From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.physics
Subject: Re: lighter than air (Summary)
Date: 23 Jan 1996 06:11:22 GMT

In article <4e0tvk$90r@geraldo.cc.utexas.edu>, glhurst@onr.com (Gerald L. Hurst) says:

>In article <4du0tj$4pr@ixnews6.ix.netcom.com>, Alan \"Uncle Al\" Schwartz
><uncleal0@ix.netcom.com> says:
>>
>>Do you want a solid which is lighter than air and still physically
>>competent?  No problem!
>>
>>Take a 99.9%-void silica aerogel, enclose it in a relatively impermeable
>>membrane, purge it with helium. and you have it.  Do you want it lighter?
>> Forget the helium and instead pump it down to vacuum.
>>
>>So, what's the big deal?
>
>Are you sure about this light solid thing, Al? It seems like a pretty
>big dealto me. I don't think 99.9% will cut it because that would
>still be denser than air. With helium it will take about 99.96% void
>volume. If you are to pull a vacuum on the material, it probably
>needs to be at somewhat lighter to allow something extra for the
>weight of the surrounding film unless you go for a largish balloon
>where the practical weight of a typical very thin and reasonably
>impermeable commercial film would be more than offset by the mass
>savings of vacuum over helium.
>
>Could the necessarily fragile aerogel support 14.7 psi? If it could,
>would it not be rather susceptible to implosion if it were to get
>a finger flick?
>
>These are quite serious questions. If the aerogels really are
>easily made with a useable crush strength exceeding 15 psi at
>a bulk density below 0.001 g/ml, I would appreciate your telling
>us more about this subject.

This topic split into two threads so closely related that I'll
repost the other comments here:

In article <31025096.429513@news.demon.co.uk>, Oz@upthorpe.demon.co.uk 
(Oz) said:

	The only thing I can imagine would fit this would be an
	evacuated aerogel (vacugel?). I have no idea if anyone 
	has attempted to do this but offhand I can't see why it
	shouldn't work as long as surface tension doesn't have 
	a bad effect and the aerogel is strong enough.

	I expect someone can give you more info, but you could 
	try a search on 'aerogel'.  

In article <3102e0c2.43740981@news.interlog.com>, borism@interlog.com 
(Boris Mohar) said:

	Could such an aerogel sphere with the skin around it  be 
	pumped down and be strong enough to support the 
	atmospheric pressure?   If so and if it is large enough 
	you would have a novel kind of lighter than air ship. For 
	that matter could a such a spherical structure be built 
	out >of anything?

 In article <4e1862$90r@geraldo.cc.utexas.edu>, glhurst@onr.com 
(Gerald L. Hurst) said:
>
	I would point out that the somewhat ambiguous term
	"20x's its own weight" implies that a cube of the material 
	with a volume of 1L would support a weight of some some 20 
	mg distributed over an area of 100 sqcm. This amounts to 
	about 2X10^-7 atmospheres and thus might be a little weak to 
	support a vacuum. Of course, a 1 cc cube would support 
	2X10^-6 atm, but that doesn't really help much

	I will ask my previous question. Do aerogels (vacugels) 
	actually exist which have a bulk density below about 0.001 
	g/cc with a crush strength above 15psi (or, say, 1 about 1 
	bar for the sprout physicists)? It is obviously possible in
	theory, but has it actually been achieved?

I should know better than to ask a question in sci.physics that
does not deal with esoteric themes that are as unanswerable
as philosophical speculations. Here is an attempt to answer my
own question:

Not having access to better resources at the moment, here is what
I have gleaned from the net. From LLNL (Lawrence Livermore) at

http://www.llnl.gov/IPandC/opportunities93/06-MATERIALS/aerogels.shtml

we have the following two extracted paragraphs with my asterisks
to save the QM and relativity folks valuable time.

	Silica aerogels are made using a silica solution (with 
	the consistency of oil) to which water, a solvent, and 
	a base catalyst are added, forming a gelatin-like 
	substance. Chemical and heat treatments remove alcohol 
	and water from the gel, leaving the finished aerogel. 
	The Laboratory's manufacturing process takes only a few 
	days, while other methods can take weeks. This feature
	offers a significant savings to those businesses that 
	hope to develop commercial aerogel products. The material 
      **can be produced over an extremely wide range of densities, 
      **from 0.6 g/cm3 down to 0.001 g/cm3, a 200-fold change. 


