Patent ID: 12191052

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a dielectric high gradient insulator and method of manufacture. The principles of the invention may be better understood with reference to the drawings and the accompanying description.

FIG.1shows a cross-sectional diagram of a prior-art metal-ceramic HGI. HGI10insulates two high-voltage electrodes15and25, and forms a vacuum seal around vacuum cavity65, which is cylindrically symmetric about a longitudinal axis Z. In a particle beam apparatus, for example, electrodes15and25may correspond to a cathode at ground potential and an anode, at +50 kilovolts, respectively. Cavity65is in high vacuum, with pressures in a range typically below 10−6torr. HGI10consists of metal layers30and dielectric layers40in an alternating arrangement. The metal layers are typically made of stainless steel, molybdenum, or Kovar, which is a nickel-cobalt ferrous alloy.

The material composition of the dielectric layers is an insulating plastic, such as polyimide and polystyrene, or an insulating ceramic, such as alumina (aluminum oxide, Al2O3). Metal layers30divide the voltage difference between the two electrodes, with a roughly linear dependence on the axial coordinate Z. The voltage standoff of HGI10is up to four times higher than that of a uniform insulator, having the same overall length and diameter. Alternatively, HGI10can be made much smaller than a uniform alumina insulator, and still provide the same voltage standoff.

FIG.2shows a cross-sectional diagram of an exemplary DHGI according to a preferred embodiment of the invention. DHGI110insulates high-voltage electrodes115and125, and forms a vacuum seal around vacuum cavity165, which may or may not be cylindrically symmetric with respect to longitudinal axis Z. Unlike HGI10, DHGI110has no metal layers; rather, it is comprised of a dielectric layer130having a high dielectric constant, ε2, and an adjacent dielectric layer140having a dielectric constant, ε1, where ε2>ε1, and the dielectric ratio ε2/ε1 is typically in a range of 10 to several thousands. For example, layer130may consist of barium titanate having a dielectric constant of ε2=500, and layer140may consist of aluminum oxide, or alumina, having a dielectric constant of ε1=7.4.

The dielectric constant, or relative permittivity, ε, of a material increases with the ability to modify a charge distribution inside the material by applying an external electric field. In the electrostatic regime, the charge distribution inside a dielectric material subject to an externally applied electric field, D, induces an internal electric field, Eint, equal to D(1/ε−1). The total field inside the dielectric is then (D+Eint), which is equal to D/ε. As ε goes to infinity, Eint approaches (−D), and the total field inside the dielectric diminishes to zero, which is the case of a perfect conductor, e.g. a metal with zero resistivity. Thus, for large values of ε2, the charge distribution on the surface of dielectric layer130is similar to that of a metal layer having the same dimensions.

By way of illustration, electrode115it taken to be a cathode at ground potential, and electrode125to be an anode at a high positive potential, for example, 50 kilovolts. Emission areas135designate areas on the surface of electrode115which are near to a “triple point” where the surface of electrode115meets the surface of layer130and vacuum165. Emission areas135are prone to secondary electron (SE) emission because of the presence of high extraction electric fields in these areas. As a result, SE's are emitted from electrode115, typically with kinetic energies higher than 10 electron volts, and with initial velocity vectors pointing in random directions. After emission, the SE's are accelerated by electric fields existing in vacuum cavity165. Once they are accelerated to energies of, say, 0.5 kilovolt or more, there is the possibility that they will generate additional SE's by colliding with the surface of one of the dielectric layers, thereby causing an electron avalanche and voltage breakdown by surface flashover.

Shaped electric field regions145, which are inside the vacuum cavity and in close proximity to the interface between layers130and140, are designed to prevent surface flashover. When ε2 is much greater than ε1, the electric field in regions145has a large component which is perpendicular to the Z-axis, and in a direction which deflects SE's away from the surface of layer140. Trajectory160illustrates one such path of an SE emitted at the surface of electrode115inside area135in a direction which would impact layer140, were it to travel in a straight line. As the SE approaches region145, a shaped electric field deflects trajectory160towards the Z-axis. As a result, the SE is absorbed on anode electrode125at a point which is located at an electron intercept distance ΔR, away from the vacuum surface of layer140.

The value of ΔR is proportional to the strength of the electric field component perpendicular to the Z-axis, in shaped electric field region145. The latter depends on the relative magnitudes of the dielectric constants ε2 and ε1, corresponding to dielectric layers130and140, respectively.

