Platinum-based material thin wire and method for manufacturing the same

Abstract

A platinum-based material element wire is coated with gold or gold alloy, and drawing-processed with a carbon-containing die. The thin wire manufactured in this manner is covered with gold or gold alloy, and the coverage of gold or gold alloy is 40% or more on an area basis. The thin wire formed of a platinum-based material is manufactured in a state of suppressing breakage in a drawing processing step, and has favorable performance in electric properties and the like. In addition, this manufacturing process is capable of efficiently manufacturing a platinum-based material thin wire while suppressing breakage when the thin wire is manufactured by drawing processing.

Claims

1. A platinum-based material thin wire having a wire diameter of 10 μm or more and 100 μm or less and formed of platinum or platinum alloy, wherein the thin wire is covered with gold or gold alloy, a coverage of the gold or gold alloy is 40% or more on an area basis, and a degree of circularity on a radial cross-section in an arbitrary longitudinal position is 0.90 or more.

2. The thin wire according to claim 1, wherein, when TCR.sup.c is a temperature coefficient of resistance of the thin wire, and TCR.sup.nc is a temperature coefficient of resistance of a thin wire which is identical to the thin wire in composition except gold and which is formed of platinum or platinum alloy that does not contain gold, a difference between the TCR.sup.c and the TCR.sup.nc is within ±0.5%.

3. The thin wire according to claim 1, comprising gold in an amount of 200 ppm or more and 1000 ppm or less on a mass basis.

4. The thin wire according to claim 1, wherein the platinum alloy is an alloy of platinum and rhodium, palladium, iridium, tungsten or nickel, or reinforced platinum.

5. A method for manufacturing the thin wire formed of a platinum-based material according to claim 1, comprising a step of performing drawing processing by passing a platinum-based material element wire through a carbon-containing die at least once, the drawing processing including passing the element wire through the die at least once in a state of being coated with gold or gold alloy in an amount of 200 ppm or more and 1000 ppm or less based on mass of the element wire.

6. A method for manufacturing the thin wire formed of a platinum-based material according to claim 1, comprising a step of performing drawing processing by passing a platinum-based material element wire through a carbon-containing die at least once, the drawing processing including passing the element wire through the die at least once in a state of being coated with gold or gold alloy in an amount equivalent to a thickness of 40 nm or more and 100 nm or less.

7. The method for manufacturing the thin wire formed of a platinum-based material according to claim 5, wherein the carbon-containing die is one of a ceramic die, an ultrahard die and a diamond die.

8. The method for manufacturing the thin wire formed of a platinum-based material according to claim 5, wherein an element wire having a diameter of 300 μm or more and 800 μm or less is coated with gold or gold alloy, and drawing-processed.

9. The thin wire according to claim 2, comprising gold in an amount of 200 ppm or more and 1000 ppm or less on a mass basis.

10. The thin wire according to claim 2, wherein the platinum alloy is an alloy of platinum and rhodium, palladium, iridium, tungsten or nickel, or reinforced platinum.

11. The thin wire according to claim 3, wherein the platinum alloy is an alloy of platinum and rhodium, palladium, iridium, tungsten or nickel, or reinforced platinum.

12. A method for manufacturing the thin wire formed of a platinum-based material according to claim 2, comprising a step of performing drawing processing by passing a platinum-based material element wire through a carbon-containing die at least once, the drawing processing including passing the element wire through the die at least once in a state of being coated with gold or gold alloy in an amount of 200 ppm or more and 1000 ppm or less based on mass of the element wire.

13. A method for manufacturing the thin wire formed of a platinum-based material according to claim 3, comprising a step of performing drawing processing by passing a platinum-based material element wire through a carbon-containing die at least once, the drawing processing including passing the element wire through the die at least once in a state of being coated with gold or gold alloy in an amount of 200 ppm or more and 1000 ppm or less based on mass of the element wire.

14. A method for manufacturing the thin wire formed of a platinum-based material according to claim 4, comprising a step of performing drawing processing by passing a platinum-based material element wire through a carbon-containing die at least once, the drawing processing including passing the element wire through the die at least once in a state of being coated with gold or gold alloy in an amount of 200 ppm or more and 1000 ppm or less based on mass of the element wire.

