ES2282036B2 - PROCEDURE FOR OBTAINING NANOMETRIC SIZE CORINDON POWDER. - Google Patents

Abstract

Process for obtaining corundum powder of nanometric size by vapor phase reaction between gaseous compounds of aluminum, Al (g) and Al {sub, 2} O (g), and an oxidizing gas, such as H {sub, 2} O, O {sub, 2} or H {sub, 2} O {sub, 2}. The partial oxidant pressure must be maintained under conditions of active oxidation of aluminum; temperatures are generally between 1200 ° C and 1800 ° C, and mixtures of corundum and aluminum powder with molar ratios between 1: 1 and 1:10 are used. The corundum dust particles obtained are applicable in catalysis processes, biomaterials manufacturing, cutting tools, and ceramics of high mechanical properties.

Description

Procedure for obtaining corundum powder of nanometric size.

The procedure consists of the phase reaction vapor between gaseous aluminum compounds and an oxidizing agent a high temperatures, in which gaseous aluminum compounds, Al (g) and Al 2 O (g), are obtained by reaction between aluminum and corundum powders. The partial pressure of oxidizing gas (H 2 O, O 2, H 2 O 2, or any other with capacity to transform Al (g) and Al 2 O (g) into Al_ {2} O_ {3}) must be calculated and adjusted precisely for ensure that the whole process takes place in conditions of active oxidation of aluminum. In this way passivation is avoided of aluminum by forming a surface layer of corundum, which would prevent the emission of gaseous aluminum compounds necessary for the process. Because of its small particle size, the resulting corundum powder is a very suitable candidate for the processing and sintering of parts and components of corundum

Corundum ceramics have a large number of practical applications, due to its high resistance mechanical, its thermal stability and high melting point, and its stability against the vast majority of chemical agents, both at Low as at high temperatures. Very common uses of these ceramics are like structural components, in the manufacture of cutting tools, in biomaterials, or as supports catalytic for high temperature processes.

The behavior of ceramics depends largely on the properties of the raw materials used in its manufacture. In general, the smaller the particle size of the starting powder and the narrower its size distribution, the better the properties of the sintered product. This unit explains the intense research carried out in the development of new procedures for the synthesis of raw material powder with controlled particle size. In the case of corundum, different dust synthesis procedures of small particle size and high purity have been developed, very suitable for subsequent sintering. Among them, the most commonly used are wet routes, in which the dust is the result of precipitation reactions within solutions. Examples of these procedures are found in J. Am. Ceram. Soc ., 69 [8] 174-75 (1986); J. Am. Ceram. Soc ., 72 [2] 352-53 (1989); J. Ceram Soc Jpn ., 99 [10] 1036-46 (1991); J. Ceram Soc Jpn ., 104 [5] 469-70 (1996); J. Am. Ceram. Soc ., 86 [8] 1321-25 (2003).

In the present invention a alternative process of manufacturing corundum powder high purity and very small particle size. The procedure it is based on the vapor phase reaction between gaseous compounds of aluminum and oxidizing gases. The result is a corundum powder of nanometric particle size, suitable for later use in the manufacture of corundum ceramic pieces.

Description of the invention

The procedure is based on the evaporation of massive amounts of gaseous aluminum compounds, followed by a vapor phase reaction that transforms said gases into nanometric particles of corundum. As a raw material for the generation of gaseous compounds, mixtures of aluminum and corundum powder are used. Any commercial aluminum and corundum powder may be used, although particle sizes smaller than
100 µm. The procedure is based on the volatility diagram of the Al-Al 2 O 3 system (Figure 1). According to this diagram, the predominant gaseous compounds in this system at high temperatures are Al (g) and Al 2 O (g). The partial pressure of Al (g) is greater than that of Al 2 O (g) over the entire range of compositions except at a single point (point P in Figure 1), where both pressures equalize.

The composition of the reactive atmosphere can controlled by modifying the composition of the precursors from of which gases are generated. The volatility diagram demonstrates that said atmosphere may consist solely of Al (g) or mixtures of Al (g) and Al 2 O (g). Is it is preferable to use mixtures of Al and Al 2 O (g), since in they the quantity of reactive gases is greater and, consequently, the Production of corundum powder of nanometric size is superior. From according to Figure 1, the maximums are found at point P partial pressures of Al (g) and Al 2 O (g) possible for this system at each temperature, since it is at this point where the equilibrium lines that separate the fields of stability of Al, Al 2 O 3, Al (g) and Al 2 O (g). In accordance with the above and with Figure 1, to achieve reactive atmospheres of Al (g) and Al 2 O (g) it is necessary to heat mixtures of powders aluminum and corundum The mixing of both species is done in a agate mill for 1 hour. These conditions ensure a optimal mixing of both powders, which is essential for that your reaction at high temperatures is maximum. Have been studied mixtures of corundum and aluminum powder with molar ratios between 1: 1 and 1:10. The best yields in terms of amount of nanometric corundum powder formed have been achieved starting from mixtures with ratios between 1: 3 and 1: 8. The mixture is placed in corundum, zirconia, or any crucibles other that is stable in the conditions of these treatments, and heat in a controlled atmosphere oven. Before starting the heat treatment, the oven chamber is inerted with a gas noble, preferably Ar. The heating is carried out under a flow of inert gas, generally not exceeding 5 liters per minute, and preferably less than 1 liter per minute. Temperatures of treatment are between 1200ºC and 1800ºC, although the Better results are reached between 1300ºC and 1600ºC. The times of treatment are variable, although they are usually between 1 and 6 hours.

