DC BULK CONDUCTIVE CERAMIC WITH LOW RF AND MICROWAVE LOSS

Abstract

A DC conductive, low RF/microwave loss titanium oxide ceramic provides, at room temperature, a bulk DC resistivity of less than 1×10.sup.11 ohm-meters and an RF loss tangent of less than 2×10.sup.−4 at 7.5 GHz and less than 2×10.sup.−5 at 650 MHz. The resistivity is reduced by oxygen vacancies and associated Ti.sup.3+ and/or Ti.sup.4+ centers created by sintering in an atmosphere containing only between 0.01% and 0.1% oxygen. The reduced resistivity prevents DC charge buildup, while the low loss tangent provides good RF/microwave transparency and low losses. The ceramic is suitable for forming RF windows, electron gun cathode insulators, dielectrics, and other components. An exemplary Mg.sub.2TiO.sub.4—MgTiO.sub.3 embodiment includes mixing, grinding, pre-sintering in air, and pressing 99.95% pure MgO and TiO.sub.2 powders, re-sintering in air at 1400° C.-1500° C. to reduce porosity, and sintering at 1350° C.-1450° C. for 4 hours in an 0.05% oxygen and 99.05% nitrogen atmosphere.

Claims

1. A method of manufacturing a DC conductive low RF/microwave loss ceramic suitable for implementation in a charged particle beam apparatus, the method comprising: a) preparing a mixture of precursor ceramic powders that includes a titanium-oxide based ceramic powder; b) pressing the mixed powders into a desired shape; and c) sintering the pressed powders in an atmosphere having an oxygen concentration that is reduced in comparison to air.

2. The method of claim 1, further comprising between steps a) and b) a further step of pre-sintering the mixture of precursor ceramic powders in air.

3. The method of claim 2, wherein the pre-sintering is at a temperature that is between 1150° C. and 1250° C.

4. The method of claim 1, further comprising between steps b) and c) a further step of re-sintering the pressed powders in an air atmosphere.

5. The method of claim 4, wherein the re-sintering is at a temperature of between 1400° C. and 1500° C.

6. The method of claim 4, wherein the re-sintering is continued until the pressed powders exhibit substantially no water absorbance, and until a porosity of the pressed powders is less than 4%.

7. The method of claim 1, wherein the sintering of step d) is at a temperature that is between 1350° C. and 1450° C.

8. The method of claim 1, wherein the precursor ceramic powders in step a) are at least 99.9% pure.

9. The method of claim 1, wherein step a) of the method further comprises grinding the mixed powders.

10. The method of claim 1, further comprising during step b) combining the mixed powders with a binder during pressing.

11. The method of claim 10, wherein the binder is a 10% solution of polyvinyl alcohol.

12. The method of claim 1, wherein the sintering in step c) is performed in an atmosphere having an oxygen concentration of between 0.01% and 0.1% oxygen.

13. The method of claim 12, wherein the between 0.01% and 0.1% oxygen of the atmosphere during the sintering of step c) is mixed only with one or more unreactive gases.

14. The method of claim 12, wherein the between 0.01% and 0.1% oxygen of the atmosphere during the sintering of step c) is mixed only with nitrogen, argon, or a combination of nitrogen and argon.

15. The method of claim 1, wherein the sintering of the pressed powders in step c) is at a temperature of between 1350° C. and 1450° C.

16. The method of claim 1, wherein step c) includes sintering the pressed powders during a time period of at least two hours.

17. The method of claim 1, wherein step c) includes sintering the pressed powders during a time period of at least four hours.

18. The method of claim 1, wherein in step c) the atmosphere consists of between 0.01% and 0.1% oxygen mixed with at least one of nitrogen and argon.

19. A composition of matter comprising a Mg.sub.2TiO.sub.4—MgTiO.sub.3 ceramic having, at room temperature, a loss tangent (tan d) of less than 2×10.sup.−4 at 7.5 GHz and less than 2×10.sup.−5 at 650 MHz, and having a DC bulk resistivity at room temperature of less than 1×10.sup.11 ohm-meters.

