MXPA97002537A - Ceram high voltage lighter - Google Patents

Abstract

A heating method, consisting of the step of providing a line voltage of between 120V and 230V through a ceramic lighter having a hot zone composition consisting of: (a) between 50 and 80 v / o of a electrically insulating ceramic zone with a resistivity of at least about 1010 ohm-cm; (b) between 10 and 45 v / o of a semiconductor material having a resistivity of between about 1 and about 108 ohm-cm; (c) between 5 and 25 v / o of a metallic conductor having a resistivity of less than about 10-2 ohm-cm, between 2.0 and 20 v / o of a resistivity enhancing compound selected from the group consisting of metal oxides, metal oxynitrides, oxides of rare earth, rare earth oxynitrides and mixtures of the same

Description

LIGHTER OF RLTQ VQLTR3E PE CERRflICR BACKGROUND OF THE INVENTION Ceramic materials have enjoyed great success as lighters in gas-burning ovens, stoves and clothes dryers. The production of ceramic lighters requires the construction of an electrical circuit through a ceramic component, a portion of which is highly "Resistive and increase in temperature when electrified by a wire conductor." A conventional igniter, the flini- Igniter ™, available from the Norton Company of Mil ord, NH, is designed for applications from 8 volts to 48 volts and has a composition consisting of aluminum nitride 5 ("PIN"), molybdenum disilicide ("MoSiss:"), and silicon carbide ("SiO"). Rl grow the attractiveness of the Mini-Igniter ™ so has the number of applications that require small lighters with rated voltages exceeding conventional 24 volts, however, when used in such applications, the 24V Mini-Igniter ™ is subject to temperature runaway and therefore requires a transformer in the control system to lower the conventional voltage line (ie, 120 volts.) Consequently, there is a need for smaller and higher voltage 5 lighters designed for both high voltage applications 120 or 230, which do not require an expensive transformer but still have the following requirements established by the domestic utensils and heating industries to anticipate the variation in line voltage: Time to design temperature < 5 eec Minimum temperature at 85% design voltage 1100 ° C Design temperature at 100% design voltage 1350 ° C Maximum temperature at 110% design voltage 1500 ° C Length of the hot zone < 38.1 m Power (U) 65-100. Because the amperage used for these high-voltage applications will be similarly comparable to that used in 24-volt applications (ie, approximately 1.0 amp), the increased voltage will be similarly noted by increasing the igniter's resistance. The resistance of any body is generally governed by the equation Rs = Ry x L / A, where Rs = Resistance; Ry = Resistivity; L = the length of the conductor; and P = the cross-sectional area of the conductor. Because the length of the single end of conventional 12V and 24V fork style lighters is already approximately 38.48 mm, it can not be significantly increased without reducing its commercial appeal.