	Remarkably light--but surprisingly strong Although LLNL 
	aerogels can be as much as 99.9% air, the aerogel can 
      **support over 1000 times its weight. The new materials 
	contain so little solid matter they are almost invisible, 
	giving rise to the nickname "frozen smoke." The open, 
	internal cell-like structure of the materials provides 
	exceptional mechanical integrity and optical clarity. 
	The silica aerogels can have a density less than that of 
	air, and are more than 30 times lighter than the lightest 
	of the earlier-generation aerogels. 

And I should mention a truly excellent Background Article: 
"Aerogel Research at LBL" which is too long to quote but well
worth reading at

http://www.lbl.gov/Science-Articles/Archive/aerogel-insulation.html

I conclude from the asterisk-marked lines above that at the preent
state of the art, an aerogel lighter than air (density ca 1mg/cc)
would only have a crush strength of about "1000 times its weight,"
which I interpret to be about 1/1000 of an atmosphere. It is 
therefore unlikely that such a material could be covered with
an impervious film and evacuated to a weight less than that of an 
equal volume of air.

Let me know if I'm wrong.

Jerry (Ico)

From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.physics
Subject: Re: lighter than air (Summary)
Date: 24 Jan 1996 10:20:06 GMT

In article <4e4aoc$1iv@pipe10.nyc.pipeline.com>, egreen@nyc.pipeline.com
(Edward Green) says:

>'glhurst@onr.com (Gerald L. Hurst)' wrote: 
> 
>>       The silica aerogels can have a density less than that of  
>>       air, and are more than 30 times lighter than the lightest  
>>       of the earlier-generation aerogels.  
> 
>Excuse me sir.  I cannot answer any of your solid engineering questions, 
>as I am one of the philosophical questions about quantum mechanics and
>relativity crowd -- to each his own,  nobody is going to pay me for this --
> but I wonder if I understand the material you quoted rightly:  "can have a
>density less than that of air"...   Nobody said anything about evacuation
>or purging with light gas here.  So does that mean the air filled product
>can be lighter than air?  I suppose it could be so...  though I doubt the
>difference is significant. 

The density they are talking about is what we pedestrian
members of the scientific proletariat call "bulk density."
The "crystal density" of a silica aerogel is probably
around 2.3 g/cc but that mass is spread out in a smoke-like
three dimensional web or matrix. The weight of a piece of
this material would be precisely the same as if it were 
reduced to a few miligrams of fine sand. That weight would
be almost exactly numerically equal (in grams force) to its 
absolute mass and would vary only insignificantly if weighed 
in a vacuum rather than in air.

Thus, if we wished the material to be buoyant in air we
would have to surround the mass of material with an 
impermeable film and either flood it with a lighter-than-air
gas such as hydrogen or helium OR pump the air out of the
material to form a vacuum.

If we want to go the vacuum route, the matrix must be capable
of withstanding the crushing pressure of the atmosphere. The
question of whether an aerogel can actually withstand such
pressure, as was originally suggested by Uncle Al, or NOT,
as has been suggested by me, is, or should be the center of 
this ongoing discussion.

The quote you cited above was part of a couple of paragraphs
extracted from writings of spokesmen for the leading aerogel 
lab, Lawrence Livermore. I have placed asterisks beside the 
three key lines which indicate that the state of the art is 
such that present aerogels are about three orders of magnitude 
too weak to withstand atmospheric pressure.

