NEG cartridge

Abstract

A non-evaporable getter (NEG) cartridge 10, in particular for an NEG pump, comprises a base element 12 having an opening 14 to receive a heater, and a lattice structure 18 at least partially surroundi

Description

The present invention relates to a non-evaporable getter cartridge and an NEG pump comprising such an NEG cartridge. Common NEG pumps comprise NEG elements made from an NEG material, wherein by the NEG material gas molecules and particles from a vessel are captured by physical sorption at the surface of the NEG elements thereby creating a vacuum in the vessel. If the complete surface of the NEG elements is occupied, reactivation of the NEG elements becomes necessary by heating of the NEG elements above a certain reactivation temperature. Hence, the NEG elements on the one hand need a large surface to increase the pump performance and on the other hand need to be arranged in contact with or close proximity to a heater in order to be able to reactivate the NEG material. Common NEG elements are shaped as disks, which are stacked along a cylindrical ceramic resistive heater. By each disk an additional surface is provided increasing the pump performance of the NEG pump. Therein, the NEG disks are fabricated in the prior art by sintering or pressing the NEG material. Hence, the possible shapes of the NEG elements are very limited and so are the possibilities to increase their surface. It is an object of the present invention to provide an NEG cartridge with an increased pump performance. The problem is solved by a non-evaporable getter (NEG) cartridge according to claim 1 and an NEG pump according to claim 16. The NEG cartridge, in particular for an NEG pump, according to the present invention comprises a preferably cylindrical base element, wherein the base element has an opening to receive a heater. In particular, the opening extends through the whole base element or at least partially into the base element along the axial direction of the base element. Further, a lattice structure at least partially, and preferably fully, surrounds the base element. Preferably, the base element is in the center of the lattice structure. In particular, the lattice structure surrounds the lateral surface of the preferably cylindrical base element and may in some embodiments extend at least at one end beyond the axial end of the cylindrical base element. In particular, if the opening of the base element is built as blind hole, the lattice structure may extend beyond the axial end of the base element of a side, which does not contain the opening of the blind hole. Hence, the heater can be inserted into the base element from the other side. According to the present invention the lattice structure is made from an NEG material. Due to the lattice structure the surface of the NEG material is increased thereby increasing the pump performance, i.e. pumping speed and capacity of the NEG cartridge. Therein, an NEG element is any material which acts as getter material by physisorption of gas particles and molecules (in particular after activation) to the surface of the NEG element. Preferably, the base element and the lattice structure is made from the same material. Hence, the NEG cartridge is completely made of one single material and thus also the base element together with the lattice structure act as getter material. Preferably, the NEG cartridge contains a base plate, wherein the base plate is arranged at an axial end of the base element, wherein the base element extends in particular perpendicular from the base plate. Therein, the base plate may serve as building platform and provides structural stability of the lattice structure. Hence, handling of the NEG cartridge is simplified and the NEG cartridge can be implemented into the existing NEG pumps replacing conventional disk cartridges of the prior art. Therein, the base plate may be shaped and adapted in order to fit into existing NEG pumps. In particular, the base plate is made from the same material as the lattice structure and thus contributes with its surface to the overall pump performance of the NEG cartridge. Preferably, the NEG cartridge has a symmetry axis coinciding with the axial axis of the NEG cartridge or the axial axis of the base element. Preferably, the base element as well as the lattice structure are built or made by additive manufacturing such as direct metal laser sintering (DMLS) or the like. In particular, the base element as well as the lattice structure are fabricated in the same fabrication step, i.e. by a layer-by-layer fabrication according to the additive manufacturing. If the NEG cartridge comprises a base plate, also the base plate is built by additive manufacturing and may be built in the same fabrication step as the base element and / or the lattice structure. Preferably, the base element and the lattice structure are integrally built. In particular, the base element and the lattice structure are built as one piece. If the base element and the lattice structure are built by additive manufacturing, the structural elements are connected to each other and integrally fabricated in order to provide structural stability. In addition, if the NEG cartridge comprises a base plate, also the base plate may be integrally built with the base element and / or the lattice structure in particular in order to provide structural stability to the lattice structure or the NEG cartridge. Thus, the whole NEG cartridge may be a single piece. Preferably, the lattice structure is optically blind, in particular in a radial direction or in a tangential direction. Herein, optical blindness refers to the property of the lattice structure that no straight line can be drawn from the lateral surface of the base element to the radial outer surface of the lattice structure or