20. A component suitable for implementation in a charged particle beam apparatus, the component comprising a structure formed from a ceramic containing titanium oxide and having, at room temperature, a loss tangent (tan d) of less than 5×10.sup.−4 at both 7.5 GHz and at 650 MHz, and having a DC bulk resistivity at room temperature of less than 1×10.sup.12 ohm-meters.

21. The component of claim 20, wherein the ceramic is a Mg.sub.2TiO.sub.4—MgTiO.sub.3 ceramic having, at room temperature, a loss tangent (tan d) of less than 2×10.sup.−4 at 7.5 GHz and less than 2×10.sup.−5 at 650 MHz, and having a DC bulk resistivity at room temperature of less than 1×10.sup.11 ohm-meters.

22. The component of claim 20, wherein the DC bulk resistivity of the component when the component is heated to 150° C. is reduced by at least three orders of magnitude as compared to the bulk resistivity of the component at room temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] FIG. 1 is a plot of resistivity (DC bulk resistivity) as a function of oxygen concentration during sintering applicable to the exemplary embodiment ceramic of the present invention;

[0053] FIG. 2 is a plot of loss tangent as a function of oxygen concentration during sintering applicable to the exemplary embodiment ceramic of the present invention;

[0054] FIG. 3A is plot of resistivity as a function of sample temperature between 80° C. and 150° C., applicable to the exemplary embodiment ceramic of the present invention;

[0055] FIG. 3B is plot of resistivity as a function of sample temperature between 80° C. and 350° C., applicable to the exemplary embodiment ceramic of the present invention;

[0056] FIG. 4 is a flow diagram that illustrates a method embodiment of the present invention;

[0057] FIG. 5A is a plot as a function of time of current flowing out from a sample of a ceramic that was manufactured by sintering in air, measured during irradiation of the ceramic sample by an electron beam; and

[0058] FIG. 5B is a plot as a function of time of current flowing out from a ceramic sample similar to FIG. 5A but manufactured according to an embodiment of the present invention, measured during irradiation of the ceramic sample by an electron beam;

DETAILED DESCRIPTION

[0059] The present invention is a conductive titanium oxide ceramic (referred to herein as a “TiO” ceramic), and a method of manufacture thereof, wherein the ceramic includes Ti.sup.3+ and/or TO.sup.+ centers due to oxygen vacancies created by sintering the ceramic in a reduced oxygen environment. Here, “TiO” refers to any ceramic that includes titanium and oxygen in any ratio, for example TiO.sub.2, Ti.sub.2O.sub.3, etc.

[0060] Before the disclosed sintering method is applied, these TiO ceramics have bulk resistivities in the range of 10.sup.13 to 10.sup.15, as is typical for dielectric materials that are conventionally used to form components implemented in charged particle beam apparatus. However, as a result of applying the process disclosed herein, the bulk resistivity of the claimed ceramic material is reduced by two to three orders of magnitude, while the loss tangent is increased by only about a factor of 2.

[0061] Some embodiments are based on an MgO—CaO—TiO.sub.2 ceramic composition (referred to herein as an “MCT” ceramic) having a dielectric constant of between 18 and 140. Other embodiments are based on an MgO—TiO.sub.2 ceramic composition (referred to herein as an “MT” ceramic) having a dielectric constant between 13 and 18. An exemplary embodiment is a Mg.sub.2TiO.sub.4—MgTiO.sub.3 ceramic composition (referred to herein as a “MgTi” ceramic), having a DC bulk resistivity at room temperature that is reduced from a pre-sintered value of about 1×10.sup.13 Ω×m down to a resistivity after sintering of only between 5×10.sup.10 and 1×10.sup.11 Ω×m. At the same time, the disclosed sintering process increases the loss tangent of the MgTi ceramic of the exemplary embodiment by only about a factor of 2, from about 8×10.sup.−5 up to about 1.7×10.sup.−4 at 7.5 GHz, and from about 7×10.sup.−6 up to about 1.4×10.sup.−5 at 650 MHz

[0062] The disclosed ceramic materials are therefore suitable for forming components that are implemented in charged particle based apparatus such as accelerators, vacuum electronic devices, electron microscopes, and such like, in that they have a sufficient bulk conductivity to avoid component damage due to DC electrical charge buildup, while at the same time maintaining a sufficiently low loss tangent to maintain RF/microwave transparency and minimize RF/microwave power loss.

