Deoxidising tantalum/columbium material

(57) The invention relates to a method for reducing the oxygen content in a tantalum or niobium material.

According to the invention, this material is heated to a temperature of between approximately 900 and approximately 2400 ° C. under an atmosphere containing hydrogen in the presence of an oxygen-active metal chosen from the group consisting of beryllium, calcium, cerium, hafnium, lanthanum, lithium, praseodymium, scandium, thorium, titanium, uranium, vanadium, yttrium, zirconium, their alloys and mixtures, and the like.

The invention is particularly applicable to the manufacture of capacitors.

© Applicant (s): Company known as: CABOT CORPORATION. - US.

© Inventor (s): Robert Amos Hard.

© Holder (s):

Agent (s): Cabinet Weinstein.

FR 2 611 368

D

Sale of leaflets at IMPRIMERIE NATIONALE. 27. rue de la Convention - 75732 PARIS CEDEX 15

Tantalum capacitors are typically made by compressing tantalum powder to form a pellet, sintering the pellet in an oven to form a porous body and then subjecting the body to anodization in a suitable electrolyte to form an oxide film continuous dielectric on the sintered body.

The development of a tantalum powder suitable for capacitors is the result of efforts by both capacitor producers and tantalum specialists to describe the characteristics required of a tantalum powder so that it can be used in the best possible way. production of quality capacitors. Such characteristics include surface area, purity, contraction, green strength and flowability.

Above all, the powder must have an adequate surface area, because the capacitance of a tantalum powder depends on the surface area; the larger the surface area after sintering, the greater the specific capacitance.

The purity of the powder is also a critical consideration. Metallic and non-metallic contamination tends to degrade the dielectric. High sintering temperatures tend to remove some volatile contaminants; however, high temperatures reduce the net surface area and thus the capacitance of the capacitor. Reducing the loss of surface area under sintering conditions is an essential condition in order to maintain the captivity of the tantalum powder.

The flowability of the tantalum powder and the resistance in the green state (mechanical resistance of the treated powder, not sintered) are critical parameters for the producer of capacitors in order to allow efficient production. The flowability of the powder allows regular feeding of the dies in rapid speed pressing operations; the resistance to the green state allows the handling and the transport of the product without excessive rupture.

As described above, the capacitance of a tantalum pellet is a direct function of the surface area of ​​the sintered powder. A larger surface area can of course be obtained by increasing the grams of powder per pellet, but price considerations have dictated that development be focused on a means for increasing the surface area per gram of powder used. As the decrease in the particle size of the tantalum powder produces more surface area per unit of weight, the effort has been extended to ways of making the tantalum particles smaller without introducing other harmful characteristics which accompany often a reduction in size. Three of the major shortcomings of a very fine powder are poor flow characteristics, excessive oxygen content and excessive loss of surface area during sintering.

For electrolytic capacitors, the oxygen concentration in tantalum is critical. When the total oxygen content of the porous tantalum pellets is above 3000 ppm, the capacitors made of such pellets may have unsatisfactory shelf life characteristics. Unfortunately, tantalum powder has a great affinity for oxygen, and thus processing steps which include heating and subsequent exposure to air inevitably result in an increased concentration of oxygen. Tantalum powder from. Electronic grade is normally heated under vacuum to partially agglomerate it. During this treatment, it usually absorbs a considerable amount of oxygen because the oxide surface layer goes into solution in the metal. The formation of a new surface layer upon exposure to air then adds to the total oxygen content. During the subsequent processing of these powders into capacitor anodes, this oxygen can recrystallize as surface oxide and contribute to a voltage break or a high leakage current of the capacitor by short-circuiting through the dielectric layer. amorphous oxide.

As the quantity of oxygen absorbed will be proportional to the surface area exposed, fine powders with high capacitance properties are even more sensitive to a reaction with atmospheric oxygen. For this reason, the oxide content of fine-grained tantalum powders is naturally higher than that of coarser-grained fractions. This is particularly true for agglomerated tantalum powders which are sintered together from the specially fine individual particles. In this case, an additional adsorption of oxygen takes place after the thermal agglomeration process with as a consequence a reactivation of the tantalum surface.

The electrical properties of tantalum capacitors would be markedly improved if the oxygen content of tantalum could be reduced before and / or after treatment as an anode of capacitors.

