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Physics

The hot and the cold that comes from magnets

A Unicamp team discovers unexpected properties of magnetic materials

New scientific concepts are changing the way home appliances goods work. First it was the television, whose image is no longer generated by the collision of highly accelerated electrons using a fluorescent screen. Equipment with liquid crystal or plasma screens, by way of electric currents or gases that originate the luminous signals, offer images that have more definition, even though they are at a price equivalent to that of a car – from R$ 20,000.00 to R$ 80,000.00. The next piece of equipment to pass through a similar metamorphosis could be the refrigerator: the next generation will not work with gas expansion and contraction, which removes the heat from the air on circulating through the tubes of the refrigerator walls, but by way of the action of magnetic fields, which should perform the same task with greater efficiency and less energy loss.

In the prototypes of magnetic refrigerators – already under development in some countries such as the United States, Japan and France – the material used to reduce the temperature is gadolinium, a metal chosen for its malleability and showing an acceptable efficiency at the temperature of the environment. But it is almost certain that gadolinium will be only provisional: for the new refrigerators materials are being sought that are capable of being cooled or heated with a greater efficiency when submitted to a magnetic field. Recent discoveries carried out in Campinas show that there could in fact be  alternatives with a better refrigerating capacity.

Physicists from the State University of Campinas (Unicamp) and from the State University of Rio de Janeiro (Uerj) have verified that a compound formed between manganese and arsenic, under high pressure, demonstrates the capacity of retaining heat at an environment twenty (20) times greater than that of gadolinium at normal pressure, one atmosphere, the same as that at which we live on the surface of the planet. This is a relevant scientific finding, even though at the present moment it is far away from any application. The high pressure to which this compound of manganese and arsenic – the Mn-As – has to be submitted in order to exhibit this result is only one of the limitations that impede this property from being immediately useful in the prototypes of magnetic refrigerators.

But something new could quickly come from this in practical terms because this so intense capacity of absorbing heat – baptized by the physicists of Unicamp as the colossal magnetocaloric effect – had not previously been observed in any other material. “This discovery opens up a research field of other compounds that show the colossal effect and manage to extract a lot more energy from the region to be cooled, without the limitations of manganese-arsenic”, comments the physicist Sérgio Gama, a researcher from Unicamp and the coordinator of this study, when writing in the edition of the 3rd of December for the magazine Physical Review Letters.

In this article, Gama also proposed an explanation for the colossal effect. As he and his team had experimentally verified, the manganese-arsenic compound cools by way of three associated effects. The first is the most common mechanism of materials of this type: the un-aligning of the spins of the electrons. Spin is a magnetic property of elementary atomic particles, with both direction and sense, just like the earth’s magnetic field. The direction of spin of a particle can be affected by an external magnetic field or by the spins of neighboring particles: when the spins align in the same sense, the temperature of any body increases – this is the principle through which it is intended to arrive at a refrigeration that is more efficient. The second mechanism that explains the colossal effect is the transformation of the atomic structure itself “the crystalline network” of the Mn-As. Finally, the third and most important artifice of Mn-As to absorb energy results from the interaction of the crystalline network with the magnetic field, expressed by way of the deformation of the material.

Up until now, the magnetocaloric effect – the capacity of a magnetic material to absorb or liberate heat when submitted to the influence of a magnetic field – was essentially explained by way of the alignment or un-alignment of the spins, without the changes in the network of atoms having been seen as very important. When submitted to the effect of a magnetic field, within a thermally isolated environment, the spins of the electrons align themselves in a single sense, thus abdicating themselves from the habitual pandemonium in which they live. This ordering means a decrease in entropy – or disorder – of a physical system. As there is no exchange of heat with the environment, the total entropy must remain constant: consequently, the atoms or molecules of the material submitted to the influence of the magnetic field become more disorganized in such a way as to compensate for the spin alignment. Greater disorder means greater atomic movement – and the temperature of the material increases.

It is by way of the alignment of spins that a piece of gadolinium warms up by 4°C when placed in a thermally isolated environment, to which is applied a magnetic field of 1.5 Tesla, close to 30,000 times greater than the earth’s magnetic field. As the effect is reversible, when the magnetic field is removed, the gadolinium cools the same 4°C. Does it appear too small to keep beer cold? It so happens that the construction of magnetic refrigerators forecasts the use of special heat exchangers, called regenerators, which widen this variation of temperature. In order to turn itself commercially viable, the magnetic refrigerator also depends on new geometric arrangements of permanent magnets – like those already used in the hard disk reader of a computer or in the motors that move the windscreen wipers of cars – so that they form intense magnetic fields and are of low cost.