[0063] The conductivity of the disclosed ceramic can be optimized according to the requirements of a specific implementation by adjusting the oxygen concentration of the surrounding atmosphere during sintering. Data that relates the percentage of oxygen during sintering to the resulting DC bulk resistivity of the disclosed ceramic for the exemplary Mg.sub.2TiO.sub.4—MgTiO.sub.3 embodiment is numerically presented in Table 1 below, and is graphically presented in FIG. 1.

TABLE-US-00001 TABLE 1 DC bulk resistivity of the disclosed MgTi ceramic as a function of oxygen concentration during sintering. %O.sub.2 Resistivity (ohm*m) ≥1.0%  2.4 × 10.sup.+13 0.1% 2.98 × 10.sup.+12 0.05% 1.06 × 10.sup.+12 1.05 × 10.sup.+11 1.082 × 10.sup.+11  1 × 10.sup.−4% (1 ppm) 1.17 10.sup.+7

[0064] FIG. 2 presents graphical data that indicates the variation of the loss tangent (tan d) for the embodiment of FIG. 1 as a function of oxygen concentration during sintering.

[0065] In embodiments, as the temperature of the disclosed TiO based ceramic is increased, its DC bulk resistivity can be further decreased by 3-4 orders of magnitude or more. As a result, varying the temperature of a component that is made from the disclosed ceramic material can also be used in embodiments as a mechanism for adjusting its conductivity to meet specific implementation requirements. With reference to FIG. 3A, when the exemplary embodiment of Mg.sub.2TiO.sub.4—MgTiO.sub.3 is heated from room temperature to 150 degrees centigrade, its DC bulk resistivity is further decreased by 3 orders of magnitude, from 10.sup.10 Ω×m to below 10.sup.7 Ω×m, while the increase in tan d is only about 20%. Conventional ceramics typically exhibit a dependence of tan d on temperature of as much as an order of magnitude over the same temperature range.

[0066] With reference to FIG. 3B, heating of the exemplary embodiment of FIG. 3A from room temperature to 300 degrees C. results in a conductivity increase of about eight orders of magnitude.

[0067] In embodiments, this variability of the bulk resistivity with temperature provides an additional method of discharging the ceramic components of charged particle beam apparatus by periodically heating the system, or by operating the system at an elevated temperature. In addition, this dependence of the conductivity on temperature can also be used as a mechanism for adjusting the bulk conductivity of a component made using the disclosed ceramic materials to meet specific implementation requirements.

[0068] The method of manufacturing the disclosed ceramic materials, according to the present invention, includes preparing a mixture of precursor ceramic powders that includes a titanium-oxide based ceramic powder, pressing the mixed powders into a desired shape; and sintering the pressed powders in an atmosphere having an oxygen concentration that is reduced in comparison to air.

[0069] With reference to FIG. 4, embodiments of a method of manufacturing the exemplary MgTi ceramic include mixing/grinding precursor MgO powder with TiO.sub.2 powder 400, pre-sintering the mixture in air at 1150°−1250° C. 402, grinding the mixture again 404, and compressing the mixture into a desired shape and size 406. The compressed mixture is then re-sintered at 1400°-1500° C. in air 408. In embodiments, this re-sintering 404 is continued until the pressed material exhibits substantially no water absorbance, having a porosity of less than 4%. Finally, the pressed material is sintered at 1350° C.-1450° C. in a reduced oxygen atmosphere, such as an atmosphere that includes between 0.01% and 0.1% oxygen combined with one or more non-reactive gases 410, such as nitrogen and/or argon. This reduced oxygen sintering 410 can be applied for a time period of at least 2 hours, and in embodiments at least 4 hours.