15. A method for manufacturing the thin wire formed of a platinum-based material according to claim 2, comprising a step of performing drawing processing by passing a platinum-based material element wire through a carbon-containing die at least once, the drawing processing including passing the element wire through the die at least once in a state of being coated with gold or gold alloy in an amount equivalent to a thickness of 40 nm or more and 100 nm or less.

16. A method for manufacturing the thin wire formed of a platinum-based material according to claim 3, comprising a step of performing drawing processing by passing a platinum-based material element wire through a carbon-containing die at least once, the drawing processing including passing the element wire through the die at least once in a state of being coated with gold or gold alloy in an amount equivalent to a thickness of 40 nm or more and 100 nm or less.

17. A method for manufacturing the thin wire formed of a platinum-based material according to claim 4, comprising a step of performing drawing processing by passing a platinum-based material element wire through a carbon-containing die at least once, the drawing processing including passing the element wire through the die at least once in a state of being coated with gold or gold alloy in an amount equivalent to a thickness of 40 nm or more and 100 nm or less.

18. The method for manufacturing the thin wire formed of a platinum-based material according to claim 6, wherein the carbon-containing die is one of a ceramic die, an ultrahard die and a diamond die.

19. The method for manufacturing the thin wire formed of a platinum-based material according to claim 6, wherein an element wire having a diameter of 300 μm or more and 800 μm or less is coated with gold or gold alloy, and drawing-processed.

20. The method for manufacturing the thin wire formed of a platinum-based material according to claim 7, wherein an element wire having a diameter of 300 μm or more and 800 μm or less is coated with gold or gold alloy, and drawing-processed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates a relationship between a drawing distance and a die wear amount when a platinum element wire and a silver alloy element wire are drawing-processed in a preliminary test.

(2) FIG. 2 illustrates a relationship between a lubricant and a die wear amount in a preliminary test.

(3) FIG. 3 illustrates a relationship between a drawing distance and a die wear amount when the processing speed is changed in a preliminary test.

(4) FIG. 4 illustrates a relationship between a drawing distance and a wire diameter of a thin wire when a platinum element wire is drawing-processed in each of a first embodiment and a comparative example.

(5) FIG. 5 illustrates a relationship between a drawing distance and an electric resistance value of the thin wire when a platinum element wire is drawing-processed in each of the first embodiment and the comparative example.

(6) FIG. 6 illustrates the result of observing a material structure on a longitudinal cross-section of the platinum thin wire of the first embodiment.

(7) FIG. 7 is a SEM photograph showing a surface state of the platinum thin wire after processing in each of the first embodiment and the comparative example.

(8) FIG. 8 illustrates a cyclic voltammogram of the platinum thin wire of the first embodiment.

(9) FIG. 9 illustrates a drawing distance and a die wear amount when three platinum element wires are drawing-processed in a third embodiment.

(10) FIG. 10 illustrates a drawing distance and a die wear amount when platinum-tungsten alloy is drawing-processed in a fourth embodiment.

(11) FIG. 11 illustrates a cyclic voltammogram of a platinum-tungsten alloy thin wire measured in the fourth embodiment.

(12) FIG. 12 illustrates a drawing distance and a die wear amount when platinum-nickel alloy and platinum-iridium alloy are drawing-processed in a fifth embodiment.

(13) FIG. 13 illustrates a cyclic voltammogram of a platinum-nickel alloy thin wire which is measured in the fifth embodiment.

DESCRIPTION OF EMBODIMENTS

(14) Embodiments of the present invention will be described below for promoting better understanding of the present invention. In the embodiments, first, processability was evaluated as a preliminary test based on factors other than coating with gold or gold alloy, such as difference in thin wire material, existence or non-existence of a lubricant and processing conditions. Thereafter, thin wire processing and product evaluation were performed as embodiments for examining usefulness of gold coating.