Sirnily, the cross-sectional area of the smaller lighter, between approximately 0.0254 n 2 and 0.0635 m 2, will probably not be diminished for manufacturing reasons. Therefore, apparently the desired increase in the resistance of the small and high-voltage small lighters will be obtained by increasing their resistivity. Because the Mini-Igniter ™ consists of an albacore resistive material (PIN), a moderately resistive material (S.LC), and a highly conductive material (M0SÍ2), an obvious route to increase the resistivity of the lighter is to reduce its contents of I0SÍ2 and Sic adding PIN. However, a drag composition, (containing approximately 78% by volume ("v / o" or "vol%") of PIN, 9 v / o of M0SÍ2 V 15 v / o of SiC), was found not to it was satisfactory since not only was it slow to reach the design temperature, (due to low levels of M0SÍ2), but it also possessed a negative temperature resistance coefficient ("NTCR") and therefore was subject to a runaway of temperature about only 1350 ° C. An NTCR means that by increasing the temperature of the lighter, its resistance decreases. This decrease makes the lighter hotter than it would be if the resistance were constant. Yes. The NTCR is very extreme, the igniter is slow and cold at 85% and unstable at 110% of the rated voltage. In fact, such a lighter could exhibit runaway below the 110% rating, in which case the amperage and temperature continue to rise even at a constant voltage until failure occurs (warming deterioration). Instead, it is preferable that the lighters have a positive resistance temperature coefficient ("PTCR") or a moderate NTCR. While a ceramic having a PCTR 5 increases in resistivity when its temperature increases from 1000 ° C to 1400 ° C, a ceramic having a moderate NTCR decreases in resistivity by less than 25% when its temperature increases from 1000 ° C to 1400 ° C. Either a PTCR or a moderate NTCR will allow a more gradual increase in temperature with a voltage in An increase, which is critical for 120V applications because, as explained above, the lighter must operate stably over a wide voltage range. The patent of E.U.R. No. 5,405,237 ("the ashburn patent"), describes compositions suitable for the area hot of a ceramic lighter consisting of: (a) between 5 and 50 v / o of M0SÍ2, and (bi between 50 and 95 v / o of a material selected from the group consisting of silicon carbide, silicon nitride , aluminum nitride, boron nitride, aluminum oxide, alu of magnesium, silicon aluminum oxynitride and mixtures thereof. However, each example described in the Uashburn patent (and in the similar U.S. Patent No. 5,085,804) uses only a) PIN or YES3 4, b) M0SÍ2 and c) SiC (some examples also adding NgC? 3) - As mentioned In the above, it is believed that these systems are not readily conductive to produce commercially viable ceramic lighters, which are stable at high voltages. Although the Uashburn patent describes a 220V lighter made of 50 v / o PIN, 42.2 v / o SiC and 7.8 v / o of M0SÍ2, ßl low level of M0SÍ2 in this lighter dramatically limits the speed with which this lighter reaches its design temperature. Accordingly, it is the object of the present invention to find a highly resistant mini-lighter composition, which does not experience thermal runaway at high temperatures and satisfies the aforementioned time and temperature constants of high voltage applications.

BRIEF DESCRIPTION OF THE INVENTION In accordance with the present invention, a heating method is provided, consisting of the step of providing a line voltage between 120V and 230V through a ceramic lighter having a hot zone composition consisting of: (a) between 50 and 80 v / o of an electrically insulating ceramic having a resistivity of at least about 10 ohm-crn; (b) between 10 and 45 v / o of a semiconductor material having a resistivity of between about 1 and about 108 ohm-cm; (c)? between 5 and 25 v / o of a metallic conductor having a resistivity of less than about 10 ~ 2 ohm-crn; and (d) between 2.0 and 20 v / o of a resistivity enhancing compound selected from group consisting of metal oxides, metal oxynitrides, rare earth oxides, rare earth oxynitrides and mixtures thereof.

DESCRIPTION OF THE FIGURES Figure 1 depicts a typical microstructure of the present invention, in which the PIN is gray, SiC is light gray, M0SÍ2 is white and (it is believed) the mixture of alumina / oxy aluminum truro is dark gray.

