Composite materials in the form of lead or its alloys reinforced with powders and/or ceramic fibres, and uses therof

Description

[0001] This invention relates to composite materials in the form of lead or its alloys reinforced with powders and/or ceramic fibres, for which some applications in various fields are described.

[0002] By suitably choosing the constituent materials of the matrix and reinforcement, their relative quantities and their arrangements, a wide range of products can be obtained possessing characteristics which can be stipulated beforehand.

[0003] The properties and behaviour of a composite obviously depend on its constituent materials, their form and arrangement, their mutual interaction and finally the methods used for its production.

[0004] It is therefore necessary to correctly evaluate all these parameters to ensure that the final product has the required properties.

[0005] The phenomena which govern the efficiency of the reinforcement on the matrix are various, but the most important can be considered to be basically those which determine optimum adhesion between the matrix and reinforcement.

[0006] The methods for producing composite materials are various, and differ according to the type of metal used as matrix, the type of reinforcement and the characteristics which are to be obtained. If powder reinforcements are used, isotropic composites, ie having uniform characteristics in all directions, are generally obtained. If a powder and/or ceramic fibre reinforcement is used, composites can be obtained by mixing the solid into the liquid or by pressurised infiltration of the liquid metal into preforms constructed of powders and/or fibres, and finally if mixtures of metal powders and ceramic reinforcements are used composites can be obtained by methods involving hot pressing, extrusion, drawing and powder metallurgy in general.

[0007] Composites obtained by mixing Al, Mg and Zn alloys reinforced with dispersed particles of Al₂O₃, SiO₂ or SiC having a particle size of between a few microns and a few hundred microns have been studied.

[0008] Other studies relate to the hot extrusion of mixtures of powdered aluminium and glass at temperatures exceeding the softening point of glass (about 500°C) to obtain a composite reinforced with discontinuous fibres produced on the spot by plastic deformation of the glass particles.

[0009] In all cases, perfect control of the process parameters is always necessary to ensure the effectiveness of the reinforcement. For example production methods which are carried out in the liquid phase have often resulted in a degradation of properties because of reaction with the molten alloy, so that this practice is restricted to a limited number of powder-matrix combinations. In many applications, including structural applications and specifically in industrial electrochemistry, materials having high resistance to chemical agents are required. For example in cells for the electrodeposition of zinc (electrowinning), unattackable lead-silver anodes (Ag between 0.5 and 0.75%) are used. Thus a considerable capital is tied up in these, in terms both of lead and silver.

[0010] It has been surprisingly found that by reinforcing the lead or its alloys with powders and/or ceramic fibres of suitable particle size distribution, or with alumina fibres, composite materials can be obtained having superior properties to those not containing said reinforcements.

[0011] In particular the tensile strength is greater than that of pure lead.

[0012] The present invention provides composite materials consisting of lead or lead alloy matrices reinforced with powders and/or ceramic fibres, said powders having a particle size distribution of between 1 and 200 µm and/or said fibres having a diameter of between 2 and 20 µm and a length of between 50 µm and 1000 µm. The powder is preferably silicon carbide, contained in a quantity of between 1% and 65% by volume and more preferably between 25% and 60%.

[0013] The silicon carbide when used in powder form is preferably of abrasive grade.

[0014] The ceramic fibres are preferably short alumina fibres and/or glass fibres, preferably contained in a quantity of between 1% and 40% by volume.

[0015] The densities of said composites are in relation to the volumetric ratio of the matrices to the reinforcements. For example for a composite containing 50% of SiC by volume the density is 7.3 kg/dm³, compared with about 11.3 for the density of lead.

[0016] If a lead and silver alloy is used it has been further found that by reinforcing said matrix with silicon carbide powder of the aforesaid particle size, composite materials can be obtained having high resistance to chemical agents present at medium-high temperature, while having physical and electrochemical properties equal to or better than those of lead or lead/silver alloy. Preferred lead/silver alloys for reinforcement with SiC are those having a silver content of between 0.3 and 1% by weight (of the alloy).

[0017] Methods which can be used to obtain said composites include the following:
- mixing powders and/or ceramic fibres with lead or its alloys in the liquid or semisolid state;
- pressurised infiltration of liquid metal into preforms of powders and/or ceramic fibres;
- sintering metal powders mixed with powders and/or ceramic fibres.

