"Spicy world of NanoScience" since 2007
NANOALUMINIUM – THE FUTURE OF HYDROGEN POWER!
The humanity directly depends on energy sources, and nowadays the stocks of used oil, gas and carbon, will be exhausted within next 100 years, by estimations of experts. That is why it is so important to find new energy carriers. The first among new sources is hydrogen.
The hydrogen power is represented the most perspective direction among known technologies of energy production and transportation. Advantages of hydrogen are known:
- it is the lightest element, that is why its power capacity counting on a mass unit - the highest – 121 kJ/g;
- there is a lot of hydrogen in the nature: in litho - and hydrosphere contains nearby 17 аt. %;
- ecologically, water – the safest product, appears at oxidation (combustion) of hydrogen;
- In case of hydrogen leak at enormous volumes of its use it does not accumulate in atmosphere of the Earth: light molecules of hydrogen at impact with molecules of gases of air "are dispersed" till first space speed and depart for limits of terrestrial gravitation.
By now fuel elements which can produce electric energy (with use of hydrogen and air oxygen) with efficiency factor = 80 % are already developed.
At the same time prospects of widespread usage of hydrogen power are not too serenely. As well as in any technology there is a number of problems:
- it is very difficult to accumulate and store hydrogen: it is flammable and forms explosive mixes with air;
- it condensates at rather low temperature (-2530C), that is why its storage in a liquidity rather expensive as cooling by liquid helium is required;
- it is very difficult to store hydrogen under the big pressure because of high penetrating power of its molecules.
The idea of using hydrogen in a hard form is tempting enough. Production of hard hydrogen could solve many problems of hydrogen power, rocket production and organic synthesis.
Problems with hydrogen transportation and storage can be avoided if to produce it at a consumption place. At the present time for this purpose the electrolysers of water are used, but its’ large enough dimensions and not enough high efficiency complicate creation of mobile sources of hydrogen. Therefore researches of ways of its production also proceed. But we will speak about one of its.
It is known that at interaction of water and water solutions many metals are oxidized with hydrogen formation:
Mg + 2H+ ↔ Mg2+ + H2 ↑,
Al + 3H+ ↔ Al3+ + 3/2H2 ↑.
At such oxidation of 24g of Magnesium (i.e. it's molar weight) 22,4l of hydrogen are formed, and oxidation 27g of Aluminium in water gives 33,6 l of hydrogen, i.e. in 1,5 times more. Aluminium powders produced by the industry react with water slowly, moreover only 20-30 % react, then the process is slowed down. Long time scientists tried to accelerate reaction by injection gallium, indium, rare-earth elements, etc. in aluminium, but serious successes was not reached.
The quantum leap on development of the electroexplosive "know-how" of ultradisperse powders has occurred in the mid-eighties. Already in the first experiences on electric explosion of conductors in the environment of argon gas it has been established that non-passivated aluminium nanopowder completely reacts with water for some seconds. At a lack of water interaction terminates with an intensive production of steam and sintering of residual nanopowder with products.
Let's consider more detailed some processes of interaction aluminium nanopowder with water. Metal aluminium – is one of the most active metals – in usual conditions it is always covered with a thin, continuous oxidation film which protects it from influence of oxygen, steams of water, dilute solutions. If integrity of oxidation film is broken, for example, at the expense of metal processing by alkalis and acids, aluminium actively interreacts with water. Formally the equation of such reaction, without the notice of possible transformations of aluminium hydrate, looks like:
2Al + 6H2O = 2Al(OH)3 + 3H2↑.
Fig. 1. A microphoto of aluminium nanopowder.
According to stoichiometric calculations, at interaction of 54g of aluminium with water, 67,2l (or 6g) of hydrogen is formed, and 156g of aluminium hydrate. At a room temperature, the speed of reaction is insignificant, as there is always a dissolved oxygen, in water, which partially passivates metal aluminium. But rise of temperature, and also presence of small amounts of alkalis, acids or salts in water increases speed of reaction.
The reserved energy.