DETAILED DESCRIPTION OF THE INVENTION It has been unexpectedly discovered that adding alumina, aluminum oxide or mixtures thereof to the hot zone of a conventional RIN-M0SÍ2 ~ SiC system will increase the resistivity of the lighter more than the comparable fraction of the PIN, thus allowing the use of higher fractions of M0SÍ2 while providing the necessary resistivity for higher voltage applications. The freedom to use high levels of poSÍ2 results in a faster time to temperature, and in some cases, a less drastic increase in temperature with an increasing voltage between 85% and 110% of the measured voltage. Accordingly, the lighter of the present invention possesses both the resistivity required for high voltage applications and the rapid time for temperature required by the electrical appliance and heating industries. In some embodiments of the present invention, the resistivity enhancing compound is a mixture of alumina and aluminum oxynitride. This mixture can be produced simply by adding alumina to the green body. In such cases, it is believed that, during the concretion, at least some of the alumina reacts with a portion of the aluminum nitride to form a crystalline phase of aluminum oxynitride. The presence of the aluminum oxynitride phase in the ceramic has been confirmed by X-ray diffraction analysis. The dissolution of the impurities in this crystalline phase is believed to increase the refractive character of the intergranular phase., resulting in a decrease in ionic conductivity through the intergranular phase with an increasing temperature. Moreover, it is believed that the addition of alumina increases the growth of the granule, resulting in a portion of the conductive phase being isolated, thus increasing the resistivity. When alumina is added to the green body, any conventional alumina powder can be selected. This is generally added to the green body as an alumina granule in an amount of between about 0.5 and 18.5 v / o, preferably between about 0.5 and 6.4 v / o, most preferably about 2.5 to 3.5 v / o. Typically, the alumina powder having an average granule size of between 0.1 and about 10 microns is used, and only about 0.2 p / o of impurities. Preferably, the alumina has a granule size of between about 0.3 and about 10 um. Most preferably, the Picoa P17 calcined alumina available from Picoa Industrial Chemicals of Bauxite, Flrkansas, is the one used. Traditionally, alumina can be introduced in non-powdered presentations, including, but not limited to, alumina sol-gel approaches and the hydrolysis of a portion of the aluminum nitride to produce a green body having from about 2-20% in volume, and preferably about 2-8% by volume of alumina. Although the examples I-III shown below each add only alumina to the conventional system of PIN-M0SÍ2-SiC, it is contemplated that the compounds that are not metal oxides, metal oxynitrides, rare earth oxides (e.g., 5 v / o) of yttria), rare earth oxynitrides and mixtures thereof, can be substituted by alumina in the green body of the present invention and still desirable results can be obtained. In general, the composition of the hot zone should include (a) between about 50 and about 80 v / o of an electrically insulating ceramic having a resistivity of at least about 10 * ° ohm-cm; (b) between about 10 and about 45 v / o of a semiconductor material having a resistivity of between about 1 and about 108 ohm-cm; (c) between about 5 and about 25 v / o of a metallic conductor having a resistivity of less than about 102 ohm-cm; and (d) between about 2.0 and about 20 v / o of a resistivity enhancing compound selected from the group consisting of metal oxides, metal oxynitrides, rare earth oxides, rare earth oxynitrides, and mixtures thereof. Preferably, the hot zone consists of 50-70 v / o of electrically insulating ceramic, 20-30 v / o of semiconductive ceramic, 6-12 v / o of conductive material, and 2-8 v / o of the resistivity enhancing compound . For the purposes of the present invention, an electrically insulating (or "insulator") ceramic is a ceramic having a resistivity at room temperature of at least about 10 * ohm-cm. If the electrically insulating ceramic component is present as more than about 70 v / o of the composition of the hot zone (when the conductive ceramic is present at about 6 v / o), the resulting composition becomes very resistive and is insufficiently slow to obtain target temperatures at high voltages. Conversely, if it is present as less than about 50 v / o (when the conductive ceramic is presented at about 8 v / o), the resulting ceramic becomes highly conductive at high voltages. Clearly, when the conductive ceramic fraction is elevated above 6 v / o, the hot zone is more conductive and the upper and lower limits of the insulating fraction can be adequately high to obtain the required voltage. Typically, the isolator is a nitride selected from the group consisting of aluminum nitride, silicon nitride, and boron nitride. It is known that commercial and typical PIN starting powders contain about 1 p / o of oxygen, or about 1.8 p / o of alumina as a coating on the PIN granules. Therefore, when the aluminum nitride is selected, the desired alumina content in the hot zone composition must be calculated taking into account this alumina impurity. For example, when about 70 v / o of PIN are used, the alumina impurity is about 1.5 v / o of the composition of the hot zone. For the purposes