In addition to high partial pressures of gaseous aluminum compounds, in order to deposit nanometric corundum dust, the coexistence of high partial pressures of an oxidizing gas in contact with aluminum gases, such as H2O (g), H_, is essential {2} O2 (g), O2 (g), or any other capable of oxidizing Al (g) and
Al 2 O (g) to corundum. These gases are injected into the current of Ar at the moment when the maximum temperature is reached in the oven chamber. According to Figure 1, the oxidizing gas also reacts with the non-evaporated aluminum present in the mixture of raw materials. The oxidation of aluminum can take place by two different and competitive mechanisms, called active oxidation and passive oxidation. While in active oxidation the reaction is continuous and the aluminum ends up completely volatilizing in the form of Al 2 O (g) and Al (g), in the passive oxidation a continuous surface layer of corundum is formed on the surface of aluminum that totally inhibits the emission of gases. Active oxidation is the dominant mechanism at low concentrations of oxidant and the passive at high. There is, therefore, a critical value in which the transition between both processes takes place, which is different for each temperature. According to this description, the partial pressure of oxidizing gas is a factor of critical importance, and should be adjusted so as to be as high as possible, but always under conditions of active oxidation of aluminum. The maximum pressures are shown in Table I partial of H 2 O (g), H 2 O 2 (g) and O 2 (g) necessary for the active oxidation of aluminum in the temperature range between 1200 ° C and 1800 ° C.

TABLE I

one

The result of the process is the deposition of spherical particles of corundum of nanometric size included between 50 nm and 500 nm. The shape of these particles and their narrow Size distribution allow the use of this material in catalytic applications, in the manufacture of ceramics of high mechanical and biomaterial performance, in obtaining of filters, etc.

Example

As raw materials a mixture of powders is used of corundum and aluminum in 1: 6 molar ratio. The aluminum used is from Panreac (impurity content: compounds of N: 0.005%, fat: 1%, Cu: 0.05%, Fe: 1%). The corundum is SC-B / 01 (San Ciprián, Lugo (Spain)), size of particle: 20 µm. The H 2 O 3 / Al mixture, homogenized in an agate mill, it is placed in corundum pots, in this case dimensions 12x3x2 cm. The size of the crucible is a function only of the dimensions of the oven chamber. He Heat treatment is performed in a Themolyne 35900 oven, equipped with a hollow cylindrical alumina tube of 1.25 m length and 4.5 cm internal diameter. The ends of the tube are closed with hermetic covers, in which holes have been made suitable for the entry and exit of gases. The test is performed under a constant flow of 0.2 liters per minute of Ar (Alphagaz? Ar \ ding {192} of Air Liquide (H 2 O <3 ppm; O2 <2 ppm; N 2 <5 ppm)), and consists of a 3 hour heating at 1450 ° C. Before starting the heating, purge with Ar inside the tube to ensure  complete elimination of O2 (g).

The oxidizing gas used in this case is steam of Water. To inject the appropriate amount of water vapor into the reaction chamber, the Ar current is diverted at the instant in that the programmed temperature is reached, and it is passed to through ice before entering the hot zone of the oven. Ice sublimation provides enough partial pressure of water vapor for corundum smoke formation reaction avoiding the passivation of aluminum. The diffraction analysis of X-rays of the nanometric corundum powder thus obtained demonstrates that the only crystalline phase present is corundum (Figure 2). In the Figure 3 shows the microstructure of the corundum produced in the reaction. This figure shows that the particles have a approximate size of 100 nm.

Claims (6)

1. Procedure for obtaining dust from corundum of nanometric size by a simultaneous process of solid-gas and gas-gas balances that consists in:

to)

Volatilization of gaseous compounds of aluminum, Al (g) and Al 2 O (g), from solid-gas balances between powder mixtures of corundum and aluminum at high temperatures; Y

b)

Immediate deposition at discharge nanometric corundum powder temperatures by reaction of said gases with a gaseous oxidizing agent, injected into the chamber of reaction at a partial pressure calculated and adjusted exactly, to ensure that the whole process takes place in conditions of active oxidation of aluminum.

2. Method according to claim 1, characterized in that the gaseous aluminum compounds are obtained by reaction at temperatures generally between 1200 ° C and 1800 ° C, and preferably between 1300 ° C and 1600 ° C, of mixtures of corundum and aluminum powder with molar ratios between 1 : 1 and 1:10, preferably between 1: 3 and 1: 8; and treatment times are greater than 1 hour and less than 6. 3. A method according to claim 1, characterized in that as an oxidizing species different gases can be used, such as O2 (g), H2O (g), H2O2 (g), or any other with the capacity to transform Al (g) and Al 2 O (g) into Al 2 O 3. Method according to the preceding claims, characterized in that the Al-Al 2 O 3 mixtures are placed in crucibles of corundum, zirconia, or any other suitable for these treatments, and introduced into controlled atmosphere furnaces. The treatments are carried out under flows generally less than 5 liters per minute and preferably less than 1 liter per minute, of an inert gas, preferably Ar. And the injection of the oxidant in the gas stream is done at the moment when the maximum temperature in the oven chamber is reached. 5. Method according to the preceding claims, characterized in that the reaction between the aluminum gases and the oxidizing gas causes massive deposition of spherical corundum particles of nanometric size, generally between 50 and 500 nm. 6. Use and application of particles nanometers of corundum powder, obtained according to the procedure of the preceding claims, in high catalysis processes  temperatures, the manufacture of filters and high ceramics mechanical performance, or in processes that require such particles