(15) In the preliminary test below, the wire diameter of an element wire was set to 0.5 mm, and the wire diameter of a target thin wire was set to 0.02 mm. A diamond die made of sintered diamond (manufactured by A. L. M. T. Corp.) and having a hole diameter of 20 μm. The number of processing (i.e. the number of passages of the element wire through the die) was set to 1, and the element wire was continuously drawn. The temperature of the processing atmosphere was normal temperature, and a lubricant was used. The lubricant was supplied by pouring the lubricant to the die with a circulation pump. For evaluation of processing results, a relationship between a drawing distance and a die wear amount was evaluated. Here, the wire diameter after processing over a predetermined drawing distance was measured, and the die wear amount was calculated on the basis of the wire diameter in an early stage of processing (drawing distance: 10 m).

(16) Preliminary test: First, element wires of a platinum-based material and a non-platinum-based material were drawing-processed, and uniqueness of die wear in processing of the platinum-based material was examined. Here, an element wire of pure platinum with a purity of 99.99% by mass and an element wire of silver alloy (Ag—Cu—Ni alloy: XP-3) manufactured by Tanaka Kikinzoku Kogyo were prepared, and drawing-processed. Here, a silver alloy element wire to be compared has a tensile strength higher by about 400 MPa than that of the pure silver wire and a Vickers hardness higher by about 200 than that of the pure platinum wire.

(17) FIG. 1 illustrates a relationship between a drawing distance (abscissa) and a die wear amount (ordinate) when drawing processing is performed under the conditions described above. In any of the element wires, the wear amount of the die increases as the drawing distance becomes larger. It is to be noted that the die wear amount for the platinum element wire is larger than the die wear amount for the silver alloy (AgCu alloy). The increase ratio (inclination) of the wear amount against the drawing distance is relatively mild for the silver alloy, whereas the wear amount increases at an accelerated rate for the platinum element wire. Thus, from comparison with the silver element wire having mechanical strength higher than that of the platinum element wire, it can be found that it is not possible to compare the die wear amount only on the basis of mechanical strength. From the behavior of the die wear amount for the platinum element wire, it can be presumed that action to promote wear in addition to simple mechanical damage caused only by friction between the element wire and the die is developed in the platinum-based material.

(18) Next, existence or non-existence of an effect of protecting the die by use of a lubricant was examined for the platinum element wire. Under the above-described processing conditions, drawing processing was performed on platinum element wires with no lubricant, with water and with a commercially available surfactant-based water-soluble oil. The die wear amount was determined from the wire diameter after the reaching of a drawing distance of 10,000 m. FIG. 2 shows the results of the comparison between these wires.

(19) From FIG. 2, it can be said that use of a lubricant has a certain suppressive effect on die wear when compared to a case where the lubricant is not used. However, it is hard to say that wear was sufficiently suppressed by use of a lubricant because the die was worn by about 0.4 μm even when the lubricant was used. In addition, there is no significant difference between the results with water and with the surfactant-based water-soluble oil. Since water is not essentially lubricant, the die wear amount reducing effect is ascribable to cooling effect of the liquid that is water. That is, it is apparent that the die wear reducing effect of the surfactant-based water-soluble oil as a lubricant is mainly due to cooling action of the liquid. Temperature elevation cannot be thoroughly suppressed even though the lubricant has cooling action, and direct contact between the element wire and the die cannot be avoided. The results of the preliminary test show that die wear cannot be sufficiently reduced only by use of a lubricant.

(20) Further, effects of the drawing speed on die wear was examined for the platinum element wire. FIG. 3 illustrates relationships between the drawing distance and the die wear amount when the drawing speed is set to 100 m/min and 500 m/min. By decreasing the drawing speed, die wear can be slightly suppressed in a region where the drawing distance is 5000 m or more. This is because the heat amount in processing depend on the processing speed. Of course, although die wear is suppressed, the wear amount is not low at all, and there is little difference in wear amount at a drawing distance up to about 5000 m. It may be difficult to suppress die wear by adjustment of the drawing speed.