of the present invention, a semiconductor (or "semiconductor") ceramic is a ceramic having a resistivity at room temperature of between about 1 and 108 ohm-cm. If the semiconductor component is present as more than about 45 v / o of the composition of the hot zone (when the conductive ceramic is on the scale of about 6-10 v / o), the resulting composition becomes very conductive for applications of high voltage (due to the lack of insulator). Conversely, if it is present as less than about 10 v / o (when the conductive ceramic is on the scale of about 6-10 v / o), the resulting composition becomes very resistive (because there is a lot of insulation). Again, at higher conductor levels, more resistive mixtures of the insulator and semiconductor fractions are necessary to obtain the desired voltage. Typically, the semiconductor is a carbide selected from the group consisting of silicon carbide (doped and undoped) and boron carbide. For the purposes of the present invention, a conductive material is one that has a resistivity at room temperature of less than about 102 ohm-cm. If the conductive component is present in an amount of more than about 25 v / o of the hot zone composition, the resulting ceramic becomes highly conductive for high voltage applications, resulting in an unacceptably hot igniter. Conversely, if it is present as less than about 6 v / o, the resulting ceramic becomes very resistive for high voltage applications, resulting in an unacceptably cold lighter. Typically, the conductor is selected from the group consisting of molybdenum disilicide, tungsten disilicide and nitrides such as titanium nitride and carbides such as titanium carbide. If the resistivity enhancing compound is present in an amount of less than about 2.0 v / o of the hot zone composition, then its resistivity improving effect is not significant. Conversely, if it is present in an amount of more than approximately 20 v / o, then the hot zone becomes very resistive for a rapid time to temperature relationship in high voltage applications. Preferably, it consists of between about 2-8 v / o of the hot zone composition, most preferably about 4-5 v / o. Typically, it is selected from the group consisting of metal oxides, metal nitrides, rare earth oxides and rare earth oxynitrides. Preferably it is selected from the group consisting of aluminum oxynitride and alumina. Preferably, the fractions of the aluminum nitride component, molybdenum disilicide and silicon carbide, described in the U.S. Patent. No. 5,045,237 ("the Uashburn patent"), the specification of which is fully incorporated herein by reference, is used to construct the hot zone of the lighter of the present intention. It has been found that the R1.N-SIC-M0SÍ2 system is a flexible system that can produce lighters having resistivities ranging from about 0.001 to about 100 ohm-cn. Preferably, the particle sizes of both the starting powders and the granules in the particular ceramic are similar to those described in the Uashburn patent. The hot zone / cold zone igniter design as described in the Uashburn patent may be suitably used in accordance with the present invention. The hot zone provides functional heating for the gas ignition. This generally has a resistivity of about 0.04 ohm-cm, preferably, of at least about 0.2 ohm-cm on the temperature scale of 1000 to 1800 ° C. Preferably, it consists of about 50 to 80 v / o of aluminum nitride and about 5-25 v / o of M0S.12 and 10-45 v / o of SiC (in a volume ratio of about one part of M0SÍ2 to about 2 parts of SiC), and approximately 2.0 to 20 v / o of the resistivity enhancing compound. Most preferably, it consists of about 50 to 70 v / o of aluminum nitride and about 6-12 v / o of M0SÍ2, 20-30 v / o of SiC (typically in a volume ratio of about .1 part of MoSÍ2 to about 2 parts of SiC), and about 2-8 v / o of the resistivity enhancing compound. In an especially preferred embodiment, the hot zone consists of approximately 60 v / o d / s PIN, 11 v / o of M S / 2, and 25 v / o of SiC and 5.5 v / o d / a mixture of aluminum / alumina oxynitride. In preferred embodiments the average granule size (dsc) of the components of the hot zone in the densified body is as follows: a) isolator (i.e. PIN): between about 2 and 10 microns; b) semiconductor (i.e., SiC): between about 1 and 10 microns; c) conductor (ie, M0SÍ2): between approximately 1 and 10 microns; and d) resistivity enhancing compound (ie, alumina / aluminum oxynitride mixture): between about 2 and 10 microns. Figure 1 describes a microstructure of the present invention. The cold zone allows the fixing of the wire conductors. Preferably, it also consists of PIN, SiC and M0SÍ2 - However, it has a significantly higher percentage of conductive and semiconducting materials (ie, SiC and M0SÍ2) than does the hot zone. Accordingly, it has typically and only about 1/5 to 1/20 of the resistivity of the hot zone composition and does not rise in temperature to the levels experienced by the hot zone. It preferably consists of about 20 to 65 v / o of aluminum nitride, and about 20 to 70 v / o of M0SÍ2 and SiC in a volume ratio of from about 1: 1 to about 1: 3. Most preferably, the cold zone consists of approximately 60 v / o of PIN, 20 v / o of SiC and 20 v / o of Mo! 3Í2 - Because this does not require a high resistivity. The cold zone need not contain the aluminum oxynitride phase required by the hot zone of the present invention.