With gadolinium the capacity to absorb or to give off heat is of purely magnetic origin: the result principally of the alignment of spins. It is in this way as well that other materials that show the classical magnetocaloric effect, the most tenuous of this category, happen. And as well the basic mechanism through which the materials that present the other known effect up until now work, the effect called the gigantic magnetocaloric effect, evidently more intense than the classic, but inferior to the colossal. Some alloys that exhibit the gigantic effect – such as the so called 5:2:2, formed by five atoms of gadolinium, two of germanium and two of silicon – can also count upon another artifice: they absorb a high amount of energy, the so called latent heat, whilst their temperature remains constant. It is the same phenomenon that can be verified with a block of melting ice: the ice absorbs heat, which is used in its transformation into the liquid phase; its temperature remains steady on reaching 0°C and only goes up again when all of the ice has melted.

Deformation
Gama experimentally demonstrated that, in the case of manganese-arsenic, as well as the alignment of the spins of the electrons and of latent heat, the very atomic structure of the material itself, that the physicists call its crystalline structure, contribute in a decisive manner to explaining this notable capacity of extracting heat from the surroundings: when submitted to pressure and to a magnetic field, the manganese-arsenic also deforms, bringing about the colossal effect, which is seven times greater that the gigantic effect. Other materials also show this property, called the magnetoelectric effect, which normally causes its deformation. However, the Mn-As is notable: it contracts by 3%. “This is an absurdly high value”, says Gama, “since the deformation of other materials is dozens or hundreds of times less”.

Although endowed with these properties, the manganese-arsenic is not a convenient material for applications because one of its characteristics generates an insuperable problem: even at room pressure, it does not return to the point of origin when submitted to a cycle of warming and cooling. When the temperature increases, the spins get out of line and its atomic arrangement modifies when reaching 43°C – this is the point in which the so called magnetic and structural transition occurs. But the Mn-As only comes back to its initial state, with the spins aligned and with a hexagonal structure,  at a temperature that is much lower, that of 35°C. it is as if water were to become vapor at 100°C and when cooled only returns to the liquid state at 80°C. If the Mn-As were to be used in a refrigerator, extra energy would be necessary to make the cycle of refrigeration close and for another cycle to begin – something extremely inefficient.

“The refrigeration cycles need to start and finish at the same temperature or with minimal variations, so as to reduce the losses in efficiency”, comments Gama. As a further complication, when the Mn-As is submitted to high pressure, this temperature difference increases even more: reaching 32°C. In search of alternatives, the Unicamp group discovered, in studies with other groups, that the temperature difference between the start and finish of the cycle, the so-called hysteresis, gets close to zero at room temperature when a small part (5%) of the arsenic is substituted by antimony. “Compounds with a higher quantity of antimony appear to be promising”, stated the Unicamp group coordinator, “although hysteresis, lagging, is still observed under pressure”.

Brazilian cell
Gama has been working with magnetocaloric materials since 2000, but began to study the behavior of materials under pressure whenever he and his group heard about the phenomenon at a congress on magnetism that took place in July of 2003 in Rome, the Italian capital city. It was there that they came in contract with a device the size of a pen – a pressure cell -, produced, as they found out afterwards, by a guy from Rio de Janeiro who had founded a company in Cambridge, in England. As the price was high – around US$ 20,0000.00 -, the physicists from Unicamp decided to build a similar cell in Brazil. They set up their own project and they managed it, making use of the ability of the technicians at the Physics Institute. They constructed three cells, already being used by other research groups in the country. Each one of them cost less than R$ 3,000.00 in material.

The Brazilian cell, which had been ready some two months after the Italian congress, is a copper-beryllium cylinder of 10 centimeters (10 cm) in length and eight millimeters (8 mm) in diameter, with two pistons locked in by two screws. The pistons pressurize a capsule containing a sample of manganese-arsenic with 1 to 2 millimeters immersed in a mineral oil.  This cylinder is placed within a piece of apparatus called a magnetometer with a superconductor sensor for quantic interference (of Squid type), which measures the magnetism of the material that is desired to be studied, placed within a Teflon capsule. Using this apparatus, Gama’s team could then measure the magnetocaloric effect under pressure, which at that time had not yet been described.

The first material to be measured under pressure was in fact manganese-arsenic, already studied by the Unicamp physicists as having presented gigantic effects: the  magnetocaloric and magnetoelastic effects. Gama was not the first to report the magnetocaloric effect under pressure because a Spanish group from the University of Zaragoza, together with another group from Prague, the Czech Republic, published before them, also in the magazine Physical Review Letters, the results that they had arrived at with a compound formed from terbium, germanium and silicon. However, this involved only the gigantic effect and not the colossal effect.

The Project
A study of the magnetocaloric effect in inter-metallic compounds (nº 01/05883-0); Modality Thematic Project; Coordinator Sérgio Gama – Unicamp; Investment R$ 2,285,714.11 (FAPESP)

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