The alkaline earth metals, aluminum, yttrium, carbon and tantalum carbide have all been previously mixed with tantalum powder in order to deoxygenate tantalum. However, there are certain drawbacks to this technique. Alkaline earth metals, aluminum and yttrium form refractory oxides which must be removed, for example by acid leaching before the material is suitable for capacitors. The amount of carbon must be carefully controlled because the residual carbon is also harmful to the capacitors, even at levels as low as 50 ppm. Other methods which have been proposed consist in using a thiocyanate treatment or in using a hydrocarbon or a reducing atmosphere during part of the tantalum treatment stages in order to prevent oxidation and thus maintain a low oxygen content.

According to the present invention, oxygen is removed from tantalum and / or niobium materials by performing a heating operation in an atmosphere containing hydrogen gas in the presence of a metal more active in oxygen than tantalum. Any material containing tantalum or niobium can be effectively treated. Such materials containing tantalum and / or niobium can typically contain metallic tantalum and niobium or alloys in any form, as well as tantalum and niobium compounds, such as oxides or hydrides, and the like. In a preferred embodiment, the dissolution of oxygen in a tantalum and / or niobium powder during standard heating operations, such as agglomeration or sintering, is prevented by undertaking these operations under the conditions of the present invention . In another embodiment, the oxygen can be removed from the tantalum and / or niobium powder which has been pressed into anodes by performing the sintering of the anodes in an atmosphere containing hydrogen in the presence of an active metal oxygen.

The appropriate oxygen active metals are chosen from the group consisting of beryllium, calcium, cerium, hafnium, lanthanum, lithium, praseodymium, scandium, thorium, titanium, uranium, vanadium, yttrium, zirconium, their alloys as a mixed metal, their mixtures and the like. Titanium and zirconium are preferred.

It is believed that the hydrogen gas reacts with the tantalum and / or niobium oxide to form water vapor which is then degassed by the active metal to oxygen to form an active metal oxide and hydrogen. Such a reaction mechanism is represented by the following equations, where tantalum is shown as the metal which is deoxidized and titanium is shown as a getter representative of the active metal with oxygen:

(A) Ta₂0₅ + 5 H₂-> 5 HgO + 2 Ta (B) H₂0 + Ti-TiO + H₂

The active metal getter need not necessarily be in physical contact with the tantalum material, but, for best results, it is preferably located very close to the tantalum. In order to present the highest surface area of ​​the metal getter, it is preferable to use a metal getter in the form of a sponge. However, the metal getter can be used in any form, such as a sheet, sponge or powdered material.

The heating treatment can be undertaken at any temperature between about 900 and about 2400 ° C (about 1173 ° K to about 2673 ° K); the degassing reaction is favored by increasing temperatures. A preferred temperature range is from about 1100 ° to 2000 ° C (about 1373 to about 2273 ° K). For the best results, it is preferable that a metal getter is used which has a melting point below the temperature which it is desired to obtain in the heat treatment process. A particularly preferred temperature range for the heat treatment of tantalum powder is between about 1250 ° and about 1450 ° C (about 1523 ° K to about 1723 ° K). For tantalum anodes, the preferred heat treatment is between about 1300 ° and about 1550 ° C (about 1573 ° K to about 1823 ° K).

The following examples are given to better illustrate the invention. The examples are intended to illustrate the nature of the invention without in any way limiting its scope.

Process for determining the capacitance and the leakage of direct current.

(A) Making the dumpling;

The tantalum powder was pressed in a commercial pellet press without the aid of binders. Typically, the pressed density was 6.0 g / cc using a powder weight of 1.2 g and a diameter of 6.4 mm.

(B) Sintering under vacuum:

The packed pellets were sintered in a high vacuum of less than 0.00133 Pa for 30 minutes (1800 seconds) at temperatures in excess of 1500 ° C (1773 ° K).

(C) Anodization:

The sintered pellets were anodized in a shaping bath at 90 + 2 ° C (353 + 2 ° K) at 100 V DC. The electrolyte was 0.1% phosphoric acid.

The anodizing speed was controlled to be 1 volt per minute. After three hours at 100 V DC, the pellets were washed and dried.