across the lattice structure in a tangential direction. Thus, gas particles entering the lattice structure are trapped within the lattice structure, thereby increasing the likelihood that the gas particles collide with a surface of the lattice structure to be absorbed by the surface of the NEG material formed by the lattice structure. In other words, when seen from the side, it is not possible to optically look through the lattice structure and the lattice structure is completely obstructing any direct and straight path through the NEG cartridge in a radial or tangential direction. At the same time, heat radiation from the central base element is efficiently captured by the lattice structure and efficient activation is enabled. Preferably, the lattice structure has a volume filling factor of less than 25% and preferably less than 15%. Therein, the volume filling factor defines the portion of the total or bulk volume Vt occupied by the structural elements of the lattice structure. In other words, the volume filling factor is the ratio of Vs / Vt with Vs the volume of the structural elements of the lattice structure. A theoretical volume filling factor of 0% would mean the absence of any lattice structure and a volume filling factor 100% would mean a bulk element of the NEG material. According to the present invention the volume filling factor is preferably between 5% and 25% and more preferably between 5% and 15%. Hence, likelihood of gas particles entering the volume of the lattice structure is increased due to the low filling factor of the lattice structure, while at the same time the surface is increased by the lattice structure such that even for such low volume filling factors a sufficient large surface of the NEG material is provided by the lattice structure. Preferably, the lattice structure is made from titanium (Ti), Ti64 or another Ti alloy. In a preferred embodiment the base element, the lattice structure and, if present, the base plate are made from the same material, preferably using additive manufacturing. Thus, the whole cartridge may be made from a single material such as Ti, Ti64 or another Ti alloy. Therein, it has been shown that Ti, Ti64 or another Ti alloy act as a getter material and can be used as NEG material after activation. At the same time, it has been shown that Ti, Ti64 or another Ti alloy can be used in additive manufacturing and in particular in DMLS. Preferably, the lattice structure is made from zirconium (Zr), Hafnium (Hf) or alloys thereof. In a preferred embodiment the base element, the lattice structure and, if present, the base plate are made from the same material with additive manufacturing. Thus, the whole cartridge may be made from a single material such as Zr, Hf or alloys thereof. Preferably, the lattice structure has a homogeneous porosity and in particular the porosity is constant over substantially the entire lattice structure. Alternatively, the lattice structure has an inhomogeneous porosity and in particular the porosity changes in the radial direction of the lattice structure. Therein, the porosity is defined as the ratio of the volume Vv of the voids to the overall or bulk volume VT. Therein Vv=1-Vs as defined with regard to the volume filling factor. In particular, the porosity is described by the ratio Vv / VT. Due to use of additive manufacturing, the porosity can be tailored to the specific application and needs. In particular, the porosity may decrease in the radial direction from the center to the outer side of the lattice structure. Hence, gas particles are efficiently trapped within the volume of the lattice structure and likelihood of a collision of the gas particles with structural elements of the lattice structure is enhanced. Due to the decreased porosity in the radial direction, it is unlikely for gas particles which have entered the volume of the lattice structure to leave the lattice structure again. Hence, pump performance is increased due to an increase of the likelihood of the gas particles being adsorbed by the lattice structure. Preferably, the lattice structure comprises a plurality of layers. In each layer substantially identical lattice elements are used to create the lattice structure. In particular, the lattice elements in a single layer are regularly and evenly arranged or distributed. Hence, the individual layers comprise a specific number of lattice elements to form the respective layer, wherein the lattice structure is built by the consecutive arrangement of layers in the radial direction. Therein, by each layer a cylindrical shell is formed providing part of the volume of the lattice structure, wherein in the cylindrical shell the respective lattice elements of the layer are arranged. Preferably, adjacent layers overlap by more than 2%, preferably by more than 5% and more preferably by more than 10% to create structural and thermal connection between the lattice elements of the adjacent layers. Consequently, a certain layer of the lattice structure extends into the adjacent layer such that a volume overlap of 2% or more, preferably, 5% or more and more preferably 10% or more is established. By this overlap, the lattice elements of one layer are merged with lattice element of the adjacent layer. Thereby, structural stability of the lattice structure is provided. At the same time, areas of overlap provide the contact surfaces necessary for heat conduction across the lattice in order to heat up the lattice structure for activation or reactivation. Preferably, one or more layers comprise a lattice element formed as a clockwise or counterclockwise helically shaped wire. Therein, the direction “clockwise” and / or “counter-clockwise” are defined when looked at the NEG cartridge from above, i.e. from the end of the NEG cartridge opposite to the base plate. Hence, one