First Embodiment

(21) On the basis of the results of the above preliminary test, a platinum element wire coated with gold was drawing-processed. In this embodiment, a platinum element wire having a wire diameter of 500 μm (0.5 mm) was coated with gold by a plating method. The coating amount was such that 450 g of the element wire was coated with 0.22 g of gold in terms of a mass (about 488 ppm, equivalent to a thickness of 68 nm). In this embodiment, the same diamond die as in the preliminary test (die hole diameter: 20 μm (0.02 mm)) was used. In this embodiment, an attempt was made to process a thin wire whose target wire diameter was 20 μm. The temperature of the processing atmosphere was normal temperature, and a lubricant (type: surfactant-based water-soluble oil) was used. The drawing speed was set to 50 m/min. As a comparative example, an element wire which was not coated with gold was processed. Continuous drawing was performed, and the wire diameter of the manufactured thin wire was measured at predetermined intervals. The electric resistance value was measured together with the wire diameter.

(22) FIG. 4 illustrates a relationship between a drawing distance and a wire diameter of a thin wire after processing in each of this embodiment with gold coating and the comparative example without coating. In the comparative example, breakage occurred at a drawing distance slightly larger than 5000 m. On the other hand, in this embodiment, the drawing distance far exceeded that in the comparative example, and it was still possible to perform processing even when the drawing distance reached 40000 m. In addition, in the comparative example where gold coating was not performed, it is apparent that the rate of increase in wire diameter was large from the early stage of processing, and thus die wear progressed. In this embodiment, it is apparent that die wear was mild even at a drawing distance of 40000 m, the increase in wire diameter was less than 0.1 μm, and thus excellent processing stability was exhibited. The above tendency can also be confirmed from the results of measuring the electric resistance of the manufactured thin wire (FIG. 5). In the comparative example, the electric resistance value considerably changes (i.e. decreases) as the wire diameter increases, whereas in this embodiment, a thin wire with a small change in resistance is manufactured.

(23) FIG. 6 illustrates the result of observing a material structure on a longitudinal cross-section of the platinum thin wire manufactured in this embodiment. As is apparent from this photograph, a fibrous structure composed of extremely thin crystal grains is formed as a result of drawing processing. The aspect ratio of a crystal grain seeming to have the lowest aspect ratio in the visual field was measured, and the result showed that the aspect ratio was 13.0. In this embodiment, all the crystal grains (area ratio: 100%) are considered to have an aspect ratio of 10 or more.

(24) FIG. 7 illustrates a photograph showing a surface state of the thin wire of this embodiment after the wire is drawn by 40000 m, and a surface state of the thin wire of the comparative example after the wire is drawn by 5000 m. The thin wire of this embodiment is a smooth wire material having a high degree of circularity. On the other hand, in the comparative example, angles and irregularities are present on the surface, and they may result from die wear. Thus, the degree of circularity of a cross-section of the thin wire of each of this embodiment and the comparative example was measured. The result showed that the degree of circularity of the thin wire of this embodiment was 0.957. On the other hand, the degree of circularity of the thin wire of the comparative example was 0.870.

(25) The above test results showed that by coating an element wire formed of a platinum-based material with gold, a high-quality thin wire with a small change in wire diameter was manufactured while die wear was suppressed.

(26) Next, the gold coverage on the thin wire was measured for the platinum thin wire manufactured in this embodiment. The measurement of the gold coverage was based on cyclic voltammetry analysis with a platinum thin wire as an electrode. Measurement of a cyclic voltammogram was performed in the following manner. A working electrode, a counter electrode and a reference electrode were connected to a measuring apparatus (trade name: HZ-5000 manufactured by Hokuto Denko Corporation). The platinum thin wire manufactured in this embodiment was used for the working electrode, and a platinum electrode and a reversible hydrogen electrode (RHE) were used for the counter electrode and the reference electrode, respectively. In addition, a 0.1 M-HClO.sub.4 solution was used as an electrolytic solution. In advance, the electrolytic solution was bubbled with nitrogen gas for 30 minutes. Cyclic voltammetry was performed at a sweeping rate of 10 mV/sec from 0.05 V to 1.7 V.

