WO1999016918A1

WO1999016918A1 - Porous nickel-containing material and process for producing same

Info

Publication number: WO1999016918A1
Application number: PCT/EP1998/006106
Authority: WO
Inventors: Sergej Konstantinovitj Gordeev; Alla Vladilenovna Grechinskaya; Boris Aleksandrovitj Zelenov; Svetlana Borisnovna Korchagina; Victor Petrovitj Kuznetsov
Original assignee: Frenton Limited; Scientific Research Center 'amt' Of Central Research Institute For Materials
Priority date: 1997-09-26
Filing date: 1998-09-25
Publication date: 1999-04-08
Also published as: RU2137262C1; AU9746798A

Abstract

The present invention relates to a process for producing a porous nickel-containing material comprising the steps of forming an intermediate body containing nickel and at least one modifier and subsequent removal of the at least one modifier. According to the invention the removal of the at least one modifier is made by treating the intermediate body with a gaseous halogen, the at least one modifier being an element which form a gaseous compound with said halogen. The invention also relates to a porous nickel material produced by said process.

Description

POROUS NICKEL-CONTAINING MATERIAL AND PROCESS FOR PRODUCING SAME

TECHNICAL FIELD

The present invention relates to a process for producing a porous nickel material comprising the steps of forming an intermediate body containing nickel and at least one modifier and subsequent removal of the at least one modifier. The invention also relates to a porous nickel-containing body produced by the method.

BACKGROUND OF THE INVENTION

Devices based on the principle of electrochemical reactions on the surface of metals are nowadays widely used in many fields of science and technology. For example, various devices of charge accumulation, such as accumulators, batteries, capacitors and fuel elements, are based on this princi- pie. In these devices charge is transferred by means of oxidation-reduction reactions on the surface of electrodes. One of the electrodes is made of a metal or a metal alloy. Porous metal bodies is also used as fuel cell electrodes, chemical catalysts and filters, for example.

Nickel is widely used in electrical technology as electrodes in electrochemical devices of charge accumulation. Electrodes having a high porosity and a large specific surface area do significantly improve the properties of such devices. The methods for producing porous nickel are today based on sintering of nickel powder, electrochemical etching or reduction of fine chloride or oxide powders by hydrogen with subsequent partial sintering.

WO 98/11974 discloses a process for producing porous nickel filters having a narrow pore size distribution, in which a nickel powder having a particle size in the range of about 0.5-5.0 μm is mixed with organic additives suitable for forming one of an extrudable composition and a castable composition. The composition is formed into a desired shape whereafter the composition is compacted by isostatic pressing at a pressure sufficient to form a green body having an open porosity of about 40-65 %. The green body is then heat treated in order to volatilise the organic additives and thereafter sintered so that a body having an open porosity of about 40- 70 % is formed.

JP 06088199 A2 discloses a method in which metal-based supports are coated with a thin film and made porous. The metal- based supports are sintered bodies comprising a metal matrix and a dispersed phase, which can be made porous. Preferably, the metal matrix is a Ni-matrix and the dispersed phase is made of C-fibres, glass fibres or Al . A porous Ni-body coated with Zr0₂ useful for electrolytic cells was manufactured by said method using C-fibres.

JP 04359869 A2 discloses a method in which Ni-powder is mixed with 0.1-5 % ground Al intermetallic compound (e.g. AlCr) as a reinforcing agent and then 20% sintering-preventing agent to give a slurry which is moulded to a tape and heat treated to give the title cathodes.

"Journal of the Korean Institute of Metals and Materials", V.34 N4, April 1996, pages 528-34, discloses fabrication of in-situ Ni and ex-situ NiO (Li) cathodes by cold pressing using Ni-powder and Li₂C0₃ (in case of the ex-situ cathode) with paraffin wax as a binder.

The drawbacks of the methods known from the above mentioned publications are large sizes of the pores produced and an irregular character of the porosity.

In "Materials for Electrochemical Energy Storage and Conversion. Batteries, Capacitors and Fuel cells. Symposium", Ma- ter. Res. Soc . , Philadelphia, PA, USA, 1995, NiO/Ni composite thin films of nano-sized particles were found to perform as good electrodes in electrochemical capacitor applications. However, the process for the production of such films does not allow a production of pores with equal sizes throughout the volume of the film and the result is dependent on the process conditions.

JP 06128787 discloses a process for producing a porous Ni- electrode comprising the successive steps of (1) coating a porous Ni-substrate with at least one element selected from Zn, Cd, B, Al, Ga, In, Si, Ge, Sn, P, As, Sb, Bi and rare earth elements (including Y) , (2) heat treatment to form an alloy and (3) etching with HN0₃ to dissolve most or all of the alloy- forming element without dissolving the Ni- substrate .