It has been discovered that the dimensions of the lighter-affect its properties and performance. In general, the single end length of the hot zone should be greater than about 17.8 mm (to provide sufficient mass for the convective cooling gas flow to not significantly affect its temperature) but less than approximately 38.1 m (to provide mechanical robustness enough). Its width must be greater than approximately 1.02 nm to provide sufficient strength and ease of frabrication. Similar entity, its thickness should be more than about 0.762 mm to provide sufficient strength and ease of fabrication. Preferably, the two-legged fork lighters of the present invention are typically between about 31.75 and about 50.8 mm in total length of single limb, and have a transverse measurement of the hot zone between about 0.025 and about 0.127 mm2 (most preferably, less than 0.0635 mm2). In certain embodiments designed for 120 V applications, the hot zone is approximately 31.75 mm in length of single end, approximately 0.762 mm in thickness and approximately 1.20 mm in width (ie, a transverse measurement of approximately 0.03581 m). It has also been found that alteration of these dimensions can produce lighters of the present invention having differently rated voltages. In particular, Table 1 sets the dimensions of the hot zone of the lighter required for voltages using a hot zone composition of about 60 a / d PIN, about 11 v / o dS SiC, and about 25 v / o M0SÍ2 , and approximately 4 v / o of aluminum oxynitride / alunine mixture: TABLE I Voltage Extree length - Width (mm) Thickness (mm) Simple size (m) 80 approx. 24.13 1.19 0.762 120 approx. 27.94 1.19 0.762 80 approx. 31.75 1.19 0.762 The processing of the ceramic component (i.e., the green body processing and concretion conditions) and the preparation of the lighter of the densified ceramic can be carried out by any conventional method. Typically, such methods are carried out in substantial accordance with the Uaehburn patent. It has been found that higher concretion temperatures (ie, about 1800 ° C) tend to produce more grain growth in the aluminum nitride component of the lighter, resulting in a more insulated conductive component and therefore more resistivity. high. However, it has been discovered that raising the The concreting temperature of about 1820 ° C results in a greater lighter to lighter variability and lower fracture resistance. The key advantages of the lighter of the present invention are that it has a higher resistivity than conventional small lighters and a moderate NTCR. It is believed that the moderate tendency towards the temperature increase produced by the moderate NTCR of these lighters is comfortably balanced by the moderate tendency towardsThe temperature drop due to the loss in radioactive heat, led to a high voltage igniter stable in temperature and self-controllable. In the d 120 120V modalities, it has been discovered that it is very insensitive to the variations of the procedure, that is, it is robust. Its hot zone resistance can be designed to be between approximately 100 and 300 ohms. Other properties of the 120V cigarette lighter of the present invention are comparable to those of the conventional 24 volt cigarette lighter. For example, the lighters of the present invention have a power load per unit area of radiant surface of between about 25 and about 35 Uatts / cm, a power consumption d between about 65-85 watts; a resistance to bending at room temperature of between about 400 and 500 MPa; a resistivity of at least about 0.2 ohm-cm. In 230V applications, the less extreme NTCR allows it to operate more stably within a high voltage regime and still obtain the performance requirements of conventional lighters. Rmbas modalities of 120V and 230V achieve the performance criteria mentioned above. However, as with all ceramic lighters, some of the selected compositions of the present invention appear to be limited in their operating regimes. For example, it has been found that in some fork lighters of the present invention having a hot zone resistivity of at least about 1.1 ohm-cm and a single limb length of less than about 31 mm, the instability appears at 1600 ° C. Further, it has also been discovered that when the lighters of the present invention reach approximately 1620 ° C, their native oxide protective coating melts and the failure ensues. The practice of the present invention can be more fully appreciated from the following non-limiting examples and comparative examples. For the purposes of the present invention, a "stable" lighter is one that maintains a constant resistivity and a constant temperature at a given voltage.