layer may comprise a clockwise helically shaped wire, wherein a directly adjacent layer may comprise a counter-clockwise helically shaped wire. In particular, also the combination of clockwise and counterclockwise helically shaped wires in a single layer is possible in order to create a meshed structure in that single layer. Preferably, the wire is round, i.e. comprises a circular cross-section. Alternatively, the wire is elliptically shaped, i.e. has an elliptical cross-section. Therein, preferably the long axis of the ellipse extends tangentially to the lattice structure and the short axis extends radially. Hence, by the elliptical shape of the wire the surface of the lattice structure is increased without substantially influencing the filling factor of the lattice structure. Of course, round and elliptical wires can be mixed in a single lattice structure such that for example the wires of one layer are formed round, wherein the wires of another layer can be formed elliptically. Thereby, the porosity can be tailored according to the specific need and in accordance with the trade-off of maximizing the parameters of surface of the lattice structure, trapping ability for gas particles, which already have entered the lattice structure and the ability of gas particles entering the lattice structure. Preferably, the layers have a different number of lattice elements and / or different shaped lattice elements and / or differently arranged lattice elements. Alternatively, the lattice elements of at least two and preferably all layers are identically shaped, i.e. may have the same cross section and / or may be all shaped as helical wires, and the layers may only differ in the number of lattice elements and / or distance therebetween. Preferably, the lattice structure comprises a plurality of groups of layers, wherein each group comprises at least two layers, wherein in each layer of one group the same number of lattice elements are present. For example, the lattice structure may comprise three groups of layers, wherein in the radial direction starting from the base element in the first group eight layers are present, wherein each layer has between 8 and 16 lattice elements. For example, each layer may have 12 wires, wherein these wires are alternating clockwise and counterclockwise helically shaped from one layer to the next within the first group. Further, the lattice structure may have a second group of layers comprises again 8 layers, wherein each layer may comprise 16-32 lattice elements. For example, each layer of the second group of layers comprises 24 wires, which are alternating arranged clockwise and counterclockwise from one layer to the next layer. In addition, the lattice structure may comprise a third group of layers containing four layers, wherein each layer comprises 24-48 lattice elements. For example, in each layer of the third of group layers 36 clockwise and counterclockwise helical shaped wires are arranged combinedly in each layer. By adaption of the number of lattice elements, the porosity of the lattice structure can be adapted. At the same time the lattice structure can be formed optically blind, improving the trapping of gas particles inside the volume of the lattice structure and increasing the effect of radiative heating. Of course, other configurations are also possible having more groups of layers or less groups of layers and also the configuration of individual layers of each group can be different as well as the number of layers in each group may be different. In particular, also the given numbers for the lattice elements of each layer in each of the groups are only examples and the present invention is not limited to these specific numbers. Further, the NEG cartridge comprises a plurality of radially extending heat fins, which are connected to the base element. Preferably, the plurality of radially extending heat fins extend along the complete axial length of the base element or at least along a part of the axial length of the base element. In particular, the radially extending heat fins extend from one end of the cylindrical base element to the opposite axial end of the base element. In particular, if a base plate is present, the heat fins may extend from the base plate in the axial direction and more particularly may extend substantially perpendicular from the base plate. By the heat fins, heat of the heater that is inserted into the opening of the base element is efficiently conducted into the lattice structure thereby improving homogeneity of the heat in the lattice structure and providing efficient reactivation or activation of the NEG material of the lattice structure. In particular, the heat fins are extending in a substantial radial direction. Thus, the heat fins are forming a star with the spikes of the star extending towards the radial outer surface of the lattice structure and in particular completely from the base element to the radial outer surface of the lattice structure. Alternatively, the heat fins are wound in a spiraling manner and thus have a radial component as well as a tangential component of their extension. In particular, by the spiraling heat fins a galaxy shape structure is formed, wherein the galaxy arms are formed by the heat fins. Consequently, a homogenous distribution of the heat is provided. At the same time an optically blind structure can be provided, in particular if the combined circumferential extension of the heat fins is larger than 180° and preferably 360° or larger. Preferably, the transversal width of the heat fins decreases from the base elements towards the outer surface of the lattice structure. Consequently, the cross-section of the heat fin decreases and is larger close to the center of the lattice structure and smaller towards the outer part of the lattice