The drawbacks of this method are the following:

- the initial formulations are complex and multi-component ; - the removal by etching of the modifiers in solution is difficult to control; in addition to the initial etching it is possible to etch during operation of the electrochemical cell and change its parameters,- in this case the dissolved components may contaminate the electrolyte and worsen the parameters of the devices;

- none- stability of the process conditions influence the character of the porosity (makes it disorderly) ;

- the most important drawback is that the earlier material requires a prolonged activation because a multiple charging- discharging cycle directly in the electrochemical cell is called for to increase its characteristics . During the cycling the modifier is removed by etching.

The object of the present invention is a process for producing a porous nickel material having a large surface area and high porosity, which is easy and cost effective to perform and which is suitable for continuous manufacturing of porous nickel material .

SUMMARY OF THE INVENTION

The object of the present invention is accomplished by a process for producing a porous nickel material comprising the steps of forming an intermediate body containing nickel and at least one modifier and subsequent removal of the at least one modifier, characterized in that the removal of the at least one modifier is made by treating the intermediate body with a gaseous halogen, the at least one modifier being an element which form a gaseous compound with said halogen. Since the step of removing the modifier can be made by con- tinuously passing the intermediate body through a furnace, such a process is suitable for manufacturing of porous nickel material in a continuous manufacturing line.

In a preferred embodiment the at least one modifier is an element chosen from the III, IV or V group of the Men- deleyev's Periodic System of Elements or a combination of such elements . The at least one modifier is preferably chosen from the group of B,Si, Ti and P. The treating of the intermediate body with a gaseous halogen is made at a temperature exceeding the temperature of forming a gaseous compound of said halogen and the at least one modifier. Preferably, a further step of heat treating the intermediate body in a reducing or an inert medium after the removal of the at least one modifier is made in order to remove possible rest prod- ucts . The intermediate body can be formed from an alloy of nickel and the at least one modifier, from a foil made of an alloy of nickel and the at least one modifier or by coating the surface of a nickel foil with an alloy of nickel and the at least one modifier.

The present invention also relates to a porous nickel- containing body, characterized by having pores formed by treating an intermediate body containing nickel and at least one modifier with a gaseous halogen, the at least one modifier being an element which form a gaseous compound with said halogen. In one embodiment such a body has pores only in a certain part or certain parts of the body.

BRIEF DESCRIPTION OF THE DRAWING

Embodiments of the present invention will now be described with reference to the enclosed Figures, of which;

Fig. 1 shows a phase diagram of the Ni-Si system,

Fig. 2 shows a phase diagram of the Ni-B system,

Fig. 3 shows a phase diagram of the Ni-Ti system,

Fig. 4a shows a X-ray diffraction analysis of the Ni-Ti intermediate body in an initial state,

Fig. 4b shows a X-ray diffraction analysis of the final body after chlorination of the Ni-Ti alloy intermediate body,

Fig. 5 is a scanning electron microscopy picture of the cross-section of a porous nickel film according to an embodiment of the invention, and

Fig. 6 shows a charge-discharge curve of an electrochemical cell having a porous nickel cathode according to an embodi- ment of the invention.

DESCRIPTION OF EMBODIMENTS

The inventors of the present invention have considered it possible to represent the desired reaction in the process for producing a porous material by the following Scheme :

AB + D → B + AD (1) The main features of the reaction by the scheme (1) are the following :

1. AB and B are solid substances; 2. D and AD are in gaseous states at the temperature of the reaction; 3. Substance B is resistant to the influence of reagent D at the temperature of the reaction.

If reaction (1) takes place without any ( or a small) change of the volume of the body during the transformation of AB into B, the formed B material will become porous.

Thus, with appropriate selection of components for the chemi- cal reaction according to scheme (1) , the process can be applied for the production of new porous materials.

The above mentioned criteria for a successful realization of the scheme (1) make it possible to distinguish a group of chemical compounds which can be used for a production of porous nickel. Nickel is known as the most resistant metal to the action of halogens, in particular chlorine. When using the scheme (1) for producing a porous nickel material, the components B should be Ni and component D a halogen, prefera- bly Cl₂ • Furthermore, component A should be selected from a group of elements which form chlorides at a relatively low boiling temperature. This criterion is satisfied by elements from the III, IV or V group of the Mendeleyev's Periodic System of Elements, preferably Ti, B, Si and P.

In physico-chemical systems Ni-Ti, Ni-B, Ni-Si several chemical compounds are possible. This is confirmed by the phase diagrams of states possible in the mentioned systems disclosed in Figures 1-3.

