A METHOD FOR CUTTING REFRACTORY METALS

Abstract

The invention relates to a method for cutting refractory metals, in which a solid body (1) made of a refractory metal is mechanically machining cut with a cutting apparatus (4, 7), wherein the cutting apparatus (4, 7) is wetted for cutting with a fluid (6) having at least 50 weight % water, wherein the cutting apparatus (4, 7) is brought to a positive electrical potential in relation to the solid body (1) during cutting. The invention also relates to a disc produced from a refractory metal using such a method, and such a disc that has an oxide layer with a thickness of between 2 nm and 1,000 nm on the cutting surface.

Claims

1. A method for cutting refractory metals, in which a solid body made of a refractory metal is mechanically machining cut with a cutting apparatus, wherein the cutting apparatus is wetted for cutting with a fluid having at least 50 weight % water, wherein the cutting apparatus is brought to a positive electrical potential in relation to the solid body during cutting.

2. The method of claim 1, wherein the cutting surface of the solid body is oxidized on the surface by the positive electrical potential of the cutting apparatus in relation to the solid body and by the aqueous fluid, so that an oxide layer forms on the cutting surface during cutting, wherein preferably an amorphous oxide layer forms on the cutting surface.

3. The method of claim 2, wherein due to the oxide layer formed during cutting, an absorption of hydrogen and oxygen through the oxide layer into the interior of the solid body is reduced or prevented, wherein the absorption of nitrogen and/or carbon and/or other gases through the oxide layer into the interior of the solid body is preferably also reduced or prevented.

4. The method of claim 1, wherein as a cutting apparatus, a blade of a saw device is used, preferably a wire of a wire saw, a thread of a thread saw, a band of a band saw, a saw blade of a hacksaw, a circular saw blade of a circular saw or a cutting disc is used, wherein in a particularly preferred manner, the wire saw or the thread saw is used.

5. The method of claim 1, wherein as a cutting apparatus, a wire of a wire saw or a thread of a thread saw is used, and as the fluid, an aqueous sludge is used, in which grinding particles are distributed, wherein the grinding particles are preferably selected from quartz particles, tungsten carbide particles and diamond particles or mixtures of these.

6. The method of claim 1, wherein the solid body is crystalline, preferably granular crystalline or monocrystalline.

7. The method of claim 1, wherein between the solid body and the cutting apparatus, an electrical voltage of at least 1 V is applied during cutting, preferably an electrical voltage of between 5 V and 200 V is applied.

8. The method of claim 1, wherein the cutting apparatus is brought to an electrically positive potential in relation to ground, wherein the solid body is preferably brought to ground potential.

9. The method of claim 1, wherein the solid body consists of a refractory metal from the main chemical group IVb, Vb or VIb, or the solid body consists of titanium, tantalum, niobium, vanadium, zirconium, molybdenum or tungsten, wherein titanium, tantalum, niobium and zirconium are particularly preferred and niobium is very particularly preferred.

10. The method of claim 1, wherein the solid body is positioned in a bath of the aqueous fluid during cutting, and the cutting apparatus is guided at least partially through the bath during cutting.

11. The method of claim 2, wherein the oxide layer on the surface of the cut disc is removed by pickling after cutting.

12. A disc produced from a refractory metal with a method according to claim 1.

13. A disc produced from a refractory metal with a method according to claim 1, wherein the disc has an oxide layer on the cutting surface with a thickness of between 2 nm and 1,000 nm, preferably between 10 nm and 500 nm.

14. The disc of claim 13, wherein the disc has an amorphous oxide layer on the cutting surface.

15. The disc according of claim 13, wherein the interior of the disc has an oxygen content of less than 20 g/g, preferably of less than 10 g/g, and/or the interior of the disc has a hydrogen content of less than 10 g/g, preferably less than 3 g/g.

16. The disc of claim 12, wherein the interior of the disc has an oxygen content of less than 20 g/g, preferably of less than 10 g/g, and/or the interior of the disc has a hydrogen content of less than 10 g/g, preferably less than 3 g/g.

Description

[0052] Below, an exemplary embodiment of the invention will be explained with reference to a schematically shown figure and a diagram, without however limiting the invention, in which:

[0053] FIG. 1: shows a schematic view of a device for implementing a method according to the invention, and

[0054] FIG. 2: shows a diagram in which the RRR value of niobium is shown in dependence on the content of O and N and the total of O and N.

[0055] FIG. 1 shows a schematic view of a device for implementing a method according to the invention. With the device, a cylindrical solid body 1 of a refractory metal is cut into discs. For example, the solid body 1 consists of highly pure Nb with gas contents (O, N, H, C) that total less than 20 g/g and residual resistance ratios RRR of over 300. Such niobium is used for particle acceleration in a superconductive state at low temperatures in hollow space resonators known as cavities. The residual resistance ratio RRR is the quotient of the electrical resistance at room temperature and of electrical resistance directly above the transition temperature to the superconductor. The RRR value is a comparative value for the cryogenic thermal conductivity of the material.