(D) Test conditions:

The anodes, after anodizing, rinsing and drying were first tested for leakage of direct current (DCL). A phosphoric acid solution was used. The anodes were immersed in the test solution to the top of the anode and the appropriate voltage was applied for 2 minutes and then the DC leakage was measured.

After completing the DC leakage measurements, the anodes formed at 200 volts were placed in a tray containing 10% phosphoric acid and allowed to soak for 30 to 45 minutes.

The anodes formed at 270 volts were washed for 3 to 5 minutes at 105 ° + 5 ° C (378 + 5 ° K) in air.

They were then soaked in 10% phosphoric acid for 30 to 45 minutes.

The capacitance was measured on the anode immersed in 10% phosphoric acid at 21 ° C (294 ° K) using a General Radio Type 161IB Test Bridge with a 0.5 volt AC signal and a DC bias of 3 volts.

Process for determining the resistance of the pellet.

(A) Manufacturing of the anode:

The tantalum powder was compressed in a commercial press without the aid of binders. The pressed density was 6.0 g / cc using a powder weight of 1.6 g and a diameter and length of 6.4 mm and 8.4 mm, respectively.

(B) Test:

The cylindrical pellet is placed between two flat plates with its longitudinal axis parallel to the plates; a constantly increasing force is applied to one of the plates until the pellet breaks. The force at the breaking point is recorded as resistance to crushing. The dimension of the anode diameter is measured before and after sintering; the percentage difference being recorded as

Contraction.

Analysis of¹ oxygen:

Oxygen analysis is done using the Og and Ng Leco TC-30 analyzer, which is an inert gas fusion technique.

BET surface area:

The total surface area of ​​tantalum is measured using a pore volume analyzer from Numinco Orr's surface area (manufactured by Numec Corporation). The BET (Brunauer-Emmet-Teller) surface areas obtained in this way include the external surface area as well as the internal surface area due to the presence of the pores.

Example 1.

Six-level tantalum equipment (2 cm space between levels) was used to support the test samples while they were being processed. The equipment was arranged so that the upper shelf holds a sintered tantalum ball serving as a target for optical pyrometer readings. 70 g of tantalum powder of US -60 mesh, containing 1340 ppm of 0₂ were regularly divided and then spread over two tantalum trays (Icmx4cmx5cm). These two trays (samples A and B) were then placed on the second and third shelves from the top of the equipment. A third tantalum tray was covered with a zirconium sheet (5x5cm) and placed on the fourth shelf below the tantalum powder trays. The equipment, which held the tantalum trays, was then lowered inside the vacuum furnace so that it was surrounded by the cylindrical tantalum heating element and the shield. A group of tantalum heat shields was then placed over the heating elements so that the tantalum equipment holding the trays was fully enclosed to ensure a uniform temperature inside the hot zone of the furnace. The oven was then closed and evacuated to 1 micron and the rate of leakage from the oven was determined. A leakage rate of -0.5 micron over a 5 minute period was considered to be acceptable when measured by a McLeod gauge. Energy was then applied to the oven and the tantalum powder heated under vacuum to 1000-1050 ° C over a period of 15 minutes. The heating rates were controlled by increasing the amperage of the oven at 2 minute intervals until the required amperage (usually 1400 amperes) was sufficient to reach 1000-1050 ° C. Temperature readings were taken with the naked eye using an optical pyrometer while looking directly at the tantalum ball placed on the upper shelf.

The tantalum powder started to emit gases at around 800 ° C, as evidenced by an increase in pressure in the oven, which was monitored by a Varian vacuum gauge in the front line of the oven. The oven pressure has typically increased to about 70 microns. By the time 1050 ° C was reached, the oven pressure had started to decrease. 1050 ° C was then maintained for 30 minutes, which allowed the pressure in the oven to decrease to 40-60 microns by measuring in the front line of the oven. It was noted that the McLeod vacuum gauge never exhibited an increase in pressure above 0.5 microns.

After completing the holding cycle at 1050 ° C, the oven temperature was gradually increased.

At around 1200 ° C, the vacuum valves in the oven were closed to isolate the interior of the oven. The interior of the oven was then filled to a pressure of 10 mm with Hg. The temperature of the oven was increased to 1250 ° C and maintained at 1250 ° C for 4 hours under the pressure of Hg.

