be produced using this exemplary time-temperature profile.
Step 212 of the method 200 involves unloading the fired spark plug insulator 14 from the kiln. Typically, as described above, the spark plug insulators are removed from the kiln 100 after reaching a thermal shock avoidance temperature so as to avoid microcracks and other undesirable defects that may impact the structural integrity or the dielectric properties of the insulator. This cooling may be accomplished in the kiln 100 itself, such as in exit zone 108, or in another insulated chamber or area. Once the fired spark plug insulator 14 is removed, it may require additional cooling. Also, the fired spark plug insulator 14 may be further shaped, worked, and/or otherwise formed using commonly known techniques like turning, grinding, cutting, sanding, polishing, buffing, etc. Decorating and glazing are the most common additional processing steps after bisque firing. Glazing and decorating require an additional firing step, which is in the range of 900° C. to 1100° C., well below the bisque firing temperature. The spark plug insulator 14 may then be inserted into a metal shell 16 and assembled into a completed spark plug 10.
In order to refine the time-temperature profiles and ensure the workability of microwave processing for alumina-based ceramic spark plug insulator compositions, dielectric testing was performed. The testing confirmed that certain alumina-based ceramic insulator compositions will directly absorb microwave energy and can benefit from microwave processing. As described above, at least some benefits include faster firing times and more uniform sintering. Dielectric properties were measured at microwave frequencies of 2.45 GHz and 915 MHz from room temperature to 1450° C. using a cavity perturbation method. The dielectric property measurements were performed in 50° C. steps to about 1450° C. A repeat measurement was obtained at about 1450° C., and then in 100° C. steps back down to room temperature. At the end of the cycle, the sample was removed and the empty holder was measured at room temperature, 600° C., 800° C., 1000° C., 1200° C., 1300° C., and 1450° C., for calibration. The sample was heated in a conventional furnace, and periodically dropped into a TM0n0 resonant cavity (constant electric to magnetic field ratio), in which the perturbation of the field by the sample (i.e., the frequency shift and the quality factor shift) was captured using a network analyzer. The frequency shift corresponded to the real part of the permittivity (∈′), while the quality factor shift was used to determine the imaginary permittivity (∈″). Die-pressed pellets of differing lengths between 4 and 7 mm comprising various alumina compositions (90 wt %, 96 wt %, and 99 wt %) were analyzed. Three of the pellets were stacked in a 4 mm ID quartz sample holder to form the measurement sample. Certain dielectric property indicia were analyzed in the dielectric property testing. As understood by those skilled in the art, microwave heating occurs through dielectric loss mechanisms. Dielectric property indicia such as the loss tangent (tan δ) and the half power depth can provide a guide to microwave heating behavior for developing an optimal microwave sintering process. Table I below shows the physical properties of the samples before and after each cycle.
The loss tangent (tan δ) is the dielectric loss (∈″) divided by the permittivity (∈′). Typically, when tan δ is less than 0.01, the material is fairly microwave transparent (i.e., the material weakly absorbs microwaves). Weak microwave absorption causes the material to remain cool, or to heat very slowly in a microwave field. A value of tan δ which is equal to or greater than 0.01 indicates that the material will absorb microwave energy and heat. The higher the value of tan δ is, the greater the microwave absorption will be, and the faster the material will heat up. If tan δ is greater than 2, the material can absorb microwave energy so well that the microwaves may not penetrate into the material very far, as indicated by the half power depth, which can lead to localized heating and “thermal runaway” which may be avoided through designed processing techniques.
The half power depth is the distance at which half of the microwave power is dissipated into the material. The half power depth is typically shorter at 2.45 GHz compared to 915 MHz due to the size of the wavelength. The wavelength at 915 MHz is approximately 3 times longer compared to 2.45 GHz, which increases the depth that the microwave energy can penetrate into a material. The equation below may be used to calculate the half power depth:
where DHP is the half power depth, c is the speed of light, ω is the angular frequency, ∈0 is the permittivity of free space, tan δ is the loss tangent, and ∈′ is the permittivity. tan δ and half power depth typically have an inverse relationship, and accordingly, with a higher tan δ there is a smaller half power depth. A higher tan δ indicates better absorption of microwave energy, which means that the energy is absorbed by the material over a shorter distance. The half power depth can be used to estimate the material thickness which will receive uniform microwave energy which will receive uniform microwave energy. For example, a half power depth of 10 mm indicates that a sample with a minimum dimension of 20 mm (e.g., a 20 mm thick part) would receive a uniform amount of microwave power. Microwave energy penetrates samples from all sides, so the smallest sample dimension is considered more important when looking at uniform power application. In the case where the sample thickness is more than twice the half power depth, the interior of the sample may receive a lower level of microwave energy compared to the exterior. Even in the case where the half power depth is less than half the sample thickness, microwave heating can still be beneficial, however.
The results of the dielectric property testing are shown in
As shown and described above, both microwave frequencies that were tested (915 MHz and 2.45 GHz) appear to be viable options for heating alumina-based spark plug insulator compositions. Comparing the dielectric data for the two microwave frequencies, 915 MHz had slight higher values of tan δ compared to 2.45 GHz, and also a longer half power depth. This indicates that direct microwave coupling or heating will start at approximately 100° C. lower for 915 MHz as compared to 2.45 GHz and have a slightly longer penetration depth, which could offer a processing advantage. The heating characteristics at 2.45 GHz also fall within an acceptable data range for adequate microwave heating and susceptability. Typically, 2.45 GHz is used for smaller, lab-scale, and low power kilns, while 915 MHz is used as power demands (e.g., greater than 30 kW) and cavity size increases. The option to use either frequency enables design flexibility for the microwave kiln.
It is to be understood that the foregoing description is not a definition of the invention, but is a description of one or more preferred exemplary embodiments of the invention. The invention is not limited to the particular embodiment(s) disclosed herein, but rather is defined solely by the claims below. Furthermore, the statements contained in the foregoing description relate to particular embodiments and are not to be construed as limitations on the scope of the invention or on the definition of terms used in the claims, except where a term or phrase is expressly defined above. Various other embodiments and various changes and modifications to the disclosed embodiment(s) will become apparent to those skilled in the art. All such other embodiments, changes, and modifications are intended to come within the scope of the appended claims
As used in this specification and claims, the terms “for example,” “e.g.,” “for instance,” “such as,” and “like,” and the verbs “comprising,” “having,” “including,” and their other verb forms, when used in conjunction with a listing of one or more components or other items, are each to be construed as open-ended, meaning that that the listing is not to be considered as excluding other, additional components or items. Other terms are to be construed using their broadest reasonable meaning unless they are used in a context that requires a different interpretation.
This application claims the benefit of U.S. Provisional Application No. 62/327,484, filed on Apr. 26, 2016, the contents of which are hereby incorporated by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62327484 | Apr 2016 | US |