On the basis of these diagrams it is necessary to consider possible use of the compounds Ni₃Si, Ni₂Si, NiSi, Ni₃B, Ni₂B, NiB, TiNi, TiNi₃ or nickel alloys containing such compounds. A similar approach may be used to choose possible compounds and alloys using other modifiers.

For a preliminary calculation of a pore volume, which can be obtained by chlorination of compounds in Ni-Ti, Ni-Si and Ni- B systems the following formula is used:

V_p = 1 - lχ _Niχp_com/p_NiχM_com (2)

where V_p = volume of pores, ccm/ccm; M_ni = molecular mass of nickel; p_Ni = density of nickel;

M_com = molecular mass of nickel compounds; Pcom = density of nickel compounds,- and 1 = quantity of nickel atoms in chemical formula of initial compound.

Formula (2) is true when the volumes of the initial and the final material are equal, i.e. when the volume of the material does not change during the thermochemical treatment. Table 1 shows the calculated results .

Table 1.

From this table it can be seen that all the compounds give a relatively high porosity in a final product, but the use of NiTi and NiSi compounds is preferable to form a porous nickel 5 material with maximum pore volume .

For chlorination of nickel compounds it is important to select the appropriate temperature of the process at which the modifier (titanium, silicon, etc.) actively interacts with w chlorine and nickel is resistant to the action of the halogen.

Several chemical reactions can take place during chlorination,- 15 NiB + 3/2Cl₂ = BC1₃ + Ni_porous

NiSi + 2C1₂ = SiCl₄ + Ni_porous NiP + 3/2Cl₂ = PCI₃ + Ni_porous NiTi + 2C1₂ = TiCl₄ + Ni_porous , etc.

0 In order to produce a porous nickel material the following steps are carried out ,-

1. An intermediate body of nickel and modifier or modifiers is produced. As a modifier, elements from III, IV or V group 5 of Mendeleyev's Periodic System of Elements are used, preferably Ti,B, Si or P, or combinations of such elements. The intermediate body is made by metallurgical, chemical and physico-chemical methods (alloying, chemical interaction, spraying, diffusion processes, etc.) Every known method for 0 producing a nickel-modifier body can be used. Shaping of the intermediate body can be made in any suitable way, for example by rolling, deformation, etc. The intermediate body can also be formed with a certain porosity. Examples of intermediate bodies are; 5 - foils prepared from Ni-modifier alloys,- - Ni foil with sprayed Ni-modifier alloy,- - Ni foil + introduced Si (or B, etc.) to produce NiSi (or NiB, etc) film on the surface;

- Ni-modifier compound films produced by electrochemical deposition.

2. The intermediate body is thereafter treated in a flow of gaseous halogen (or of mixtures with inert gas) at elevated temperatures . The process conditions are chosen so that a high rate of forming of gaseous halogens due to interaction between the halogen and the modifier (s) is (are) provided, the rate of interaction between the halogen and the nickel is, however, very small. The duration of the heat treatment depends on the type of modifier (s) and also on the type of porous nickel material to be produced. For example, a short duration of the heat treatment will lead to body with a practically unaffected central part having a surface layer of porous nickel whereas a long duration of the heat treatment will lead to a body being porous throughout its volume. Thus, it is possible to control the process so that porous layers of a desired thickness can be produced in the intermediate body. It is of course also possible to produce a partly non- porous body by deposition of a layer of a nickel-modifier alloy on a solid nickel body and use the composite body as an intermediate body.

3. In order to ensure removal of possible rest products, such as halogenides and/or impurities, remaining in the intermediate body after treatment of the body in a gaseous halogen a further step of heat treatment in a reducing or inert medium or in vacuum can be used.

The performance of said sequence of process steps will lead to a porous nickel material in these part of the intermediate body in which halogenization took place. The produced porous metallic body has good physico-chemical properties, in particular high electrical capacitance in electrolyte solutions . such bodies can therefore be effectively used as electrodes for charge accumulation and storage.

The following examples characterise the essence of the pres- ent invention,-

Example 1.

An intermediate body is produced by cold rolling of NiTi alloy containing 45 %wt of Ti and 55%wt of Ni . Three pieces of foil with a thickness of 200 microns and a size of 25x20 mm were prepared in this way. The foils were chlorinated at ambient pressure in a chlorine flow of 0.3 1/min. The chlorination conditions for the different samples are shown in Table 2.

Table 2

Table 2 shows that with increasing temperature the rate of the process of modifier (titanium) removal from the intermediate body increases. The theoretical value for the decrease of sample weight due to transformation of the intermediate body into porous nickel is 45 %wt . Thus, the produced samples are not fully transformed but contain porous nickel only in some parts .