When the 4 hour hold time was over, the oven temperature was increased to 1450 ° C over a period of 5 minutes. It was then kept at 1450 ° C for one hour, the oven still being at a pressure of 10 mm Hg. When the holding cycle at 1450 ° C was finished, the oven was evacuated to 0.5 micron and the powder tantalum cooled to room temperature under vacuum.

The deoxidized tantalum powder from each tray was treated separately to a US -40 mesh and chemically analyzed. The US -40 tantalum mesh from each tray was then combined and chemically analyzed a second time.

Deoxidation test conditions:

minutes at 1050 ° C under vacuum hours at 1250 ° C and 10 mm Hg

1250-1450 ° C at 10 mm Hg hour at 1450 ° C and 10 mm Hg.

Chemical analysis showed the following:

initial content of Og in tantalum powder: 1340 ppm.

Depending on the deoxidation:

Sample A Sample B

Composite of sample A and sample B

1085 ppm 1095 ppm

1095 ppm

The deoxidation treatment applied to samples A and B resulted in a significant decrease in the 0 content_2> Standard vacuum heat treatment of the same tantalum powder resulted in an increase in powder content of 0₂ from 300-500 ppm.

Example 2.

70 g of tantalum powder (the same food charge as that used in Example 1) were deoxidized with H₂ gaseous in the presence of a zirconium band. The process used to load the oven equipment and check the oven leak was the same as used in Example 1. The tantalum powder was heated in vacuo to 1050 ° C and held for 30 minutes until that the degassing of the powder is complete and that the oven pressure has decreased to 40-60 microns by measuring at the front line of the oven.

After completing the degassing cycle and at 1050 ° C, the vacuum valves of the oven were closed and the oven filled to 10 mm pressure with H₂· The oven temperature was then increased to 1250 ° C over a period of 9 minutes. When 1250 ° C was reached, this temperature was maintained for 4 hours. When the hold time of 4 hours was finished, the oven was evacuated to 0.5 micron. The oven temperature was then increased to 1450 ° C and maintained at this temperature for 30 minutes. When the cycle at 1450 ° C was finished, the power of the oven was stopped and the tantalum powder cooled to room temperature under vacuum. The tantalum powder was treated in the same manner as in Example 1.

Deoxidation test conditions:

minutes at 1050 ° C under vacuum hours at 1250 ° C at 10 mm H₂

1250-1450 ° C under vacuum minutes at 1450 ° C under vacuum Chemical analysis has shown the following:

Initial content of Og in tantalum powder: 1340 ppm Following deoxidation;

<td>Sample C</td><td> — °₂- * 1310 ppm</td>

<td>Sample D</td><td>1335 ppm</td>

<td>Composite of sample C</td><td></td>

<td>and sample D</td><td>1335 ppm</td>

Deoxidation treatment of samples C and D accomplished prevention of absorption of 0₂ by Ta powders. Similar heat treatment of the same Ta powders using standard vacuum conditions resulted in an increase in the 0 content₂ 300-500 ppm powder.

Example 3.

70 g of tantalum powder (the same food charge as that used in Example 1) were deoxidized with H₂ gaseous in the presence of a titanium strip as a degassing agent. The process used to load equipment into the oven and check for oven leakage was the same as used in Example 1.

The tantalum powder was heated under vacuum to 1050 ° C and kept under vacuum for 30 minutes until degassing of the powder was complete. At 1055 ° C, the vacuum valves of the oven were closed and the oven filled with Hg up to 10 mm pressure. The oven temperature was increased to 1250 ° C over a period of 9 minutes. When 1250 ° C was reached, this temperature was maintained for 2 hours. When the 2 hour holding time has ended, the oven has been evacuated to

0.5 micron. The oven temperature was then increased to 1450 ° C and maintained at this temperature for 30 minutes. When the cycle at 1450 ° C was finished, the power of the oven was stopped and the tantalum powder cooled to room temperature under vacuum. The tantalum powder was treated in the same manner as in Example 1.

Conditions of the deoxidation test:

minutes at 1050 ° C under vacuum hours at 1250 ° C at 10 mm H₂

1250-1450 ° C under vacuum minutes at 1450 ° C under vacuum Chemical analysis has shown the following:

Initial 0 content₂ in tantalum powder: 1340 ppm.