structure. Thus, more heat is conducted closer to the center and less heat is conducted by the outer parts of the heat fins according to the specific requirements of heat in the specific parts of the lattice structure. Preferably, the heat fins are made from the same NEG material as the lattice structure. In particular, the heat fins are built by additive manufacturing at the same time the lattice structure and / or the base element is built. In particular, the base element, the lattice structure and the heat fins are integrally built in particular as one piece. In particular, the heat fins are made from Ti, Ti64 or another Ti alloy. Preferably, the maximum distance of any point of the lattice structure to one of the heat fins is smaller than 5cm, preferably smaller than 3cm and more preferably smaller than 2cm. Hence, the maximum distance of any point of the lattice structure is small such that each point of the lattice structure can be sufficiently heated by the heat fins due to the close distance between any point of the lattice structure and the heat fins conducting the heat from the base element towards the outer parts of the lattice structure. In another aspect of the present invention an NEG pump is provided comprising an NEG cartridge as described before and a heater extending into the opening of the base element to reactive or activate the NEG material of the lattice structure. Preferably the heater is rod shaped. Preferably, the heater is a ceramic and / or resistive heater. Preferably, the opening has a size conforming with the size of the heater such that close contact between the base element and the heater is provided. Hence, by the NEG cartridge according to the present invention an improved NEG cartridge is provided which can replace existing and conventional NEG cartridges comprising disk shaped NEG elements. Therein, by the lattice structure made from the NEG material, improved pump performance is provided compared to conventional disk shaped NEG elements. In the following the present invention is described in more detail with reference to the accompanying figures. The figures show: Figure 1 a first embodiment of an NEG cartridge according to the present invention, Figures 2A, 2B a detailed view of the NEG cartridge according to the present invention, Figure 3A, 3B another detailed view of the cartridge according to the present invention, Figure 4 a top view of the schematic NEG cartridge comprising layers, Figure 5 top view of the schematic NEG cartridge comprising groups of layers, Figure 6A, 6B an embodiment of an NEG cartridge comprising heat fins in a first embodiment and Figure 7A, 7B comprising heat fins in a second embodiments. Referring to Figure 1 showing an NEG cartridge 10 in particular for an NEG vacuum pump. The NEG cartridge 10 comprises a cylindrical base element 12. The base element 12 comprises an opening 14 which is configured to receive a heater and in particular a rod shaped heater such as a resistive heater for activation and reactivation of the NEG cartridge 10. The base element 12 is surrounded by a lattice structure 18. Structural stability is further provided by a base plate 16 which is connected to an axial end of the base element 12, wherein the base element 12 extends in particular perpendicular from the base plate 16. Therein, the lattice structure 18 as well as the base element 12 are built by additive manufacturing and in particular DMLS. Therein, the lattice structure 18 and the base element 12 may be built during the same fabrication step. In particular, the base element 12 and the lattice structure 18 are built from the same material. The lattice structure 18 is built from an NEG material, which provides getter activity in order to provide a pumping action of the NEG cartridge. Therein, the lattice structure is built from titanium (Ti), Ti64 or another Ti alloy. It has been shown that these materials can be used in an additive manufacturing process such as DMLS and at the same time, after activation, are able to act as NEG material. In the case that the NEG cartridge 10 contains a base plate 16, also the base plate 16 can be built by additive manufacturing together with the lattice structure 18 and / or the base element 12. Alternatively, the base plate 16 can be provided as a building platform for the base element 12 and / or the lattice structure 18. In particular, the whole NEG cartridge 10 may be a single piece which is robust and easy to handle compared to sintered metal disks of conventional NEG cartridges. As shown in Figure 1, the lattice structure 18 is built as periodic pattern providing a small filling factor. In particular, the volume filling factor of the lattice structure 18 is less than 25% and preferably less than 15%. At the same time, due to the lattice structure 18, the surface of the NEG material provided by the lattice structure 18 is increased, increasing the pump performance of the NEG cartridge 10. Further, as visible in Figure 1, the lattice structure 18 is optically blind. This means that no straight line can be drawn from the outer surface to the base element 12 or in a tangential direction to the lattice structure 18. Hence, due to the open structure of the lattice structure 18 gas particles can easily enter into the lattice structure 18 and are trapped within the volume. Thus, by the lattice structure 18 likelihood of a collision of the gas molecules with a surface of the NEG material provided by the lattice structure 18 is enhanced. Consequently, the pump performance is improved. Therein, the NEG cartridge 10 of the present invention may be configured such that it may replace NEG cartridges of existing NEG vacuum pumps comprising NEG elements that are disk shaped which only provide an inferior pumping activity compared to the NEG cartridge 10 of the present