[0070] In embodiments, the precursor powders are at least 99.9% pure. The powders can be mixed and/or ground 400, 404 in a grinder (for example, in an attritor or other grinder). Either or both of the grinding steps 400, 404 can be performed for a period of at least three hours. The mixture can be combined with a binder during pressing 408, which can be a 10% solution of polyvinyl alcohol.

[0071] In an exemplary embodiment, MgO and TiO.sub.2 powder of at least 99.95% purity are mixed and ground 400 in an attritor for three hours. After drying and pre-sintering 402, the mixture is re-ground in the attritor 404 for an additional three hours. The mixture is then hydraulically pressed 406 with a 10% polyvinyl solution used as a binder. The pressed mixture is pre-sintered 408 in air at 1400-1500 degrees centigrade, followed by sintering 410 at 1350-1450 degrees centigrade in an atmosphere of 0.05% oxygen and 99.95% nitrogen for four hours. The resulting material has a dielectric constant of approximately 15, a bulk resistivity of approximately 1.4×10.sup.11 w×m, a tan d (loss tangent) of approximately 1.4×10.sup.−5 at 650 MHz and a tan d of approximately 1.7×10.sup.−4 at 7.5 GHz.

[0072] Method of use embodiments of the present invention include determining an optimal conductivity for a specified application, determining the corresponding oxygen concentration during sintering that will provide the determined optimal conductivity, and forming the component from the disclosed ceramic, wherein the sintering is performed at the determined oxygen concentration. In embodiments, the method further comprises adjusting the bulk resistivity of the component to a desired value after implementation by controlling the temperature of the component, for example in the range of 20 to 150 degrees centigrade.

[0073] FIGS. 5A and 5B present results of a comparative test performed on an MgTi ceramic sample prepared according to the exemplary embodiment described above, and on an MgTi ceramic sample that was otherwise identical, except that the final sintering step 410 was also performed in air. In each case, the ceramic sample to be tested was mounted in an aluminum holder that was electrically isolated from the rest of the system. A copper wire was clamped between a nut on the top plate of the holder and run to a high voltage meter configured to measure current. The holder and sample were mounted within a vacuum chamber and exposed to a continuous beam from an electron gun.

[0074] For each of the samples, the electric current flowing from the ceramic sample was monitored, indicating the degree of electrical charging of the ceramic as it was exposed to the electron beam. The electron gun filament was maintained at a fixed current to ensure beam stability and reproducibility. The beam was tightly focused to ensure that charge was imparted only to the ceramic sample. In each case, data was collected for 2.5 hours to study the time dependence of charging and discharging. FIG. 5A shows the data recorded using the sample that was sintered entirely in air, and FIG. 5B shows the data recorded using the sample for which the final sintering step was performed in an atmosphere of 0.05% oxygen and 99.05% nitrogen.

[0075] It can be seen from the figures that there is very little variation in the measured current, and hence very little variation in the DC electric charge, for the sample where the final sintering step was in 0.05% oxygen (FIG. 5B), indicating that the ceramic that was prepared according to the present invention was able to efficiently discharge the electrons that it received from the electron beam. In contrast, there is much more structure in the measured current of the sample that was sintered entirely in air (FIG. 5A). In particular, some initial charge buildup 500 occurred during approximately the first 5 minutes, followed by a discharge 502 indicated by the increased current. Over the next approximately 2.25 hours, a non-periodic behavior of charging (decreases in current) and discharging can be seen in FIG. 5A.

[0076] The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. Each and every page of this submission, and all contents thereon, however characterized, identified, or numbered, is considered a substantive part of this application for all purposes, irrespective of form or placement within the application. This specification is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of this disclosure.

[0077] Although the present application is shown in a limited number of forms, the scope of the invention is not limited to just these forms, but is amenable to various changes and modifications without departing from the spirit thereof. The disclosure presented herein does not explicitly disclose all possible combinations of features that fall within the scope of the invention. The features disclosed herein for the various embodiments can generally be interchanged and combined into any combinations that are not self-contradictory without departing from the scope of the invention. In particular, the limitations presented in dependent claims below can be combined with their corresponding independent claims in any number and in any order without departing from the scope of this disclosure, unless the dependent claims are logically incompatible with each other.