Following deoxidation:

-°2—

Sample E 1425 ppm

Sample F 1445 ppm

Composite of Sample E and Sample F 1,555 ppm

Sample deoxidation treatment

E and F resulted in minimal absorption of Og compared to absorption of Og of 300-500 ppm under heat treatment conditions under standard vacuum.

Example 4.

70 g of a tantalum powder of US -60 mesh containing 1625 ppm of Og with Hg gas were deoxidized in the presence of a zirconium band as degassing agent. The process used to load the oven equipment and check the oven leak was the same as used in Example 1. The tantalum powder was heated under vacuum to 900 ° C and kept at this temperature until that the evidence of degassing is complete. At 1160 ° C, the vacuum valves of the oven were closed and the oven filled to 10 mm pressure with Hg. The oven temperature was raised to 1250 ° C in 6 minutes. At 1250 ° C, the oven temperature was gradually increased over a period of one hour to 1450 ° C. When 1450 ° C was reached, the oven was evacuated to 0.5 micron and the energy of the oven stopped.

The tantalum powder was cooled to room temperature under vacuum and treated in the same manner as in Example 1.

Deoxidation test conditions:

degas the powder under vacuum at 900-1000 ° C under vacuum heat for one hour at the rate of 1250-1450 ° C at 10 mm H₂.

Chemical analysis showed the following:

Initial 0 content₂ in tantalum: 1625 ppm.

Following deoxidation:

Sample G Sample H

Composite of samples G and H

Heat treatment of samples G and H resulted in the 0 content₂ in the powder remained essentially unchanged.

A standard vacuum heat treatment of this high surface powder resulted in an absorption of 0₂ from 300 to 500 ppm.

Example 5.

70 g of the tantalum powder (the same food charge as in Example 4) were deoxidized with H₂ in the presence of a titanium strip as a degassing agent. The process used to load the equipment with powder

1630 ppm 1635 ppm 1640 ppm. deoxidation of the oven and check the oven leak was the same as used in Example 1. The tantalum powder was heated under vacuum to 900 ° C and kept at this temperature until evidence of degassing be finished. At 1200 ° C, the vacuum valves of the oven were closed and the oven filled to 10 mm of pressure with Hg. At 1250 ° C, the temperature of the oven was gradually increased over a period of 3 hours up to 1450 ° vs. When 1450 ° C was reached, the oven was evacuated to 0.5 micron and the energy of the oven stopped. The tantalum powder was cooled to room temperature under vacuum and treated with a US -40 mesh. The tantalum powder from samples I and J was combined and analyzed chemically.

Deoxidation test conditions:

degas powder under vacuum at 900-1000 ° C heat 3 hours at a rate of 1250-1450 ° C at 10 mm Hg.

Chemical analysis showed the following:

Initial content of Og in tantalum powder: 1625 ppm.

- Og——

Composite of sample I and sample J 1370 ppm

The deoxidation conditions including a time extension of the temperature ramp from 1250 to 1450 ° C resulted in a further decrease in the Og content in the tantalum powders.

Example 6.

60 g of tantalum powder (the same food charge as that used in Example 4) were deoxidized with Hg gas in the presence of a titanium sponge as degassing agent. The titanium sponge was degassed under vacuum at 800 ° C in a separate oven before using it as a degassing agent. The process used to deoxidize the tantalum powder, that is to say introducing the equipment into the oven and checking the oven leakage, was identical to Example 1 except that the titanium sponge was used instead of the titanium band. The tantalum powder was heated under vacuum to 900 ° C and maintained at this temperature until evidence of degassing was complete. At 1150 ° C, the vacuum valves of the furnace were closed and the furnace filled to a pressure of 10 mm by H ,,. At 1250 ° C, the oven temperature was gradually increased over a 3 hour period to 1450 ° C. When 1450 ° C was reached, this temperature was maintained for an additional 1 hour. When the 1 hour hold time was over, the oven was evacuated to 0.5 micron and the oven energy stopped. The tantalum powder was cooled to room temperature under vacuum and treated with a US -40 mesh. The tantalum powder from samples K and L was combined and chemically analyzed.