invention. Therein, the porosity of the lattice structure 18 can be tailored to the specific needs thereby maximizing the parameters of surface of the lattice structure, trapping ability for gas particles, which already have entered the lattice structure and the ability of gas particles entering the lattice structure. In the following it is referred to Figures 2A and 2B. In the Figures 2A and 2B for simplicity only the base element 12 and individual lattice elements 20, 22 are shown. Therein, the lattice elements 20, 22 are provided in the shape of clockwise or counter-clockwise helically shaped wires winding around the cylindric base element 12. Therein, as indicated in Figure 4 the individual lattice elements are arranged in layers 32. The lattice elements 20, 22 in each layer 32 may be identical or almost identically shaped. Therein, the lattice elements of adjacent layers may be differently shaped or identically shaped. As shown in Figures 2A, 2B in a first layer 32 a first lattice element 20 is arranged which is in direct contact with the base element 12. Figure 2B shows a second lattice element 22 is provided which is arranged in an adjacent or second layer 32 and thus the second lattice element 22 is radially distant or apart from the lateral surface of the cylindrical base element 12. Therein, adjacent layers may overlap by more than 2% and preferably more than 5% and more preferably by more than 10%. Due to the overlap at the crossing point 23 of the first lattice element 20 and the second lattice element 22 of adjacent layers 32, structural and thermal connection is established which, on the hand provides structural stability of the overall lattice structure 18 and, on the other hand enables heat conductance across the lattice structure 18 from the base element 12 towards the outer parts of the lattice structure 18. Contrary, in Figures 3A, 3B the clockwise lattice element 20 and the counterclockwise lattice element 22' are arranged in the same layer such that at the crossing point 23 a full overlap between the first lattice element 20 and the second lattice element 22' exists. Therein, the directions "clockwise" and "counter-clockwise" are defined when viewed from above along the axial axis of the base element 12 or the cartridge 10. Figure 5 shows a schematic structure of the lattice structure 18 comprising a plurality of groups of layers 34, wherein each group comprises at least two layers 32 as exemplified in Figure 4. Therein, each layer 32 of one group 34 may have the same number of lattice elements. Thus, a first group 34 may have 12 instances of lattice elements each, wherein each layer 32 of a second group 34 may have 24 instances and a third group 34 may have 36 instances of lattice elements. Therein, the lattice elements may also be clockwise and / or counterclockwise helically shaped wires as exemplified in Figures 2A, 2B and Figures 3A, 3B. Thus, by arranging the layers 32 into a plurality of groups 34 of layers, porosity of the lattice structure 18 can be adapted to the specific needs of the application. At the same time, despite a regularly patterned lattice structure being provided, due to the plurality of groups 34 an optically blind lattice structure 18 can be provided. In the following it is referred to Figures 6A and 6B. In this embodiment the NEG cartridge 10 comprises a plurality of heat fins 36 which are radially extending from the base element 12 in a star like manner. By the heat fins 36 heat from the base element 12 is conducted towards the outer most parts of the lattice structure 18. Therein, the lateral width of the respective heat fins may decrease towards the outer surface of the lattice structure 18 in accordance with the heat requirement to be conducted from the heater arranged in the central opening 14 (Figure 1) towards the outer parts of the lattice structure 18. In Figures 7A and 7B another embodiment is shown, wherein the heat fins 36' are shaped spiraling or in a winding manner. Therein, the cross-section has a galaxy shape, wherein the arms of the galaxy are provided by the heat fins 36'. Also in the embodiment of Figures 7A and 7B the lateral cross-sections of the heat fins are decreasing towards the outer surface in accordance with the requirement of heat conductance. Thus, the volume occupied by the lattice structure 18 can be maximized. Therein, the heat fins 36 in the embodiment of Figures 7A and 7B extend for an angle of about 360° providing an optical blind NEG cartridge. In the embodiments of Figures 6A, 6B and 7A, 7B, the heat fins 26 may extend at least partially and preferably along the whole axial length of the base element. In the case a base plate is present, the heat fins 36 may extend from the base plate 16 and in particular extend substantially perpendicular from the base plate 16. Therein, the heat fins 36, 36' are also fabricated by an additive manufacturing process. In particular, also the heat fins 36, 36' are fabricated in the same process as the lattice structure 18 and / or the base element 12. Therein, preferably also the heat fins 36, 36' are made from the same material as the lattice structure 18, i. e. preferably Ti, Ti64 or another Ti alloy and thus also contribute to the pump performance of the NEG cartridge with their surfaces. In addition, in the embodiments of Figures 6A, 6B and 7A, 7B, the maximum distance of any point of the lattice structure to one of the heat fins 36, 36’ is small increasing the efficiency of heating of the lattice structure 18 either by heat conductance via the crossing points 23 in the lattice structure 18 or via heater radiation. Reference List 10 NEG cartridge 12 base element 14 opening 16 base plate 18 lattice structure 20 lattice element 22, 22’ lattice element 23 crossing point 32 layer 34 group of layers 36, 36’ heat fin