E3EHPL0 1 A hot zone composition consisting of about 60 parts by volume of PIN, about 11 parts by volume of M0SÍ2 / about 25 parts by volume of SiC and about 4 parts by volume of PI2O3 were mixed in a high shear mixer. A cold zone composition consisting of about 20 parts by volume of R1N, about 20 parts by volume of M0SÍ2 and about 60 parts by volume of SiC were similarly mixed. These powder mixes were then loaded into contiguous volumes of a hot press and heat pressed to form an ingot of approximately 60% theoretical density. This ingot was subsequently machined in green to form double zone blocks that were approximately 76.2 x 50.8 x 5.08 rn. Then, the machined blocks were subjected to hot isostatic pressing in which the blocks were impregnated at 1790 degrees centigrade and 2.109 kg / crrfc for 1 hour. After submerging under pressure, the dense block was machined with diamond until a fork lighter design (ie, a single end length of 38.1 mm x a thickness of 0.762 mm x 1.19 mm in width of width with a width of 15.24 mm groove This lighter showed good performance at 120 V. It had a sufficiently high resistivity (0.3+ 0.05 ohm-cm at 1300 ° C), a temperature relationship at low time (4 seconds at 1100 ° C) and € >Stable up to 132V).

EXAMPLE 2 Lighters were prepared in a manner similar to that described in Example 1, except that the composition was 60 v / o of PIN, 10 v / o of M0SÍ2 and 25 v / o of SiC and 5 v / o of alumina (? umitomo PKP-30). This lighter showed good performance at 230. It had a sufficiently high resistivity (1.2 ohm-cm at 1300 ° C), a temperature relationship at low time (5 seconds at 1100 ° C) and was stable up to 250V).

EXAMPLE 3 Blocks prepared in accordance with Comparative Example 2 were exposed to water with a temperature of 95 ° C for 20 minutes. After drying, these blocks showed a weight gain of about 1% as a result of hydrolysis of the PIN which formed alumina under heating at about 900 ° C. The blocks were then densified and the lighters formed as described in example 1. This lighter showed good performance at 150V. It had a sufficiently high resistivity (0.4 ohm-cm at 1300oC), a temperature relationship at low time (less than 5 seconds at 1100 ° C) and was stable up to 180V).

COMPARATIVE EXAMPLE 1 Lighters were prepared in a manner similar to that described in Example 1, except that the composition was 66-71 v / o PIN, 8.5-9 v / o dβ M0SÍ2 and 20.5-25 v / o SiC. No alumina was used in this composition. In a 120V application, this lighter possessed a temperature relationship at moderate time (6-7 seconds at 1100 ° C).

COMPARATIVE EXAMPLE 2 Lighters were prepared in a manner similar to that described in Comparative Example 1, except that the blocks were densified at an impregnation temperature of 1815 ° C. In a 230V application, this lighter was not only slow (10 seconds at 1100 ° C), but also unstable at 245V.

COMPARATIVE EXAMPLE 3 Lighters were prepared in a similar manner to that described in example 1, except that the composition was 65 v / o of PIN, 10 v / o of M0SÍ2 and 25 v / o of SiC. No alumina was used in this composition.

In a 120V application, it was found that this lighter possessed a resistivity of only about 0.1 ohm-cm, reaching 1300 ° C to only about 90V. It had this low resistivity even though it had less M0SÍ2 than in Example 1 and the same concentration of M0SÍ2 as in Example 2. The lighters of the present invention can be used in many applications, including lighting applications d € > Gas phase fuel such as ovens and cooking utensils, base plate heaters, boilers and stove tops.

Claims (10)

NOVELTY OF THE INVENTION CLAIMS

1. - A heating method, consisting of the step of providing a d-line voltage between 120V and 230V through a ceramic igniter with a hot zone composition consisting of: (a) between 50 and 80 v / o of an electrically insulating ceramic having a resistivity of at least about 1010 ohm-cm; (b) between 10 and 45 v / o of a semiconductor material having a resistivity d between about 1 and about 108 ohm-cm; (c) between 5 and 25 v / o of a metallic conductor having a resistivity of less than about 10-2 ohm-cm; and id) between 2.0 and 20 v / o of a resistivity comp compactor selected from the group consisting of metal oxides, metal oxynitrides, rare earth oxides, rare earth oxynitrides and mixtures thereof.

2. The method according to claim 1, further characterized in that the electrically insulating ceramic is selected from the group consisting of aluminum nitride, silicon nitride and boron nitride.

3. The method according to claim 2, further characterized in that the semiconductor material is selected from the group consisting of silicon carbide and boron carbide.

4. - The material of the method according to claim 3, further characterized in that the metallic conductor is selected from the group consisting of molybdenum disilicide, tungsten disilicide and titanium nitride.

5. The method according to claim 4, further characterized in that the electrically insulating material is aluminum nitride.