[0056] A refining of standard Nb (gas contents approx. 200 g/g) occurs by multiple electron beam remelting under vacuum conditions of better than 10.sup.5 mbar and temperatures of approx. 3,000 C. The resonator cells are usually welded from deep-drawn, rolled sheets. In order to minimise the contamination of the highly pure Nb during the process of removal from the melt block to the sheet, a considerable number of grinding and pickling procedures are required.

[0057] An electrical pole of an electrical voltage source 2, the electrical voltage of which can be measured with a voltmeter 3, is connected to the solid body 1 made from the refractory metal, in particular from Nb. The solid body 1 is cut with a wire saw. The wire saw comprises a copper-plated steel wire 4, which is guided over movable deflection rollers 5. The copper-plated steel wire 4 is here guided through a bath of an aqueous slurry 6. The slurry 6 contains water and particles of a grinding agent, and is formed by slurrying the component parts.

[0058] The steel wire 7 wetted with the slurry 6 cuts the solid body 1 into discs with the aid of the particles of the grinding agent. For this purpose, the copper-plated steel wire 7 that is wetted with slurry is suitably guided through the solid body 1 with the deflection rollers 5. The slurry 6 and thus also the steel wire 4, 7, is electrically connected to the other pole of the voltage source 2, so that the steel wire 7 wetted with slurry lies opposite the solid body 1 on positive electrical potential. However, a converse polarity can also be used with the structure for test purposes (see below). For this purpose, the electrical voltage can be variably set with the voltage source 2.

[0059] An anodic switching of an Nb solid body 1 or refractory metal solid body 1 appears at first to be illogical. However, due to the targeted structuring of a closed, amorphous NbO layer or refractory metal oxide layer, a further inward diffusion of O and other impurities, such as N and H, is surprisingly prevented. Here, the creation of a passivating oxide layer is largely responsible, which prevents a chemical cracking of water molecules on the refractory metal surface. As a result, the subsequent method steps, such as a titanium getter annealing, for cleaning the Nb discs or refractory metal discs that have been produced, can be avoided.

[0060] Wire sawing is actually intended and suitable for brittle materials, such as for Si wafers in the chip industry, or for quartz glass. Through adaptation of the cutting parameters, it is possible to cut the very soft Nb on a wire saw with thickness tolerances of 0.03 mm to 0.07 mm. Separation is achieved with this type of sawing by a grinding means in an aqueous solution, which is carried along by the cutting wire 4 and which thus slowly grinds the wire 4, 7 through the initial material 1.

[0061] Subsequently, on the basis of measurements, the content of 0 and H is determined in discs cut from the solid body with a method according to the invention with a wire saw and an aqueous slurry.

[0062] Grinding tests were conducted on samples of Ze, Ta and Nb, with different voltage potentials. The applied voltage was selected at 50 to +50 volts.

[0063] The samples were ground with water for 1 minute each.

[0064] The initial contents of the samples:

[0065] Zr: O content 630 g/g H content: 2 g/g

[0066] Ta: O content 9 g/g H content: 1 g/g

[0067] Nb: O content 6 g/g H content: 1 g/g

[0068] Table 1: In Table 1 below, the measured values for the oxygen content (O content) after grinding are given in g/g for discs cut from the solid body that are made of zirconium (Zr), tantalum (Ta) and niobium (Nb), wherein the cuts were made with the exemplary structure shown in FIG. 1 and with different electrical DC voltages. Here, the electrical potential difference between the solid body 1 and the wire 4, 7 of the wire saw is given as the electrical voltage in volts.

TABLE-US-00001 Voltage [V] Zr Ta Nb 50 753 119 197 45 944 125 224 40 817 92 174 35 764 107 185 30 882 83 169 25 759 67 182 20 732 112 143 15 798 48 177 10 744 54 164 5 695 27 121 0 672 15 98 5 641 9 27 10 633 8 8 15 628 10 6 20 625 9 6 25 628 7 5 30 634 9 6 35 619 10 6 40 622 9 7 45 631 10 5 50 627 8 6

[0069] Table 2: In Table 2 below, the measured values for the hydrogen content (H content) after grinding are given in g/g for discs cut from the solid body that are made of zirconium (Zr), tantalum (Ta) and niobium (Nb), wherein the cuts were made with the exemplary structure shown in FIG. 1 and with different electrical DC voltages. Here, the electrical potential difference between the solid body 1 and the wire 4, 7 of the wire saw is given as the electrical voltage in volts.

TABLE-US-00002 Voltage [V] Zr Ta Nb 50 29 19 22 45 32 25 24 40 28 22 19 35 29 17 21 30 33 29 18 25 28 23 21 20 33 24 23 15 31 26 20 10 28 23 18 5 17 18 12 0 6 15 7 5 2 5 4 10 1 2 1 15 2 1 1 20 2 1 1 25 2 1 1 30 2 1 1 35 1 1 1 40 2 1 1 45 2 1 1 50 1 1 1

[0070] It can clearly be seen that with an applied voltage of +20 volts, no absorption of gases occurs during the aqueous grinding process. Without an applied voltage (U=0V) the gas absorptions of Zr and Ta are generally low. With Nb, there is already a drastic rise in O and H content.