Deoxidation test conditions:

degass under vacuum at 900-1000 ° C. heat 3 hours at 1250-1450 ° C at 10 mm H₂ hour at 1450 ° C at 10 mm Hg.

Chemical analysis showed the following:

initial 0 content₂ in tantalum powder: 1625 ppm.

- o₂—

Composite of sample K and sample L 1430 ppm

It has been shown that the titanium sponge is an effective degassing agent, as part of the present deoxidation mechanism for treating tantalum powder.

Example 7.

To demonstrate the effectiveness of the present deoxidation process for the treatment of tantalum after having shaped it into anodes, two samples of tantalum powder were compressed into pellets in a commercial press. A group of these anode pellets was then sintered under a standard high vacuum of 0.133 Pa for 30 minutes at temperatures above 1500 ° C. A second group of anodes was then sintered under 10 mm

Hg in the presence of metallic Zr. As shown in the following table, the decrease in oxygen content was accomplished during sintering of the anode, without significant effect on the electrical properties, using the treatment of the present invention, while a significant increase in the oxygen content was noted after standard vacuum sintering treatment.

Tantalum powder Og initial 1580 ppm

Sintered anode indicator

Sample M Anodes des-

<td colspan="2"> 5</td><td>under vacuum</td><td colspan="2">oxidized to H,</td>

<td></td><td></td><td>1560 ° C x30 minutes</td><td>1650 ° C x30 minutes</td><td>1560 ° C x30 minutes</td>

<td></td><td>Capacitance (pFV / g)</td><td> 12050</td><td> 92 20</td><td> 9470</td>

<td> 10</td><td>DCL (nA / pFV)</td><td> 0,35</td><td> 0,12</td><td> 0,13</td>

<td></td><td>Contraction, % diameter</td><td> 2,0</td><td> 4,1</td><td> 4,3</td>

<td> 15</td><td>Density sintered</td><td> 5,1</td><td> 5,7</td><td> 5,5</td>

<td></td><td> 0₂ (ppm)</td><td> 1920</td><td> 1925</td><td> 1170</td>

<td></td><td>Ng (ppm)</td><td> 65</td><td> 40</td><td> 30</td>

Example 8.

This example further illustrates that the electrical properties of the anodes are not adversely affected to a degree marked by the deoxidation treatment which is applied to the tantalum powder used to produce the anodes. Two samples of the same tantalum powder material (initial 0 content₂of 1280 ppm) were individually heat treated. The control sample was heat treated under standard vacuum conditions then pressed into anodes and sintered. The other sample was heat treated using the deoxidation process of the present invention then pressed into anodes and sintered in the same manner as the control sample. As reported below, the control sample absorbed 435 ppm of 0₂ during treatment, while the sample treated according to the present invention showed an increase in its 0 content₂ which was only 135 ppm.

Initial Og tantalum powder 1280 ppm

<td></td><td>Witness</td><td>Sample N</td>

<td>Treatment</td><td>1475 ° Cx30 minutes</td><td>1250 ° Cxl20 mid-</td>

<td>thermal</td><td>under vacuum</td><td>nutes under H₂</td>

<td></td><td></td><td>1450 ° C x 30 minutes under vacuum</td>

<td>(ppm)</td><td> 1715</td><td> 1385</td>

<td>Anode</td><td>pressed at 6.25 g / cc</td><td>pressed to</td>

<td>preparation</td><td rowspan="2">sintered at 1595 ° C x30 minutes</td><td rowspan="2">6.25g / cc - sintered 1595 ° Cx30 minutes</td>

<td>Properties electric Capacitance</td>

<td>(juFV / g)</td><td> 10852</td><td> 10760</td>

<td>contraction diameter (%) Current leak continuous (DCL)</td><td> 2,6</td><td> 2,6</td>

<td>(nA / pFV) Resistance to</td><td> 0,24</td><td> 0,36</td>

<td>crushing (kg) Surface area</td><td> &gt; 22,7</td><td> 21,3</td>

<td>BET (m²/ g)</td><td> 0,21</td><td> 0,19</td>

Example 9.