[0018] The lead-based composite materials of the present invention find application where the chemical and physical properties of pure lead in combination with superior mechanical properties and lower weight and cost are desirable. One example of such applications is plates for stationary acid batteries. Conventional lead plates tend to deform and to bend under their own weight. As a consequence short circuits are created resulting in rapid deterioration.

[0019] Alloying lead with other metals on the one hand improves mechanical characteristics but on the other hand can cause problems in electrochemical applications. In short, these composite materials enable the matrix to be used purely for chemical or electrochemical purposes, while deriving their mechanical properties from the addition of reinforcements. A further potential application of these lead-matrix composites is as antifriction materials. Incorporating fibres in a lead (or lead alloy) matrix considerably improves surface fatigue strength. In this respect the use of lead-based composites as protection against ionizing radiation produced by any means, and against acoustic pollution (sound-absorbent and sound-insulating materials) can be cited. In these specific applications, the strength of the fibres obviates the need for external supports for wide lead sheets, so saving space and weight.

[0020] With particular regard to the Pb-Ag-SiC composite, it should be noted that the silicon carbide besides acting as reinforcement for the alloy is also effective as an electrocatalyst, acting positively in the "secondary" electrochemical reactions at the electrode-solution interface, ie when used as an unattackable anode for the electrowinning or electrorefining of metals. Some examples are given hereinafter to better illustrate the invention, it being however understood that this is in no way to be considered limited thereby.

EXAMPLE 1

[0021] A composite material was prepared by pressurised infiltration consisting of Pb-Ag alloy containing 0.7% of Ag by weight reinforced with SiC powder having a content of 50% by volume and a particle size distribution of between 70 and 180 µm.

EXAMPLE 2

[0022] A composite material was prepared by pressurised infiltration consisting of Pb-Ag alloy containing 0.5% of Ag by weight reinforced with SiC powder having a content of 50% by volume and a particle size distribution of between 40 and 70 µm.

EXAMPLE 3

[0023] A composite material was prepared by infiltration consisting of Pb-Ag alloy containing 0.8% of Ag by weight reinforced with SiC powder having a content of 50% by volume and a particle size distribution of between 20 and 60 µm.

EXAMPLE 4

[0024] A composite material was prepared by infiltration consisting of Pb-Ag alloy containing 0.6% of Ag by weight reinforced with SiC powder having a content of 50% by volume and a particle size distribution of between 5 and 25 µm.

EXAMPLE 5

[0025] Example 4 was repeated using only pure lead. The tensile strength was found to be 26 MPa.

EXAMPLE 6

[0026] A composite material was prepared by mixing, consisting of Pb-Ag alloy containing 0.5% of Ag by weight reinforced with SiC powder having a content of 30% by volume and a particle size distribution of between 20 and 60 µm.

EXAMPLE 7

[0027] 

[0028] Example 1 was repeated but using a pure lead matrix with a reinforcement consisting of 15% by volume of short Al₂O₃ fibres having an average diameter of 3 µm and an average length of 500 µm.

EXAMPLE 8

[0029] Example 7 was repeated but using a reinforcement consisting of 20% by volume of glass fibres having a diameter of 20 µm and an average length of 5 mm.

EXAMPLE 9

[0030] Example 7 was repeated but using a reinforcement consisting of 15% by volume of short Al₂O₃ fibres and 10% by volume of SiC having a particle size distribution of between 5 and 25 µm.

EXAMPLE 10

[0031] Two electrolytic cells containing an aqueous sulphuric acid solution (150 g/l) at 45°C were arranged in parallel:
Cell 1) Pb-Ag anode (Ag 0.5% by weight)
Cell 2) Pb-Ag anode (Ag 0.5% by weight) + SiC (30% by volume) (as Example 6)
Anodic current density 500 A/m²
Operated for 200 hours
Voltage of cell 1) 1.83 V initial, 1.80 V final
Voltage of cell 2) 1.70 V initial, 1.70 V final
Percentage weight loss at anode cell 1): 0.6
Percentage weight loss at anode cell 2): ≃ 0

EXAMPLE 11

[0032] Example 10 was repeated but adding zinc sulphate (Zn²⁺ = 90 g/l) (45°C) to the sulphuric solution.
Voltage of cell 1) 1.84 V initial, 1.82 V final
Voltage of cell 2) 1.71 V initial, 1.71 V final
Percentage weight loss at anode cell 1): 0.5
Percentage weight loss at anode cell 2): 0.02

