Study On The Preparation And Mechanism Of High Performance Molybdenum Powder And Sintered Molybdenum

The preparation of high performance molybdenum powder and sintered molybdenum is an important link in the powder metallurgy process of molybdenum and also a key for deep processing of molybdenum. In this paper, the phase change process,reaction kinetics and mechanism in the preparation process of molybdenum powder were studied with SEM,TEM,XRD,TGA,DTA and so on. The preparation technology of molybdenum powder was optimized through the test method designed by the quadratic general rotary unitized design. The properties of sintered molybdenum strengthened and toughened by composite particles of La₂O₃ /MoSi₂ were tested through the thermal simulator and electronic universal testing machine. Furthermore, the strengthening and toughening mechanisms were studied.1. The dynamic calcining law of different ammonium molybdenum raw materials is announced, in which it is presented that the roasting decomposition of ammonium dimolybdate has two steps but the ammonium tetramolybdate's has only one step. The rate equation of ammonium dimolybdate in the thermal decomposition reaction is setuped.The first step is:dα/(dT)=4.99×10¹²/βe^-123743/(RT)·2(1-α)[-ln(1-α)]^1/2,And the second step is:dα/(dT)=1.13×10¹³/βe^-180875/(RT))·3(1-α)[-ln(1-α)]^2/3,While the rate equation of ammonium tetramolybdate in the thermaldecomposition reaction is:dα/(dT)=2.344×10¹²e^-121556/(RT)·3/2(1-α)[-ln(1-α)]^1/3,It is illustrated that the raw materials' particle size is hereditary, which means that the change law of products' particle size corresponds to that of the raw materials'. However, the morphology of raw materials has no inheritance, which means it has little effect on the morphology of the products.2. A new intermediate compound discovered is blue molybdenum in the cause of reduction of MoO₃. Its composition is MoO₃, Mo₄O₁₁ and Mo₉O₂₆, and its appearance does not have the crack existence, which is different with the blue tungsten.3. It is revealed that the interface chemical reaction is the major control step in the reduction of MoO₃ to MoO₂, in which the apparent activation energy is 58.1KJ/mol. And internal diffusion is the major control step in the reduction of MoO₂ to Mo, in which the apparent activation energy is 30.1KJ/mol. In addition, single particle of MoO₃ and MoO₂ both follow the chemical vapor migration model in the transition process of hydrogen reduction.4. Based on the quadratic general rotary unitized design, the regression equation is setuped between properties of Mo powder (average size,oxygen content,apparent density) and process parameters (reduction temperature,bed depth,boat speed), which is significant in the statistic test. And the optimum parameters which are reasonable by practice test are established as follows:a. the optimum reduction temperature range for the preparation of Mo powders is 950-1000℃;b. the optimum bed depth range is 1.8-2.0cm;c. the optimum boat speed is 90-100min/boat.5. A new idea is presented that sintered Mo can be strengthened and toughened by composite particles of La₂O₃ /MoSi₂. It is found that with the addition of La₂O₃ /MoSi₂ composite particles, the high temperature tensile strength,room temperature bending strength,density and hardness are all better than those of the sintered Mo only added La₂O₃, but the room temperature tenacity is a little worse than the latter's. When the content of La₂O₃ /MoSi₂ composite particles is 1.0wt.%, the integrated performance index is the best. And the main strengthening and toughening effects are contributed by the grain refining and the second phase particles.