X-Message-Number: 18667
From: 
Date: Wed, 27 Feb 2002 15:47:41 EST
Subject: New magnetic refrigerators.

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Short Article from New York Times... February 19, 2002
Building a Better Refrigerator, With Magnets
By KENNETH CHANG    
The Astronautics Corporation of America in Milwaukee has been working on 
prototypes of magnetically operated refrigerators.

In 1881, Emil Warburg, a German physicist, placed a piece of metal near a 
strong magnet. The metal warmed up.

Present-day scientists and engineers hope to take advantage of that 
phenomenon not for heating, but the opposite: for building a new type of 
refrigerator that is quiet and efficient. 

They say large commercial refrigerators or air-conditioning systems based on 
the technology may be no more than a year or two away from the market.

A company in Wisconsin has a prototype cooling unit plugged in and working.

The cooling power of today's refrigerators comes from the repeated 
compression and expansion of a gas. As the gas expands, it cools and is 
cycled around an insulated compartment, chilling the contents. By contrast, 
magnetic refrigerators cool by repeatedly switching a magnetic field on and 
off.

In certain metals, atoms act like tiny bar magnets pointing in random 
directions.

When placed in a magnetic field, the bar magnets quickly pivot, so they are 
parallel with the field. That is a lower energy state, and the surplus energy 
makes the atoms vibrate, producing heat.

In other words, the metal warms up.

Engineers realized decades ago that they could turn around this process to 
draw heat away from an object and, thus, cool it. Magnetic refrigeration has 
been used in laboratories to cool within a degree above absolute zero.

Now the same principles can be applied at everyday temperatures. Here is how 
a spinning metal disk, a magnet and some water could one day chill your food:

The magnet straddles one part of the disk.

As a part of the disk spins into a magnetic field, the tiny bar magnets in 
the disk line up, and the temperature rises. Water circulates over that part 
of the disk, cooling it.

When that part of the disk leaves the magnetic field, the bar magnets are no 
longer forced into lining up.

Part of the heat energy is dissipated into jostling the bar magnets back into 
random directions, cooling the disk below room temperature. A second stream 
of water runs over the disk, and that cool water is used to chill the 
refrigerator.

Although the concept is straightforward, researchers have been refining the 
details, first looking for metals that maximize the magnetocaloric effect. 
The current prototype uses a disk, about the size of a CD, made of 
gadolinium, a metal used in the recording heads of video recorders.

Earlier prototypes also used superconducting magnets     which themselves have 
to be chilled to very low temperatures     to generate the magnetic field.

In the latest prototype, the scientists fashioned a permanent magnet that 
generates a field nearly as strong.

"This is getting closer to a real machine that you put in a real device," 
said Dr. Karl A. Gschneidner Jr., a senior metallurgist at the Ames 
Laboratory in Iowa, who has been working on the prototypes with colleagues at 
Ames and a company, the Astronautics Corporation of America in Milwaukee.

The use of a permanent magnet is "definitely a nice advance," said Dr. Robert 
D. Shull, leader of the magnetic materials group at the National Institute of 
Standards and Technology in Gaithersburg, Md. But Dr. Shull said he wanted to 
know more details.

"I don't know what the efficiency of it is," he said. "That's one of the 
critical numbers one needs to know whether it can be commercialized."

There is still room for improvement.

In an article published in the journal Nature last month, scientists at the 
University of Amsterdam reported that they had created an iron- based 
compound that also exhibits a large warming effect in a magnetic field. Iron 
and the other ingredients in the compound are considerably less expensive 
than gadolinium.

Dr. Ekkes Br  ck, a physics professor and an author of the paper, called the 
compound "probably more feasible" for production, "because it is a lot 
cheaper."

Dr. Shull said another advantage of the iron compound was that it worked at 
warmer temperatures, operating in 100-degree heat, when gadolinium might 
falter. 

"That is what is particularly nice about it," Dr. Shull said. "It has these 
very large effects at slightly larger temperatures."

But Dr. Gschneidner worried that an ingredient in the iron compound was the 
poison arsenic, while gadolinium is harmless to animals and plants.

"I just wouldn't want that much arsenic floating around in the world," he 
said. 

Dr. Shull said he doubted that the arsenic would pose a health risk. It would 
be bound to the other atoms. 

Dr. Br  ck noted that cellphones had gallium arsenic.

Because gadolinium and the magnet are not cheap, a magnetic refrigerator 
would cost more than a conventional one.

But it would also be more energy efficient, costing less to operate. And the 
magnetic type would not use chlorofluorocarbons, which eat away ozone in the 
upper atmosphere that protects Earth from harmful ultraviolet radiation.

"It's environmentally much more friendly," Dr. Gschneidner said.

Something else is missing, said Robert P. Herman, a senior engineer at 
Astronautics, the hum and whir of present-day compressors.

"The only thing you may hear is a very low noise from the motor," Mr. Herman 
said. "That's about it. Once in an enclosure, you don't even hear that."

    
    

 

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