Method for Producing Ag-MeO Electrical Contact Material FIELD OF THE INVENTION The present invention relates to the field of electrical functional 5 materials, and particularly to a method for producing a silver-based electrical contact material. BACKGROUND OF THE INVENTION The Ag-MeO (metal oxide) contact material came out between 1920s 10 and 1930s. At the end of 1930s, F. R. Hensel and his colleagues manufactured the first Ag-CdO material. At the end of 1960s, the Ag-MeO contact material, such as Ag-SnO 2 , Ag-ZnO or the like, started to appear. Ag-CdO has been widely applied in the engineering due to its excellent properties. However, "cadmium poisoning" pollution occurs during production and usage stage of the Ag-CdO material, which 15 attracts the attention of the governments around the world. In the development of medium or low-pressure electrical contact material, it is an important trend to develop a new Ag-MeO which can replace the Ag-CdO contact, and to further improve the alloy properties. Nevertheless, the difficult-to-machine problem prevails in the new Cd-free Ag-MeO contact material, such as Ag-SnO 2 , Ag-ZnO or the like. Therefore, it 20 is necessary to develop a new manufacturing process with high efficiency and low cost. The current methods for producing Ag-MeO contact material mainly comprise alloy internal oxidation and powder metallurgy. The powder metallurgy comprises powder mixing, co-depositing, and coating. In the method of powder 25 mixing, a silver powder and a metal oxide powder are mixed, molded, and sintered. It is required in such a process that the powders of raw materials must be very fine to provide a homogeneous structure. However, too small powders are prone to agglomerate, thus making it difficult to mix homogeneously. Besides, it is relatively difficult to prepare a raw material powder with a grain size less than 5 im. Therefore, 30 this process is limited in applications. During preparing an Ag-MeO composite powder by co-depositing or coating, pollution like waste water or gas will occur. Besides, there is a poor wetting between silver and the metal oxide in the Ag-MeO contact material, which results in a low resistance to arc erosion of such a material. In addition, the oxide particles deposited on the surface of contact increase the contact 5 resistance of the material. The internal oxidation of an alloy bulk is suitable for mass production, but is has the following disadvantages. During the internal oxidation, oxygen atoms have to diffuse in the dense body of alloy, and the gradient of its density will lead to change in microstructure of the material, which is unfavorable for electrical properties of the material. At the same time, there are limitations to the size 10 of product and the content of oxide. By the method of alloy powder oxidation, it is possible to avoid the above problems. However, the Ag-MeO composite powder, which is obtained by oxidizing the silver alloy powder, is required to go through pressing, sintering, re-pressing, re-sintering, and repetitive extrusion and drawing to complete a finished product. The production flow is long. Besides, due to hardening 15 during extrusion and drawing, MeO particles easily agglomerate together, and the generated stress is concentrated, so that it is relatively difficult to further process the material. SUMMARY OF THE INVENTION 20 It is an object of the present invention to solve the problems of difficult-to-oxidize sufficiently, difficult-to-machine, and long-production-flow of Ag-MeO material. Also, it is an object of the present invention to enhance the binding between the dispersed oxides and the silver matrix, improve the overall properties of the products, shorten the production flow, and reduce the manufacturing cost. 25 The solution of the present invention resides in a method for producing Ag-MeO electrical contact material, which comprises the steps of: melting Ag and Me in a designed ratio; atomizing Ag and Me with an atomization device into Ag-Me alloy powders with homogeneous composition and fine grain size; 30 - performing an internal oxidization on the alloy powders to form Ag-MeO 2 composite powders; and processing the Ag-MeO composite powders by die processing and then powder hot extrusion to form an electrical contact material. The metal Me in said Ag-Me alloy may be one or more of Sn, Zn, Cu, La, 5 Ce, Sb, Bi, Mo, Al, Ti, Mg, Y, wherein Ag in the Ag-Me alloy amounts to 85%-92% by weight. Preferably, the oxidation parameters for said internal oxidation comprise a temperature of 400-800 'C and an oxygen pressure of 0.21-50 atm. The duration of the internal oxidation is 3-6 hr. 10 The Ag-MeO composite powder is processed by die processing into a green body with a relative density of 65%-80%. As for preferred conditions for the hot extrusion, the temperature for extrusion is 600-850 *C, and the extrusion ratio is 12:1-200:1. The Ag-Me alloy powder can be dried and screened before internal 15 oxidization. The present inventor prepares Ag-Me alloy powders with