We may reasonably conclude that if a cube of aerogel floats
into a room where relativity is being discussed, the 
participants need only chant the following mantra: 

"That floating aerogel isn't strong enough to support a vacuum, 
so it must be filled with helium or the like." :)

Jerry (Ico)
 

From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.physics
Subject: Re: lighter than air
Date: 24 Jan 1996 06:29:32 GMT

In article <4e42b3$9o4@aklobs.org.nz>, rtomes@kcbbs.gen.nz (Ray Tomes) says:

>glhurst@onr.com (Gerald L. Hurst) wrote:
>
>>Alan \"Uncle Al\" Schwartz <uncleal0@ix.netcom.com> says:
>>>Do you want a solid which is lighter than air and still physically
>>>competent?  No problem!
>...
>
>>Are you sure about this light solid thing, Al? It seems like a pretty
>>big dealto me. I don't think 99.9% will cut it because that would
>>still be denser than air. With heliun it will take about 99.96% void
>>volume. If you are to pull a vacuum on the material, it probably
>>needs to be at somewhat lighter to allow something extra for the
>>weight of the surrounding film unless you go for a largish balloon
>>where the practical weight of a typical very thin and reasonably
>>impermeable commercial film would be more than offset by the mass
>>savings of vacuum over helium.
>
>I don't know about the vacuum part, but can confirm the rest of
>what Al said.  There was an article last year about people actually
>making such a solid with a density about half of air.

Alas, your information comes too late. I just broke that record
this evening when I constructed a porous cube of just over 16.4L
with a density of 0.00056 g/cc or about 0.43 the density of STP
air. The cube is not yet as strong as current aerogels by a long
shot, but it compares favorably with early aerogel values, being
capable of supporting up to 25-50 times its weight, and thus, like
the aerogels, is very light but not quite ready to be coated with
an impermeable film and evacuated.

From my limited research, I can already predict that vast 
improvement can be made in the crush strength without sacrificing
low weight by utilizing advanced pasta technology to reduce the
diameter of the spaghetti strands thus allowing diagonal 
spaghettini brace installation rather than the simple eight-strand
cube geometry of the fettucinetti prototype. A thin-wall hollow 
rice noodle construction of roughly mee hun diameter would quite 
possibly permit structures with strength approaching that of some 
organic aerogels.

Although I am excited by the prospect of possibly matching at
least some of the records set by the aerogels, I am afraid the
dream of creating the evacuated air-buoyant solid may continue
to elude both the Lawrence Livermore group and my kitchen for
some time to come.

I think we should look at Uncle Al's description of the floating
solid not as a thing of today but as a prophecy of things to
come - sort of in the fashion of a latter-day Nostradamus.

Incidently, I hope you boron filament composite guys out there
will have the decency to lay off until I set the Guiness Book
record using my high-gluten Italian microbore durham semolina 
angel hair. 

Jerry (Ico)

From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.physics
Subject: Eat your heart out, aerogels
Date: 24 Jan 1996 22:18:23 GMT

There has been an ongoing discussion regarding the low bulk density
of aerogels and the impressive strength of the Lawrence Livermore
products which are air-like but capable of supporting a supposedly
impressive "1000 times their own weight."

Harrumph. The so-called "Mylar" metallized balloons, which were
invented in about 1974, have evolved to the point where the 
biaxially oriented nylon film and LLDP polyethylene seals
of the 14" diameter envelopes, when inflated with helium to a
rigid structure, will easily support over 250 lbs, a weight 
equivalent 8,000 times the mass of the envelope and helium. The
absolute density of these units is approximately 1.07E-3, less
than that of air. The larger balloons (28") are also stronger
than the values reported for aerogels and have absolute densities
of about 6.8E-4 which is just about half that of air. 

It is interesting to note that the combination of low mass and
strength of this product has been ignored, probably because of
its status as a "toy" rather than a "scientific invention."

Jerry (Ico)

Aerogel Research at LBL: From the Lab to the Marketplace

Aerogels produced at LBL are 96-percent air mixed with a wispy matrix
of silica. Despite their lack of substance, these materials are the
world's best solid insulator, transmitting only one hundredth the heat
of normal glass. 


Summer 1993

By Jeffery Kahn (jbkahn@lbl.gov)


On first sight, silica aerogels cause most people to do a double take.
Observers perceive a ghost-like substance, what looks like fog that
somehow has been molded into a distinct form yet fog which is encased
by no evident means. Almost like solid smoke, an aerogel resembles a
hologram, appearing to be a projection rather than a solid object. 

Aerogels are advanced materials yet also are literally next to nothing.
They consist of more than 96 percent air. The remaining four percent is
a wispy matrix of silica (silicon dioxide), a principal raw material
for glass. Aerogels, consequently, are one of the lightest weight
solids ever conceived. 