Claims

1. NEG cartridge, in particular for an NEG pump, comprisinga base element, wherein the base element has an opening to receive a heater,a lattice structure at least partially surrounding the base element, wherein the lattice structure is made from an NEG material.

2. NEG cartridge according to claim 1, wherein the base element and the lattice structure are built by additive manufacturing.

3. NEG cartridge according to claim 1 or 2, wherein the base element and the lattice structure are integrally built.

4. NEG cartridge according to any of claims 1 to 3, wherein the lattice structure is optically blind.

5. NEG cartridge according to any of claims 1 to 4, wherein the lattice structure has a volume filling factor of less than 25% and preferably less than 15%.

6. NEG cartridge according to any of claims 1 to 5, wherein the lattice structure is made from Ti, Ti64 or another Ti alloy.

7. NEG cartridge according to any of claims 1 to 6, wherein the porosity changes in the radial direction and preferably decreases in the radial direction.

8. NEG cartridge according to any of claims 1 to 7, wherein the lattice structure comprises a plurality of layers, wherein in each layer substantially identical lattice elements are used to create the lattice structure.

9. NEG cartridge according to claim 8, wherein adjacent layers overlap by more than 2%, preferably by more than 5% and more preferably by more than 10% to create a structural and thermal connection between the lattice elements of the adjacent layers.

10. NEG cartridge according to claim 8 or 9, wherein one or more layers comprise as a lattice element a clockwise and / or counter-clockwise helically shaped wire.

11. NEG cartridge according to claim 10, wherein the wire is elliptical, wherein preferably the long axis of the ellipse extends tangentially and the short axis extends radially.

12. NEG cartridge according to any of claims 8 to 11, wherein the lattice structure comprises a plurality of groups of layers, wherein in each layer of one group the same number of lattice elements are present.

13. NEG cartridge according to any of claims 1 to 12, wherein a plurality of radially extending heat fins is connected to the base element.

14. NEG cartridge according to claim 13, wherein the heat fins are extending in the radial direction or wound in a spiraling manner.

15. NEG cartridge according to claim 13 or 14, wherein the maximum distance of any point of the lattice structure to one of the heat fins is smaller than 5 cm, preferably smaller than 3 cm and more preferably smaller than 2 cm.

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