homogeneous composition and fine grain size by gas atomization, then internally oxidizes the powders to form Ag-MeO composite powders, and further processes the Ag-MeO composite powders by die processing and then powder hot extrusion to form an 20 electrical contact material. By means of the above process innovation, the cumbersome sintering, re-pressing, re-sintering, and repetitive extrusion and drawing in the prior art process are omitted, so that the production efficiency is increased and the manufacturing cost is reduced. Since the materials in the powder green body still substantially stay in the powder state and have certain deformation and mobility under 25 extrusion force. As compared with extrusion of dense metal materials, the required extrusion force is small and the range of temperature and speed for extrusion is large. Therefore, the difficult-to-machine problem of Ag-MeO material is solved, the microstructure of the Ag-MeO contact material is improved, and thus the overall properties are enhanced. 30 According to the present invention, the specific processes comprise: 3 melting Ag and Me in a designed ratio in a medium frequency induction furnace; - atomizing the alloy in an atomization device with compressed air; screening the atomized alloy powders; 5 - loading powders with a grain size below 325 mesh into an oxidation furnace to be oxidized, with the temperature for oxidation being controlled at 500-800 'C, the pressure for oxygen being kept at 0.21-50 atm, and the oxidation lasting for 3-6 hr; processing the powders, which have been tested and qualified in terms of 10 composition and microstructure, by die processing into green bodies with a relative density of 65%-80%; loading the green bodies, which have been heated to 600-850 'C, into an extrusion mold which has been preheated to 300-500 'C; and performing hot extrusion to form Ag-MeO rods or wires. 15 The advantages of the present invention comprise: the atomized alloy powders are more easily oxidized that cast alloys, so that the duration for oxidation can be decreased and the occurrence of metal-poor oxide region can be prevented, and the resulting MeO particles are dispersed in the composite powders. Besides, since the powder hot extrusion process is applied in the present invention, the cumbersome 20 sintering, re-pressing, re-sintering, and repetitive extrusion and drawing can be omitted, the manufacturing cost can be saved, and the difficult-to-machine problem of Cd-free Ag-MeO material can be solved. Besides, the microstructure of Ag-MeO contact material is improved, and the overall properties are increased. By means of the improvement and innovation to the complete process accomplished by the present 25 inventor, the complete process flow becomes simple and can effectively solve the problems of difficult-to-oxidize sufficiently, difficult-to-machine, and long-production-flow of Ag-MeO material. In addition, the binding between the dispersed oxides and the silver matrix is enhanced, the overall properties of the products are improved, the production flow is shortened, and the manufacturing cost 30 is reduced. 4 It is further preferred in the present invention that Ag and Me are mixed in a designed ratio and then melt in a medium frequency induction furnace, so that it is ensured to effectively provide a homogeneous composition of the alloy. 5 BRIEF DESCRIPTION OF THE DRAWINGS Fig. I is a flow chart showing a specific process of the present invention; Fig. 2 shows in a cross section a microstructure of a product of the present invention, which is magnified by 100; Fig. 3 shows in a longitudinal section a microstructure of a product of 10 the present invention, which is magnified by 100; Fig. 4 shows in a cross section a microstructure of a product of a comparative example, which is magnified by 100; Fig. 5 shows in a longitudinal section a microstructure of a product of a comparative example, which is magnified by 100. 15 DETAIL DESCRIPTION OF THE EMBODIMENTS The following embodiments intend to illustrate, but not further restrict the present invention. Embodiment 1 20 Silver 2004 g, tin 112.5 g, and antimony 76.5 g are weighed and put into a medium frequency induction furnace to melt. Upon melting, the alloy is atomized in an atomization device with compressed air. The atomized alloy powders are screened, and the powders with a grain size below 325 mesh are loaded into an oxidation furnace to be oxidized. The temperature for oxidation is controlled at 800 'C, and the 25 pressure for oxygen is kept at 50 atm. The oxidation lasts for 6 hr. The powders are then processed by die processing into green bodies with a diameter of 28 mm and a relative density of 70%. Then, the green bodies, which have been heated to 800 'C, are loaded into a mold which has been preheated to 500 'C, and are subject to hot extrusion to form products with a diameter of 4 mm. 30 Embodiment 2 5 Silver 2000 g, zinc 223 g, lanthanum 105 g, and yttrium 25 g are weighed and put into a medium frequency induction furnace to melt. Upon