Arlon Hunt was working in LBL's solar energy and energy conversion
research program in 1981 when he first saw an aerogel which had been
brought to the Laboratory by a visiting Swedish professor. Hunt says
that immediately, he was fascinated by the material and its manifest
possibilities. 

"I was intrigued by how lightweight, transparent, and amazingly porous
the stuff was," recalled Hunt. "Porous materials scatter light and
almost always are opaque or whitish. The near transparency of the
material implied extremely fine pore structure. Later, I found out just
how fine." 

As Hunt learned of the unique thermal, optical, and acoustical
properties of aerogels, he became further intrigued. Since 1982, the
Applied Science Division researcher has explored fundamental questions
about the properties of aerogels and developed processes for creating
thermally and optically-enhanced versions. Over the past decade, Hunt
also has evaluated aerogels for many applications and developed
chemical production methods suitable for commercial manufacturing. 

Made of inexpensive silica, aerogels can be fabricated in slabs,
pellets, or most any shape desirable and have a range of potential
uses. By mass or by volume, silica aerogels are the best solid
insulator ever discovered. Aerogels transmit heat only one hundredth as
well as normal density glass. Sandwiched between two layers of glass,
transparent compositions of aerogels make possible double-pane windows
with high thermal resistance. Aerogels alone, however, could not be
used as windows because the foam-like material easily crumbles into
powder. Even if they were not pulverized by the impact of a bird, after
the first rain they would turn to sludge and ooze down the side of the
house. 

Aerogels are a more efficient, lighter-weight, and less bulky form of
insulation than the polyurethane foam currently used to insulate
refrigerators, refrigerated vehicles, and containers. And, they have
another critical advantage over foam. Foams are blown into refrigerator
walls by chlorofluorocarbon (CFC) propellants, the chemical that is the
chief cause of the depletion of the earth's stratospheric ozone layer.
The ozone layer shields life on Earth from ultraviolet light, a cause of
human skin cancer. According to the Environmental Protection Agency,
4.5 to 5 percent of the ozone shield over the United States was
depleted over the last decade. Based on the current levels of
ultraviolet exposure, the agency projects that more than 12 million
Americans will develop skin cancer and more than 200,000 will die of
the disease over the next 50 years. 

Replacing chlorofluorocarbon-propelled refrigerant foams with aerogels
could help reduce this toll. Exchanging refrigerant foams with aerogels
reportedly would reduce CFC emissions in the U.S. by 16 million pounds
per year. 

Aside from their insulating properties, aerogels have other promising
characteristics. Sound is impeded in its passage through an aerogel,
slowed to a speed of 100 to 300 meters per second. This could be
exploited in a number of ways, as for example, improving the accuracy
and reducing the energy demand of the ultrasonic devices used to gauge
distances in autofocus cameras and robotic systems. A layer of aerogel
on a camera's ceramic piezoelectric transducer could considerably
improve the efficiency with which it generates ultrasonic waves. 

Aerogels also have a number of novel applications. Currently, they are
components of Cerenkov radiation detectors used in high- energy physics
research at CERN near Geneva, Switzerland. Another scientific
application currently under consideration involves utilizing aerogels
in space like a soft, spongy net to capture fast-moving micrometeroids
without damaging them. 

The new generation of aerogels that Hunt is creating is based on the
groundwork laid down in the 1930s by Stanford University's Steven
Kistler. Kistler worked with gels and in a 1932 paper published in
Nature, resolved many of the basic questions about this odd form of
matter. Kistler showed that a gel is an open structure composed of a
matrix of solid pore walls and a liquid fill. Subsequently, he invented
a way to dry a gel of its liquid contents without collapsing or
shrinking it.  Kistler called his new material an aerogel. 

Aerogels -- the name pays tribute to the near paradoxical
accomplishment of creating a hybrid between a gel and thin air -- are
not known to exist in nature. Jellyfish are a non-manmade example of a
gel, and a dead jellyfish washed up on the beach and baked by the sun
is an illustration of what happens to a gel when it is dried in nature.
Unlike aerogels which start out as a gel and do not lose volume as they
dry out, a dead, desiccated jellyfish ultimately shrinks to 10 percent
of its former size. 