FIG.3shows a representative semi-log plot having a linear vertical scale for ΔR, in arbitrary units of length, and a logarithmic horizontal scale for ε2; the value of ε1 is fixed at 7.4. The value of ΔR is calculated by a computer simulation, in which electric fields are computed numerically including the effect of space charge inside the vacuum cavity, and SE trajectories are calculated using a conformal finite-element mesh. As ε2 approaches 10000, the curve approaches an asymptotic limit at about ΔR=7.5. However, such a large intercept distance is not needed to prevent the creation of electron avalanches; it is sufficient to choose a material having a more moderate dielectric constant, such as ε2=500, as indicated by point P inFIG.3. Generally, the fabrication of materials with more moderate values of dielectric constant requires less high dielectric filler material and presents fewer issues of thermal compatibility as will be explained further in the following sections.

FIG.4shows a cross-sectional diagram of an exemplary DHGI210according to a preferred embodiment of the invention, consisting of five dielectric layers230having a high dielectric constant ε2, such as ε2=500, and four dielectric layers240having a relatively lower dielectric constant ε1, such as ε1=7.4, arranged in an alternating structure. In this case there is a multiplicity of shaped dielectric field regions245, each of which can deflect SE's towards the Z-axis, and thereby inhibit voltage breakdown due to surface flashover.

FIG.5shows a cross-sectional diagram of an exemplary DHGI310, according to a preferred embodiment of the invention, consisting of a variable dielectric layer330having a gradual change in dielectric constant and a dielectric layer340with a fixed dielectric constant ε1, such as ε1=7.4. Furthermore,FIG.5illustrates that the geometry of DHGI310may be non-cylindrical; for example, it may be conical or some other shape. In variable dielectric layer330, the dielectric constant decreases from a high value of ε2A for material330A, such as ε2A=500, to a moderate value of ε2B for material330B, such as ε2B=125, and to a still lower value of ε2C, for material330C, such as ε2C=25. That is, the values (ε2A, ε2B, ε2C, ε1) are monotonically decreasing. The advantage of this arrangement is that the bonding of material330C to material340, at the interface between layers330and340, involves two materials whose physical properties, e.g. dielectric constants and thermal coefficients, are may be closely matched, so as to simplify the thermal processing.

By combining the features ofFIG.4andFIG.5, one may form more complicated embodiments of the invention, such as a DHGI with more than two dielectric layers having different values of dielectric constant arranged in an alternating structure and with at least one layer having a gradual variation in dielectric constant.

Dielectric Materials

High and variable dielectric material layers may preferably be made by casting and/or printing a mixture composed of a low dielectric matrix and high dielectric or metallic filler particles followed by a densification (sintering in ceramic materials) stage. The proportion of matrix to filler material is selected to achieve a desired dielectric constant value or profile.

The mixture can be in the form of a powder or a slurry. The material of the low dielectric matrix may be, for example: alumina (Al2O3), aluminum nitride (AlN), silicon dioxide (SiO2), silicon nitride (Si3N4), titanium dioxide (TiO2), polyamide, polystyrene, polyethylene, polyvinyl chloride (PVC), and plexiglass (PMMA, or Polymethyl methacrylate).

The material of the high dielectric filler particles may be, for example: BaTiO3, PbTiO3, LaTiO3, SrTiO3, doped NiO, CaCu3Ti4O12, doped TiO2or αFe0.5β0.5O3, where α represents the elements Ba, Sr, or Ca and β represents the elements Nb, Ta, or Sb.

DHGI Method of Manufacture

FIG.6shows a block diagram of a method600of manufacturing a DHGI having two or more dielectric layers. The method consists of:step610A—providing a low dielectric matrix material;step610B—providing a filler material comprising high dielectric or metallic particles;step610C—preparing mixtures of matrix and filler materials;step610D—casting and/or printing layers with a pre-determined composition;step610E—aligning and hot-pressing dielectric layers to form a stack; andstep610F—applying a densification process to the stack.

A co-sintered structure may have the advantage of providing superior vacuum tightness and mechanical stability, with fewer processing steps. In the case of ceramic dielectric layers, the densification process in step610F typically includes a sintering process. After step610F, the stack is cooled gradually to minimize thermally induced mechanical stresses.

An optional additional step610G may be desirable which consists of treating one or more surfaces of the dielectric layers that are subject to incident SE bombardment with an insulating material having a low SE emission yield; that is, a material whose SE emission yield is less than unity over a wide range of incident SE energies. Examples of such materials are metal oxides or nitrides, where the metal may be, for example, titanium, chromium, or vanadium. The treatment is preferably done by chemical or physical vapor deposition (CVD or PVD) or by doping the matrix material.

It will be appreciated that the above descriptions are intended only to serve as examples, and that many other embodiments are possible within the scope of the present invention as defined in the appended claims.