This example further demonstrates the effectiveness of the deoxidation of the tantalum powder after having formed it into anodes. 275 anodes (total weight 23 g), representing four different groups of anodes with a variable Og content, were placed in two tantalum trays and loaded into a vacuum oven as previously described in Example 1. A piece of zirconium sheet, placed at the top of a third tantalum tray, was also placed inside the hot zone of the oven according to Example 1.

The tantalum anodes were heated under vacuum to 1200 ° C. At this temperature, the vacuum valves of the oven were closed and the oven filled to 200 mm of pressure with Hg. The temperature of the oven was then increased to 1500 ° C over a period of 9 minutes. When 1500 ° C was reached, this temperature was maintained for 60 minutes. When the cycle at 1500 ° C was finished, the energy of the oven was stopped and the tantalum anodes were cooled to room temperature under vacuum. The chemical analysis of the anodes before and after deoxidation is shown below:

Anode group

No.

Initial Og Level (ppm)

Deoxidized Og level (ppm)

2200 1115

2200 895

2800 895

3600 1040

Example 10.

An experiment similar to that described in Example 9 was also carried out at 1400 ° C for 60 minutes at 20 mm Hg in the presence of a sheet of Zr.

Similar reductions in the Og content of the anodes have also been accomplished as shown below.

Anode group No.

8

Initial level of Og (ppm)

2200

2200

2800

3600

Deoxidized Og level (ppm)

1345

935

955

955

Example 11.

1.362 kg of tantalum hydride powder were regularly distributed in three tantalum trays and loaded into a vacuum oven which also contained metallic Zr as a degassing agent. The tantalum hydride, which contained 1140 ppm of Og, was heated under vacuum to 1000 ° C to allow removal of chemically bound Hg. When this was accomplished as evidenced by the lack of degassing of the tantalum powder, the temperature of the oven was raised to 1200 ° C. At 1200 ° C, the vacuum valves of the oven were closed and the oven chamber filled to a pressure of 20 mm Hg. The oven temperature was allowed to rise to 1250 ° C which was held for 60 minutes. When the 60 minute holding cycle was completed, a vacuum was formed in the furnace chamber to remove Hg gas and the energy of the furnace elements was turned off.

The oven was then filled with argon gas to allow the oven chamber and the tantalum powder to reach room temperature.

The surface of the tantalum powder placed in each tray was removed and analyzed separately from the mass of the rest of the tantalum powder. Analysis of the surface of the powder and the mass of the samples is shown.

Mass 0 ^ deoxidized (ppm)

<td>Tray</td><td> 1</td><td> 1140</td>

<td>Tray</td><td> 2</td><td> 1185</td>

<td>Tray</td><td> 3</td><td> 1155</td>

<td>Area</td><td>composite</td><td> 1145</td>

The above deoxidized powder was then subjected to a second deoxidation treatment at 1450 ° C. x 60 minutes at 20 mm Hg. The tantalum powder was loaded into the furnace at the same time as metallic Zr as previously described. The powder was heated to 1250 ° C under vacuum. At 1250 ° C, the oven chamber was isolated and filled to 20 mm Hg. The oven temperature was then increased to 1450 ° C and held at this temperature for minutes. The surface of the tantalum powder was sampled and analyzed as well as the mass of the powder from each tray.

Mass

Deoxidized Og (ppm)

<td>Tray</td><td> 1</td><td> 1495</td>

<td>Tray</td><td> 2</td><td> 1340</td>

<td>Tray</td><td> 3</td><td> 1340</td>

<td>Area</td><td>composite</td><td> 1075</td>

The data shows that a double heat treatment of the tantalum powder in the presence of Hg gas and a metallic degassing agent helped to control the increase in the Og content in the powders.

Example 12.

As another example of Og control during the heat treatment of tantalum powder, a comparison was made between the heat treatment of the same tantalum powder under vacuum as a function of deoxidation with Hg. 100 g of tantalum powder originally containing 950 ppm of Og at 1100 ° C for 6 hours. Chemical analysis of the powder showed that its Og content had increased to 1535 ppm of Og. A tantalum powder of the same type and in the same amount was also heat treated at 1100 ° C for 6 hours at a pressure of 200 mm Hg and in the presence of about 10 g of a strip of Zr. This tantalum powder exhibited a smaller increase in Og of 1360-1410 ppm, indicating that the increases in Og were reduced due to the conditions of deoxidation with Hg.