After Kistler brought aerogels into the world, they remained a
forgotten phenomenon for three decades. Briefly, they reemerged in the
scientific literature in the 1960s but aerogels were not fully
resurrected as an object of significant scientific curiosity until the
1980s. That was when Hunt first encountered aerogels and immediately,
he saw their potential.  Hunt also realized that Kistler's aerogels had
drawbacks. As they were at the time, aerogels were commercially
worthless. 

Aerogels were cloudy rather than totally transparent. Before they could
be used in double-pane windows or skylights, clarity had to be
improved. Aerogels were splendid insulators but in order for them to
become a cost-effective alternative to existing products, they had to
be made even more thermally resistant. From the standpoint of
fabrication, several obstacles emerged. The extant chemistry and
processing technology was too expensive and it was potentially
explosive.  Finally, processing required toxic compounds which
presented yet another impediment. Taken together, a formidable phalanx
of technological barriers prevented aerogels from making the leap from
the laboratory to the consumer. 

Over the past eight years, Hunt has confronted each of these obstacles.
Fundamental studies he has conducted have resulted in applied advances
toward resolving each of the major shortcomings. Along the way, Hunt
founded a private firm which has been licensed by the Laboratory to
manufacture and sell aerogels. Thermalux, L.P. is the only U.S. aerogel
firm and has set up a development-stage pilot plant in Richmond,
California. Currently, a Swedish company that produces aerogels for use
in radiation counters, is the only other commercial aerogel
manufacturer in the world. 

Whether they are the commercial aerogels Thermalux is fabricating for
tests and assessment by the refrigeration industry or the experimental
compounds Hunt is producing in his laboratory, all aerogels start out
as a gel. A gel consists of chains of linked particles or polymers
permeated by a liquid. To transform a gel into an aerogel, the liquid
must be removed without collapsing the solid framework. This is a
tricky proposition. 

The gel lattice consists of solid pore walls filled by a liquid. When
liquid is evacuated from a gel, normally surface tension overwhelms the
porous network, causing it to collapse. As air replaces the liquid
inside each pore, surface tension inexorably pulls the sides of the
pores together and the gel shrinks. 

Kistler discovered the secret to drying a gel without collapsing it. He
dried his gels at elevated temperatures and pressures, transforming the
liquid to a supercritical state wherein there is no longer a
distinction between a liquid and a gas. After cranking up the
temperature and pressure to create supercritical conditions, pressure
is slowly released. The supercritical fluid is vented out of the gel
matrix without any surface tension effects. What remains is an aerogel
that is more than 96 percent air. 

Aerogels were exquisite structures but they were formulated with a
standard starting compound known to damage the cornea of the eye. The
toxic material, tetramethylorthosilicate (TMOS), had been introduced in
the 1960s as a means of reducing the preparation time for aerogels from
several weeks to a few hours. Hunt and his colleagues Rick Russo, Mike
Rubin, Kevin Lofftus, Paul Berdahl, and Param Tewari experimented,
looking for safer preparations and processes. 

One known alternate compound favored by Russo was
tetraethylorthosilicate (TEOS). However, the only aerogels ever made
with TEOS were less transparent and more shrunken than the aerogels
made with TMOS. The LBL group conducted a number of experiments with
TEOS and focused on the base catalysis process. Ultimately, they tried
ammonium fluoride, an acid catalyst. Voila, the result was a clearer
aerogel and less shrinkage. 

Sven Henning, one of the few other scientists in world then doing
aerogel work, was visiting Hunt's group in 1984 when word arrived that
his small aerogel manufacturing facility in Sweden, the world's first,
had exploded. Gases escaping the autoclave aerogel drying apparatus had
ignited, blown the roof off the plant, demolished the building, and
injured several employees who were hospitalized. 

Hunt was motivated to explore alternate aerogel drying processes. 

The drying process in use at the time relied on alcohol. When the gel
was ready for drying, it was loaded into a pressure vessel, alcohol was
added, and heat was applied. At 280 C and 1800 pounds per square inch
of pressure, the alcohol was a supercritical fluid. After reaching that
plateau, pressure was slowly released and the supercritical alcohol
gradually was vented from the vessel. 