6. The method according to claim 5, further characterized in that the semiconductor material is silicon carbide.

7. The method according to claim 6, further characterized in that the metallic conductor is molybdenum disilicide.

8. The method according to claim 7, further characterized in that the aluminum nitride consists of between 50 and 7D v / o of the hot zone composition.

9. The method according to claim 8, further characterized in that the silicon carbide consists of between 20 and 30 v / o of the composition dß hot zone. 10.- The method according to the claim 9, further characterized in that the molybdenum disilicide consists of between 6 and 12 v / o of the hot zone composition. 11. The method according to claim 10, further characterized in that the resistivity enhancing compound is selected from the group consisting of aluminum oxide, aluminum oxynitride and mixtures thereof. 12. The method according to claim 4, further characterized in that the resistivity enhancing compound is selected from the group consisting of aluminum oxide, aluminum oxynitride and mixtures thereof. 13. The method according to claim 12, further characterized in that the resistivity enhancing compound consists of between 2 and 8 v / o of the hot zone composition. 14 - The method according to the claim 12, further characterized in that the resistivity improving compound consists of about 4 v / o of the hot zone composition. 15, - The method according to claim 12, further characterized in that the hot zone composition consists of 0.5 to 18.5 v / o of the resistivity enhancing compound. present in the form of granules with an average granule size dβ between 2 and 10 microns. 16. The method according to claim 11, further characterized in that the hot zone composition consists of 0.5 to 6.5 v / o of the resistivity enhancing compound, present in the form of granules with an average granule size of between 2 and 10 mieras 17. The method according to claim 12, further characterized in that the line voltage is 120 V, and the hot zone composition has a transverse measurement of less than 1.27 mm2 and a single end length of less than 38.1 mm. 18.- The method of compliance with the claim 12, further characterized in that the line voltage is 230 V, and the hot zone composition has a transverse measurement of less than 1.27 mm2 and a single end length of less than 38.1 mm. 19. A green body consisting of: (a) between 50 and 80 v / o of an electrically insulating ceramic granule with a resistivity of at least approximately lQ * or ohm-crn; (b) between l and 45 v / o of semiconductive ceramic granules having a resistivity of between about 1 and about 108 ohm-cm; (c) between 5 and 25 v / o of metallic conductor grains having a resistivity of less than about 10 ~ 2 ohm-cm; and (d) between 2.0 and 20 v / o of a resistivity enhancing compound selected from the group consisting of metal oxides, metal oxynitrides, rare earth oxides, rare earth oxynitrides, and mixtures thereof. 2) densify the green body to form a ceramic. 20. The green body according to claim 19, further characterized in that the resistivity enhancing compound is alumina. 21. The green body according to claim 20, further characterized in that the electrically insulating ceramic is aluminum nitride and the alumina is present essentially as a coating on the aluminum nitride granules. 22. The green body according to claim 20, further characterized in that between 0.5 v / o and 18.5 v / o of the alumina is present as granules having an average size of between 2 and 10 microns. 23. A concreted ceramic having a hot zone composition consisting of: (a) between 50 and 80 vol% of an electrically insulating material consisting essentially of aluminum nitride; ib) between 10 and 45 vol% of a semiconductor material selected from the group consisting of boron carbide and silicon carbide, and mixtures thereof; (c) between 5 and 25 vol% of a metallic conductor selected from the group consisting of molybdenum disilicide, tungsten disilicide and titanium nitride, and mixtures thereof; and (di) between 0.5 and 18.5 vol% of a resistivity enhancing compound selected from the group consisting of aluminum oxide, aluminum oxynitride and mixtures thereof, and with an average particle size of between about 2 and 10 microns. . 24, - A concreted ceramic having a hot zone composition consisting of: (a) between 50 and 80 vol% of an electrically insulating material consisting essentially of aluminum nitride; (b) between 10 and 45 vol% of a semiconductor material selected from the group consisting of boron carbide and silicon carbide, and mixtures thereof; (c) between 5 and 25 vol% dβ a metallic conductor selected from the group consisting of molybdenum disilicide, tungsten disilicide and titanium nitride, and mixtures thereof; and id) between 2 and 20 vol% of a resistivity enhancing compound selected from the group consisting of aluminum oxide, aluminum oxynitride and mixtures thereof.