melting, the alloy is atomized in an atomization device with compressed air. The atomized alloy powders are screened, and the powders with a grain size below 325 mesh are 5 loaded into an oxidation furnace to be oxidized. The temperature for oxidation is controlled at 400 'C, and the pressure for oxygen is kept at 10 atm. The oxidation lasts for 3 hr. The powders are then processed by die processing into green bodies with a diameter of 28 mm and a relative density of 75%. Then, the green bodies, which have been heated to 600 *C, are loaded into a mold which has been preheated 10 to 450 *C, and are subject to hot extrusion to form products with a diameter of 8 mm. Embodiment 3 Silver 2000 g, copper 110.5 g, antimony 82.9 g, and cerium 20 g are weighed and put into a medium frequency induction furnace to melt. Upon melting, the alloy is atomized in an atomization device with compressed air. The atomized 15 alloy powders are screened, and the powders with a grain size below 325 mesh are loaded into an oxidation furnace to be oxidized. The temperature for oxidation is controlled at 700 *C, and the pressure for oxygen is kept at 25 atm. The oxidation lasts for 5 hr. The powders are then processed by die processing into green bodies with a diameter of 28 mm and a relative density of 80%. Then, the green bodies, 20 which have been heated to 830 'C, are loaded into a mold which has been preheated to 300 'C, and are subject to hot extrusion to form products with a diameter of 4 mm. Embodiment 4 Silver 2005 g, cerium 160 g, bismuth 10 g, and lanthanum 5 g are weighed and put into a medium frequency induction furnace to melt. Upon melting, 25 the alloy is atomized in an atomization device with compressed air. The atomized alloy powders are screened, and the powders with a grain size below 325 mesh are loaded into an oxidation furnace to be oxidized. The temperature for oxidation is controlled at 650 0 C, and the pressure for oxygen is kept at 20 atm. The oxidation lasts for 5 hr. The powders are then processed by die processing into green bodies 30 with a diameter of 28 mm and a relative density of 80%. Then, the green bodies, 6 which have been heated to 800 'C, are loaded into a mold which has been preheated to 450 'C, and are subject to hot extrusion to form products with a diameter of 2 mm. Embodiment 5 Silver 1980 g, lanthanum 155 g, molybdenum 43.7 g, magnesium 10 g, 5 and antimony 15 g are weighed and put into a medium frequency induction furnace to melt. Upon melting, the alloy is atomized in an atomization device with compressed air. The atomized alloy powders are screened, and the powders with a grain size below 325 mesh are loaded into an oxidation furnace to be oxidized. The temperature for oxidation is controlled at 750 'C, and the pressure for oxygen is kept at 10 atm. 10 The oxidation lasts for 3 hr. The powders are then processed by die processing into green bodies with a diameter of 28 mm and a relative density of 75%. Then, the green bodies, which have been heated to 820 *C, are loaded into a mold which has been preheated to 400 'C, and are subject to hot extrusion to form products with a diameter of 6 mm. 15 Embodiment 6 Silver 2003 g, tin 157 g, aluminum 43.7 g, yttrium 5 g, and titanium 10 g are weighed and put into a medium frequency induction furnace to melt. Upon melting, the alloy is atomized in an atomization device with compressed air. The atomized alloy powders are screened, and the powders with a grain size below 325 20 mesh are loaded into an oxidation furnace to be oxidized. The temperature for oxidation is controlled at 600 'C, and the pressure for oxygen is kept at 5 atm. The oxidation lasts for 3 hr. The powders are then processed by die processing into green bodies with a diameter of 28 mm and a relative density of 70%. Then, the green bodies, which have been heated to 850 'C, are loaded into a mold which has been 25 preheated to 450 'C, and are subject to hot extrusion to form products with a diameter of 4 mm. Comparative example 7 Silver 2004 g, tin 112.5 g, and antimony 76.5 g are weighed and put into a medium frequency induction furnace to melt. Upon melting, the alloy is atomized in 30 an atomization device with compressed air. The atomized alloy powders are screened, 7 and the powders with a grain size below 325 mesh are loaded into an oxidation furnace to be oxidized. The temperature for oxidation is controlled at 800 'C, and the pressure for oxygen is kept at 50 atm. The oxidation lasts for 6 hr. The oxidized powders are pressed and then sintered at 830 'C for 2 hr. The sintered blanks are 5 forged at 820 *C to complete the products. Performance Index for the embodiments and the comparative example Products Relative Hardness Resistivity Tensile Elongation density (%) (HV/MPa) (pQ -cm) strength rate (%) (MPa) Embodiment 1 98.9 950 2.3 380 23 Embodiment 2 99.0 870 2.5 340 30 Embodiment 3 99.4 700 2.2 300 30 Embodiment 4 99.5 650 2.1 280 36 Embodiment 5 99.6 800 2.2 320 28 Embodiment 6 99.7 750 2.1 360 25 Comparative 98.5 1000 2.5 400 25 example 7 8