In addition to the evident potential for explosions, Hunt realized that
this process was too costly for successful commercialization. The high
pressures and temperatures required massive, expensive fabrication
chambers, and beyond that, it was an energy-hungry process. Hunt looked
for a substitute for alcohol. The surrogate substance had to become
supercritical at a lower temperature and pressure and it had to be
nonflammable. 

Liquid carbon dioxide proved to be the ideal supercritical fluid. Under
pressure, it becomes liquid at near room temperature. And whereas
alcohol can be bomb-like, carbon dioxide is fire- quenching. Hunt's
carbon-dioxide aerogel drying process has been patented. 

From scratch, Hunt's aerogel process begins with the mixing of TEOS and
water. To allow these two immiscible fluids to loosen up and mix,
alcohol is added. The water breaks apart the TEOS, attacking the
silicon bonds, and creating an intermediate ester that condenses into
pure silica particles. With the assist of a catalyst, ammonium
fluoride, and a solution of ammonium hydroxide to control the pH, the
silica particles grow and link, forming an alcogel. A clear gel, the
alcogel is sufficiently strong so that when a bottle is half filled
with it and turned upside down, it will not flow. 

The gel is then inserted into a pressure vessel where liquid carbon
dioxide flushes out and replaces the alcohol in the gel, reducing
potential fire risks in the process. Pressure is increased, the carbon
dioxide becomes supercritical, and as it is slowly vented, the alcogel
dries into an aerogel. 

Hunt says he continues to fine-tune the drying process. "In principle,"
he says, "the carbon dioxide process is straight- forward, but you have
to practice the process. At 600-800 pounds per square inch, there are a
whole world of things going on inside that pressure vessel. It's like
driving a sports car on a mountain road. You have to slow down, speed
up, make adjustments in the pressure and temperature. You can crack-up
in a car and you can fracture aerogels or, you can make them
crack-free." 

With these multiple refinements, Hunt and company had created a safer,
more energy-efficient process that required less massive and costly
equipment. He turned next to the problem of clarity. 

Aerogels were transparent but they were not transparent enough to be
used in double-paned windows. They scatter light through a natural
process first described by Lord Rayleigh in the late 19th century. This
phenomenon -- Rayleigh scattering -- is why the sky looks blue against
the dark background of outer space and why the same sky looks yellow
when viewed in the direction of a setting sun. Hunt's aerogels scatter
light in a similar manner. Placed against a dark background, they
appear bluish whereas against a light background, they are yellowish. 

Hunt decided to tweak his recipe, altering the quantities of the five
compounds that go into the gel plus the variable of temperature in an
effort to increase clarity. Some 500 formulations were tested and
additional variations were evaluated using a powerful experimental
technique called factorial design analysis that helps pinpoint the
roles that different ingredients play. 

Additionally, Hunt drew on his doctoral thesis work, employing a
beloved, mothballed device he had devised to measure light scattering.
The scientist retrieved his trusty, old scanning polarization modulated
nephelometer. The nephelometer measured several of the 16 separate
elements of the light scattering matrix of various experimental
formulations of aerogels, allowing Hunt to isolate and identify the
structures responsible for scattering. 

"My measurements revealed that the largest of the pores was responsible
for the scattering and the haziness in aerogels. The cross-linked
silica particles are extremely fine, 20-40 angstroms in diameter. That
is smaller than the wavelengths of visible light and too small to cause
scattering, which is good news. The average pore size was 200 angstroms
but the largest in our TEOS gels were 3,000 angstroms. The large pores
are the problem." 

By filtering out impurities in the starting solution, improving the
overall cleanliness of operations, and providing more uniform gelling
conditions, pores larger than 500 angstroms have been eliminated. This
has considerably improved the clarity of Hunt's aerogels, making them
suitable for use in skylights or atrium coverings. But further research
and development is necessary before aerogels are totally transparent.
Until then, the promise of aerogel-insulated double-paned windows will
remain just out of sight. 

On the other hand, aerogels could make their debut as insulation in
refrigerators within several years. 