[0071] FIG. 2 shows a diagram in which the RRR value of niobium (Nb) is shown in dependence on the content of O and N and the total of O and N.

[0072] Due to the melting under a vacuum of the electron beam, which occurs eight times in total, Nb is produced with particularly low gas contents. This is necessary for use in superconductive, high-frequency hollow space accelerations, in order to set the highest possible residual resistance ratio RRR. The RRR value is a comparative value for the cryogenic thermal conductivity of the material. The RRR value is significantly influenced by the proportion of interstitial elements (gas contents). In order to achieve an RRR of 300, the gas contents must be significantly less than 20 g/g in total.

[0073] The graphic in FIG. 2 shows the dependency of the RRR value on the interstitial impurities in niobium.

[0074] The rough cylindrical solid body used has a diameter of 305 mm and a length of 1,600 mm. The gas content of the solid body is 4 to 7 g/g N, 1 to 2 g/g O, 1 to 2 g/g C and less than 1 g/g H. The solid body has an RRR value of 400.

[0075] In a first partial test, a portion of the solid body (length 300 mm) was sawed with water using a wire saw without application of a voltage (U=0V) into discs with a thickness of 2.8 mm. The structure is shown in FIG. 1. The cutting process lasts approximately 72 hours.

[0076] After cutting, the following gas contents and the resulting RRR value are found:

[0077] N content: 4 . . . 7 g/g

[0078] O content: 35 g/g

[0079] C content: 1 . . . 2 g/g

[0080] H content: 40 g/g

[0081] RRR: 260

[0082] In a second partial test, a further partial piece (length 300 mm) of the same solid body was also sawed with an applied voltage of +30 V, into discs with a thickness of 2.8 mm. Here, the process time was also 72 hours.

[0083] After the voltage-protected cutting, the following gas contents and the resulting RRR value are found:

[0084] N content: 5 . . . 7 g/g

[0085] O content: 3 . . . 7 g/g

[0086] C content: 1 . . . 2 g/g

[0087] H content: 25 g/g

[0088] RRR: 340

[0089] In this second partial test, after 600 C. vacuum annealing, an H content of less than 1 g/g was achieved; the RRR value then reached 480.

[0090] Third partial test: A further Nb solid body with a diameter of 480 mm with the following initial data (gas contents) was cut into discs of 4.6 mm.

[0091] N content: 11 . . . 13 g/g

[0092] O content: 7 . . . 18 g/g

[0093] C content: 1 . . . 2 g/g

[0094] H content: 1 g/g

[0095] RRR: 288 . . . 336

[0096] A voltage of U=+30V was also applied. The cutting duration was 100 hours.

[0097] The results after sawing:

[0098] N content: 11 . . . 14 g/g

[0099] O content: 17 . . . 25 g/g

[0100] C content: 1 . . . 2 g/g

[0101] H content: 20 . . . 25 g/g

[0102] RRR: 220 . . . 290

[0103] In Table 3 below, the results are compiled, wherein here, the average values have been calculated with multiple measurements:

TABLE-US-00003 Start Sawing U = 0 V Delta Sawing U = 30 V Delta N 5.5 5.5 0 5.5 0 O 1.5 35 33.5 5 3.5 C 1.5 1.5 0 1.5 0 H 1 40 39 25 24 RRR 400 260 140 340 60 N 12 12.5 0.5 O 12.5 21 8.5 C 1.5 1.5 0 H 1 22.5 21.5 RRR 312 255 57

[0104] Both the oxygen absorption and the hydrogen absorption can be prevented through an applied voltage for the aqueous separation of refractory metals.

[0105] In cases of long process times, with non-voltage-protected cutting, an oxygen absorption of 33.5 g/g and a hydrogen absorption of 39 g/g occurs.

[0106] Due to the applied voltage, the oxygen absorption can be reduced to 10 to 30%, even with extremely long process times.

[0107] For wire sawing of refractory metals, due to the thickness-dependent high process times (in the exemplary embodiments, 72 and 100 hours), a minimal oxygen absorption of 3 to 8.5 g/g and a noticeable hydrogen absorption of 29 to 25 g/g occurs.

[0108] The absorbed oxygen can no longer be reduced from the structure components. The hydrogen can be entirely removed by vacuum annealing, with T>600 C.

[0109] The features of the invention disclosed in the above description, and in the claims, figures and exemplary embodiment, can be essential both individually and in any combination desired for the realisation of the invention in its different embodiments.

LIST OF REFERENCE NUMERALS

[0110] 1 Solid body/refractory material [0111] 2 Voltage source [0112] 3 Voltmeter [0113] 4 Copper-plated steel wire [0114] 5 Deflection roller [0115] 6 Slurry [0116] 7 Copper-plated steel wire with slurry