EXAMPLE 12

[0033] Two electrolytic cells containing an aqueous sulphuric acid solution (150 g/l) (45°C) were arranged in parallel:
Cell 1) Pb-Ag anode (Ag 0.5% by weight)
Cell 2) Pb-Ag anode (Ag 0.5% by weight) + SiC (50% by volume) (as Example 2)
Anodic current density 500 A/m²
Operated for 200 hours.
Voltage of cell 1) 1.83 V initial, 1.80 V final
Voltage of cell 2) 1.73 V initial, 1.70 V final
Percentage weight loss at anode cell 1): 0.5
Percentage weight loss at anode cell 2): ≃ 0

EXAMPLE 13

[0034] Example 12 was repeated but adding zinc sulphate (Zn²⁺ = 90 g/l) (45°C) to the sulphuric solution.
Voltage of cell 1) 1.84 V initial, 1.82 V final
Voltage of cell 2) 1.72 V initial, 1.73 V final
Percentage weight loss at anode cell 1): 0.8
Percentage weight loss at anode cell 2): 0.01

EXAMPLE 14

[0035] Two electrolytic cells containing an industrial zinc solution (H₂SO₄ 105 g/l, Zn²⁺ 80 g/l, Mn²⁺ 7 g/l) (45°C) were arranged in parallel:
Cell 1) Pb-Ag anode (Ag 0.5% by weight)
Cell 2) Pb-Ag anode (Ag 0.5% by weight) + SiC (30% by volume) (as Example 6)
Anodic current density 500 A/m²
Operated for 300 hours
Voltage of cell 1) 1.86 V initial, 1.80 V final
Voltage of cell 2) 1.68 V initial, 1.70 V final
Percentage weight loss at anode cell 1): 0.7
Percentage weight loss at anode cell 2): ≃ 0

EXAMPLE 15

[0036] Example 14 was repeated but using a different cell 2) and a different operating time.
Cell 2) Pb-Ag anode (Ag 0.5% by weight) + SiC (50% by volume) (as Example 2)
Voltage of cell 1) 1.74 V initial, 1.73 V final
Voltage of cell 2) 1.60 V initial, 1.61 V final
Percentage loss at anode cell 1): 3.1% after 1200 hours
Percentage loss at anode cell 2): 0.05% after 1200 hours.

[0037] Other tests carried out at temperatures within the range of 25 to 55°C but other than at 45°C showed that the results obtained are independent of temperature.

[0038] Said examples show a more than 10% energy saving resulting from the lower voltage encountered in all cells 2) compared with cells 1).

Claims

1. Composite materials consisting of lead or lead alloy matrices reinforced with powders and/or ceramic fibres, said powders having a particle size distribution of between 1 and 200 µm and/or said fibres having a diameter of between 2 and 20 µm and a length of between 50 µm and 1000 µm.

2. Composite materials as claimed in claim 1, wherein the powder is silicon carbide contained in a quantity of between 1 and 65% by volume.

3. Composite materials as claimed in claim 2, wherein the silicon carbide powder is contained in a quantity of between 25 and 60% by volume.

4. Composite materials as claimed in claim 2, wherein the silicon carbide powder is of abrasive grade.

5. Composite materials as claimed in claim 1, wherein the ceramic fibres are short alumina fibres and/or glass fibres contained in a quantity of between 1 and 40% by volume.

6. Composite materials as claimed in claim 1, wherein the alloy contains lead and silver.

7. Composite materials as claimed in claim 6, wherein the silver is present in the alloy in a quantity of between 0.3 and 1% by weight.

8. A method for preparing the composite material claimed in claim 1, comprising infiltration of liquid metal under pressure into preforms of powders and/or ceramic fibres.

9. A method for preparing the composite material claimed in claim 1, comprising sintering metal powders mixed with powders and/or ceramic fibres.

10. A method for preparing the composite material claimed in claim 1, comprising mixing powders and/or ceramic fibres with lead or its alloys in the liquid or semisolid state.

11. Use of the composite material claimed in claim 2 as unattackable anodic material for electrowinning or electrorefining of metals.

12. Use of the composite material claimed in claim 1 as protective material against ionizing radiation produced in any manner.

13. Use of the composite material claimed in claim 1 as antifriction material.

14. Use of the composite material claimed in claim 1 as sound absorbent and/or sound insulating material.