Refrigerators and freezers account for about 20 percent of residential
electricity use in the U.S. Because of a vast potential for energy
savings through the use of available, cost-effective technology,
Congress passed the National Appliance Energy Conservation Act in 1987.
Implementing the act, the Department of Energy (DOE) has announced
rules requiring improved energy efficiencies in appliances with the new
standards for refrigerators taking effect January 1, 1993. Only a
handful of the 2,000 models now on the American market meet the 1993
standards. The DOE has pledged it will impose yet more stringent
standards in the future as soon as new and affordable technology makes
this practical. (The 1993 DOE standards are based on technical and
economic analysis performed by Isaac Turial and Jim McMahon in LBL's
Applied Science Division.) 

Every year in the U.S., 300 million square feet of insulation are used
in new refrigerators. Currently, the insulation of choice is
polyurethane foam which is expanded into refrigerator walls by
chlorofluorocarbons (CFCs). Refrigerators account for an estimated two
percent of the U.S.' annual CFC usage. The U.S., through an
international treaty and the Clean Air Act of 1990, has committed to
halt its production of ozone-destroying CFCs by the end of the century. 

Three insulating materials, all silica-based, are the leading
candidates to replace foams. The competition pits aerogels against
silica powder and glass beads which are sealed inside steel sheets. All
three insulating systems would be sealed in a partial vacuum to
increase their thermal resistance. 

In a partial vacuum, aerogels outperform silica powder and glass beads.
Inch-thick aerogels have the same R value (a measure of thermal
resistance) as inch-thick foams. But when 90 percent of the air is
evacuated from a plastic-sealed aerogel packet, the R-7 value nearly
triples to R-20 per inch. To match the R-value of aerogels at this
vacuum of one-tenth of an atmosphere, silica powder has to be evacuated
to a few thousandths of an atmosphere. Glass beads require
one-billionth of an atmosphere. 

Achieving a vacuum of one-tenth of an atmosphere and sustaining it for
the lifetime of a refrigerator is a piece of cake.  Existing plastic
vacuum packing techniques can do the job. Maintaining a vacuum of
one-thousandth of an atmosphere or better is a major technological
challenge. 

Whereas Hunt doesn't have to worry about vacuum sealant technology, he
is under pressure to reduce the cost of aerogels.  About $20 of foam
goes into a 1991 model refrigerator using 40- square-feet of
polyurethane insulation. Insulating the same refrigerator with aerogels
would cost in excess of $80. The aerogels, however, would have double
the R-value of foam and in two years, the energy saved would recoup the
$60 in additional costs. 

Hunt has conducted fundamental studies on how heat is transmitted
through aerogels in an effort to improve the material.  The less
aerogel necessary for a given application, the lower the cost. 

Research shows that the little remaining heat which is conducted
through an aerogel under vacuum is attributable to solid conduction
through the silica lattice and to radiant heat transfer. The solid and
the radiative component each account for about half of the heat that
passes. Focusing on neutralizing the radiative element, Hunt conducted
analysis which pinpointed the spectra of infrared energy which aerogels
conduct. Whereas aerogels block the passage of most wavelengths, they
are transparent to infrared radiation between the wavelengths of three
and eight microns. 

Hunt began a Cinderella-like search for an additive that would block
the infrared energy in this wavelength region. The substance had to fit
the job at hand and no less than a perfect fit would do. The
perfectly-proportioned additive would absorb infrared radiation in the
three to eight micron region, be available in small particle sizes, not
interfere with the gelation or drying process, disperse uniformly
without clumping, and be non-toxic and inexpensive. 

Hunt tried carbon black. The slipper fit. Doped (mixed) with carbon,
aerogels turn black and become better insulators.  Inch- thick
carbon-doped aerogels have been tested and rated at R-25 per inch. 

All these years later, Hunt remains entranced by aerogels. They were
the best solid insulator known when he first saw them and he has made
them even more impervious to heat. Today, Hunt continues to work on
improving aerogels. Currently, he is contriving to fabricate them using
still less raw material so that they are yet cheaper and lighter, just
a wisp of solid within a filigree of air. Beautiful as Hunt finds the
new aerogels, the scientist intends to create ever more elegant
aerogels, materials that consumers and manufacturers will find
absolutely irresistible.