Since 1980th years high interactions of ultradisperse powders of aluminium with oxygen and water induced thought on the reserved energy in metal nanopowders. Really, different samples of copper and silver nanopowders, heated up to 3000C and to 6000C accordingly, allocated heat without weight change. But with aluminium such experiments was not possible to carry out: even in vacuum 0,01Pa, the weight of samples increased. The methods of burning in oxygen under pressure in isometric conditions thermal effects had been measured. Its’ excess over rating value (i.e. the reserved energy) reached 80 kJ/mole of initial nanopowder. If we consider that it contains 92-94 mas. % of metal aluminium - the excess will be approximately 100 kJ. To estimate eccentricity of the revealed effect, it is enough to recollect that melting heat of aluminium is equal 13,6 kJ/mol in a massive condition. Under thermodynamics laws, this result has no explanation: aluminium should melt If we inject energy of 13,6 kJ into 1 mole of aluminium.
The further researches have confirmed thought of the reserved energy in nanopowders with diameter of particles from 60 to 150 nanometers. Energy reserves in the form of a double electric layer with high pseudo-capacity which was generated in the conditions of electric explosion and has increased at passivation process. By results of X-ray structural analysis of nanopowders it was found out that the x-ray density of particles is lowered – on the average on 0,2 % (such size is usually reached by heating of massive
aluminium up to 700С). We want to underline that the reserved energy is not connected with energy of a surface: for particles of such diameter it does not exceed 4 kJ/mol. At burning and course of chemical reactions the reserved energy acts in a role of «a starting dope» (the trigger mechanism), lowering temperature thresholds of processes.
Advantages of aluminium nanopowders.
Metal powders, are produced by the industry – powder metallurgy. It is manufactured by most different methods and make materials which cannot be produced in other ways. We have compared speeds of reactions (or that the same, speeds of allocation of hydrogen) of powder aluminium: industrial (with diameter of particles = 100micron) and nanosized – with water.
At use of an industrial powder hydrogen is allocated with a speed only 1,38*10-4 l/s*g (0,138 ml/s on 1g. of powder). To final product – a mix of oxides and hydroxides of aluminium - thus only 20-30 % of initial aluminium converts. In our researches aluminium nanopowders in reactionary ability surpassed an industrial powder in ten times. If reaction of aluminium nanopowder with the distilled water was spent, the speed of allocation of hydrogen depended on temperature: at 600С it was 3 ml/s*g., at 800С – 9,5 ml/s*g. (It is remarkable that last indicator exceeds speed of allocation of hydrogen at hydrothermal synthesis approximately in 70 times).Besides the level of transformation of aluminium reached 98-100 % (depending on temperature). Moreover, injection into distilled water even small amounts of alkali led to considerable increase of speed of allocation of hydrogen: at pH 12 and temperature of 250С only it increases to 18 ml/s*g.. With the industrial powder it makes only 1 ml/s*g even in solution of NaOH at concentration 8 g/l and the same temperature.
All this data shows clear advantages of aluminium nanopowders in comparison with the industrial powder on speed of reaction, and on level of transformation of aluminium.
The reaction of aluminium with water is exo-metric: at full interaction of 1mole it is allocated 418 кJ of heat energy. It is enough for (without heat removal in environment) products of that reaction have heated up to ~23000С. Naturally, it is necessary to avoid such heating in practice – at the expense of superfluous of water. Distinctive features of the thermal mode of process in which aluminium nanopowder co-operates with water, lead to occurrence of new effects which were not known for reaction with participation of large aluminium powders.
First of all is a self-warming of nanoparticles up to the temperatures exceeding temperature of surrounding water on hundreds of degrees. What reasons of this effect? As it was found out, it is connected with the small sizes of particles as shares of atoms of aluminium on a surface of nanoparticle and in its’ volume are comparable. Heat, allocated in reaction, accumulates in a metal component of nanoparticle. Now, direct methods of definition of temperature inside nano objects is not developed. Nevertheless, we managed to find an indirect way of definition of the maximum temperature of self-heating.
Please, find full text of research (including various spheres of application of nanoaluminium) at www.neoecosys.com
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