The produced samples and the intermediate body were studied by X-ray diffraction methods on a DRON-3 diffractometer with a copper anode. The X-ray pictures are presented in Figures 4a, 4b. Figure 4a shows that the intermediate body consists of crystalline Ni-Ti compounds. Figure 4b shows that the porous zone of the final body, i.e. the produced sample, consists of practically pure nickel with a negligible amount of impurities. On the basis of the data from the X-ray analysis and using the known equation of Selyakov-Sherer, the sizes of nickel crystallites in the film under different conditions were calculated by the formula:

DNI = 0.89λ/B(l/2x2θ)xcosθ₀ (3) where D_ni = crystal diameter λ = 0.1541 nm - wave length of CuKα

B(l/2χ2θ) = half -width of Ni (111) line, radian θ₀ = position of line maximum, radian.

The results are shown in Table 3

Table 3

It follows from Table 3 that a temperature increase leads to a decrease of the size of the nickel crystallite down to about 20 nm. This size decrease can be explained by the fact that the nickel formation occurs under more non-equilibrium conditions at high temperatures. In accordance with crystallite sizes of 20-50 nm, the produced nickel films belong to fine crystalline films.

The produced sample 3 was studied by electron microscope JEOL-100. Figure 5 shows a cross-section of the porous zone of this sample. As can be seen, the structure of the produced porous nickel is highly porous and its porosity can be estimated to 40-60 vol%. Its body is built of fragments of a size less than 500 nm which are clearly separated from each other and which form the porous body with a uniform structure.

Example 2

A sample of porous nickel were produced in the same way as in

Example 1. The different was that the heat treatment in chlorriinnee wwaass ccaarrrriieedd oouutt aatt aa temperature of 400 C up to a change of mass of 22 wt%.

The produced sample was used as a cathode in an electrochemical cell. As anode an electrode with a diameter of 20 mm and a height of 1 mm made of a nanoporous carbon material manufactured in accordance with the teaching of WO 97/20333, was used. The electrolyte in the cell was a 25% aqueous solution of KOH. The cell was charged with 10 mA DC and was then discharged through an external resistor 100 Ohm. The obtained charge-discharge curve is shown in figure 6. This Figure shows that an electrode with porous nickel yields high effi- ciency for energy accumulation. The exhibited electrical capacitance of the cell was 21 F, while a capacitance of the same cell with a carbon cathode in accordance with the teachings of WO 97/20333 was 10 F.

Thus, the described process makes it possible to produce highly porous metallic materials having a microporosity. These materials can be used in adsorption technology, for catalysis and in electro-chemical devises. When using said materials as electrodes in electrochemical cells a high spe- cific energy of the cells is achieved due to a highly developed surface of metal pores which is highly accessible for the electrolyte. High electrical conductivity of the produced porous material provides for high discharge currents, i.e. high specific power. Such parameters are important for a development of electromobile technology, electrical starting devices, etc. The described process is simple and easy to perform and is suitable for continuous manufacturing lines. Furthermore, the porosity of the porous nickel material produced can be determined by a suitable choice of the start material and the pore sizes and crystallite sizes can be controlled by the conditions of the chlorination step (temperature, duration) .

Claims

1. A process for producing a porous nickel-containing material comprising the steps of forming an intermediate body containing nickel and at least one modifier and subsequent removal of the at least one modifier, characterized in that the removal of the at least one modifier is made by treating the intermediate body with a gaseous halogen, the at least one modifier being an element which form a gaseous compound with said halogen.

2. A process according to Claim 1, characterized in that the at least one modifier is an element chosen from the III, IV or V group of the Mendeleyev's Periodic System of Elements or a combination of such elements.

3. A process according to Claim 2, characterized in that the at least one modifier is chosen from the group of B, Si, Ti and P.

4. A process according to any one of Claims 1-3, characterized in that the treating of the intermediate body with a gaseous halogen is made at a temperature exceeding the temperature of forming a gaseous compound of said halogen and the at least one modifier.

5. A process according to any one of Claims 1-4, characterized by the further step of heat treating of the intermediate body in a reducing or an inert medium after the removal of the at least one modifier in order to remove possible rest products .

6. A process according to any one Claims 1-5, characterized in that the intermediate body is formed from an alloy of nickel and the at least one modifier.

7. A process according to any one of Claims 1-6, characterized in that the intermediate body is formed from a foil made of an alloy of nickel and the at least one modifier.

8. A process according to any one of Claims 1-5, characterized in that the intermediate body is formed by coating the surface of a nickel foil with an alloy of nickel and the at least one modifier.

9. A porous nickel-containing material, characterized by having pores formed by treatment of an intermediate body containing nickel and at least one modifier with a gaseous halogen, the at least one modifier being an element which form a gaseous compound with said halogen.

10. A porous nickel-containing material according to Claim 9, characterized by having pores only in a certain part or certain parts thereof.