(27) FIG. 8 illustrates a cyclic voltammogram of the platinum thin wire of this embodiment. In the cyclic voltammogram of FIG. 8, the peak at about 0.65 to 0.7 V (vs. RHE) indicates formation/reduction of a platinum oxide film, and originates from platinum forming the thin wire. On the other hand, the peak at about 1.15 to 1.2 V (vs. RHE) indicates formation/reduction of an oxide film, and originates from gold covering the thin wire.

(28) The gold coverage based on the cyclic voltammogram is calculated in the following manner. First, the amounts of electricity (Q.sub.Pt and Q.sub.Au) at the peaks (platinum and gold) in the cyclic voltammogram are determined. The amount of electricity is calculated by time integration of current values at the peaks, and the calculation can be performed with general spreadsheet software or analysis software. Next, from the obtained amounts of electricity (Q.sub.Pt and Q.sub.Au) and the electric capacitances for oxide layer reduction with platinum and gold (Q.sub.Pt-O(red): 420 μC/cm.sup.2 and Q.sub.Au-O(red): 390 μC/cm.sup.2), the areas of platinum and gold (SA.sub.Pt and SA.sub.Au) are calculated. The area ratio (SA.sub.Au/(SA.sub.Pt+SA.sub.Au)) calculated from the respective areas is defined as a gold coverage. The gold coverage on the platinum thin wire having a diameter of 20 μm of this embodiment, based on the cyclic voltammogram of FIG. 8, was 65.6%.

(29) Further, the temperature coefficient of resistance (TCR) was measured for the platinum thin wire manufactured in this embodiment. In this embodiment, the reference temperature and the test temperature were set 0° C. and 100° C., respectively, the resistance values at these temperatures (R.sub.100 and R.sub.0) were measured, and TCR in this embodiment with gold coating and TCR.sup.nc in the comparative example without gold coating were measured.

(30) The results of measuring the TCRs showed that TCR (TCR.sup.c) of the thin wire of this embodiment was 1.3857 (ppm/° C.), whereas TCR (TCR.sup.nc) of the thin wire of the comparative example was 1.3888 (ppm/° C.). In the thin wire of this embodiment, the thin wire is covered with gold, and therefore the TCR value is slightly lower than that of the thin wire of the comparative example without gold (the thin wire of the comparative example is identical in composition to the thin wire of this embodiment except that gold is not present). However, there is an extremely small difference of −0.22% between TCR.sup.c and TCR.sup.nc. It is considered that the platinum thin wire of this embodiment is a practically acceptable level of TCR, and can be used as such for the above-described purposes. Comparison between the resistance values in this embodiment and the comparative example at a drawing distance of about 0 m, with reference to FIG. 5, shows that there is no significant difference between the resistance values of the thin wires. Accordingly, it may be preferable that rather than the resistance value, TCR is applied for rigorous examination of the electric properties of the thin wire according to the present invention.

Second Embodiment

(31) Here, various platinum thin wires were manufactured while the coating amount of gold on an element wire and the final hole diameter of a die were changed. The element wire to be processed is the same platinum element wire (500 μm) as in the first embodiment. In addition, the coating amount of gold was 320 ppm in terms of an element wire mass ratio and was equivalent to a thickness of 44 nm. As the die, a diamond die was used (drawing distance: 500 m) for each of the thin wired.

(32) Regarding the manufactured thin wire, the actual wire diameter was measured, and the cyclic voltammogram was measured in the same manner as in the first embodiment to determine the gold coverage. Table 1 shows the manufacturing conditions and the measured values for the manufactured thin wires for the platinum thin wires manufactured in this embodiment.

(33) TABLE-US-00001 TABLE 1 Manufacturing conditions Gold coating amount Thin wire measured values Pt element Mass Thickness Die hole Wire diameter No. wire diameter ratio equivalent diameter (actual size) Au coverage 1 500 μm 320 ppm 44 nm 72 μm 72 μm 47.3% 2 46.5 μm   47 μm 58.5% 3 30 μm 30 μm 57.5% 4 11.1 μm   11 μm 51.7%

(34) As shown in Table 1, the platinum thin wires manufactured in the second embodiment each had a small deviation in wire diameter with respect to the target wire diameter (i.e. die hole diameter). In any of the thin wires, breakage did not occur during processing. The gold coverage (i.e. area ratio) on each thin wire was 40% or more.

(35) TCR.sup.cs (R.sub.100 and R.sub.0) of the thin wires of the second embodiment were measured, and the results showed that for all the thin wires, the difference was within the range of ±0.5% with respect to the value (1.3888 (ppm/° C.)) for the thin wire without gold coating in the comparative example in the first embodiment.

Third Embodiment

(36) In this embodiment, effects of the gold coating amount on drawing processing were examined. In this regard, a platinum element wire processed to a wire diameter of 800 μm was coated with gold in an amount of 400 ppm (equivalent to a thickness of 44 nm) or 200 ppm (equivalent to a thickness of 88 nm) to manufacture a platinum thin wire. In addition, an element wire not coated with gold was processed by drawing processing. The drawing speed was set to 50 m/min. A relationship between a drawing distance and a die wear amount was examined.

(37) FIG. 9 shows the results of this embodiment. As described above, die wear is reduced by coating the platinum element wire with gold. In this embodiment, drawing processing was performed with the gold coating amount reduced by half, and it was shown that the die wear amount increased as the amount of gold decreased. However, even when the gold coating amount is reduced by half, wear is considerably reduced as compared to the die wear amount without gold coating.

Fourth Embodiment

(38) In this embodiment, effects of gold coating on drawing processing of platinum alloy. An element wire having a diameter of 500 μm of platinum-tungsten alloy (Pt-8% by mass W alloy) was coated with gold in an amount of 410 ppm in terms of an element wire mass ratio (equivalent to a thickness of 57 nm) to manufacture a thin wire. The drawing speed was set to 50 m/min. A relationship between a drawing distance and a die wear amount was examined. For determining the gold coverage on the platinum alloy thin wire, the cyclic voltammogram was measured in the same manner as in the first embodiment.

(39) FIG. 10 shows the results of the drawing distance and the die wear amount in this embodiment. The results showed that effects of gold coating were exhibited in drawing processing of not only pure platinum but also platinum alloy. FIG. 11 shows the results of measuring the cyclic voltammogram. The platinum alloy thin wire of this embodiment was coated with gold at a coverage of 73%.

(40) Further, TCR's (R.sub.100 and R.sub.0) of the platinum-tungsten alloy thin wire of this embodiment were measured, and the results showed that the difference was within the range of ±0.5% with respect to a platinum-tungsten alloy thin wire of the same composition which was manufactured from an element wire without coating.

Fifth Embodiment

(41) In this embodiment, platinum alloy was drawing-processed. In this regard, element wires having a diameter of 500 μm of platinum-nickel alloy (Pt-7% by mass Ni alloy) and platinum-iridium alloy (Pt-10% by mass Ir alloy) were coated with gold in an amount of 420 ppm in terms of an element wire mass ratio (equivalent to a thickness of 58 nm) to manufacture a thin wire. The drawing speed was set to 50 m/min.

(42) FIG. 12 shows the results of the drawing distance and the die wear amount for the platinum alloy thin wires. In drawing processing of the platinum alloy wires, the die wear amount is extremely low. For these wires, the die wear amount is expected to be less than 0.1 μm even when the drawing distance reaches 10000 m. FIG. 13 illustrates a cyclic voltammogram of a platinum-nickel alloy thin wire which is measured under the same conditions as in the first embodiment. The coverage on the platinum-nickel alloy thin wire was 90%. In this embodiment, TCR.sup.cs (R.sub.100 and R.sub.0) of the thin wires were measured, and the results showed that for all the wires, the difference was within the range of ±0.5% with respect to an alloy thin wire of the same composition which was manufactured from an element wire without coating.

INDUSTRIAL APPLICABILITY

(43) As described above, according to the present invention, a product of high quality can be manufactured while breakage during processing is suppressed in manufacturing of a platinum-based material thin wire by drawing processing. The present invention can adapt to reduction of the wire diameter of a thin wire, so that a thin wire having a wire diameter of 10 μm can be efficiently produced. The thin wire according to the present invention can be used for sensors such as hydrogen gas sensors, and other various articles such as medical equipment and devices, various electrodes, heaters and probe pins.