This month's science essay is prompted by several questions
about
nanofactories that I've received over the past few months.
I'll discuss
the way in which nanofactories combine nanoscale components
into large
integrated products; the reason why a nanofactory will probably
take
about an hour to make its weight in product; and how to cool
a
nanofactory effectively at such high production rates.
In current nanofactory designs, sub-micron components are
made at
individual workstations and then combined into a product.
This requires
some engineering above and beyond what would be needed to
build a single
workstation. Tom Craver, on our [blog], suggested that there
might be a
transitional step, in which workstations are arranged in
a
two-dimensional sheet and make a thin sheet of product. The
sheet of
manufacturing systems would not have to be flat; it could
be V-folded,
and perhaps a solid product could be pushed out of a V-folded
arrangement of sheets. With a narrow folding angle, the product
might be
extruded at several times the mechanosynthetic deposition
rate.
http://crnano.typepad.com/crnblog/2005/11/what_does_extre.html#comment-11311211
Although the V-fold idea is clever, I think it's not necessary.
Once you
can build mechanosynthetic systems that can build sheets
of product,
you're most of the way to a 3D nanofactory. For a simple
design, each
workstation produces a sub-micron “nanoblock” of
product (each dimension
being the thickness of the product sheet) rather than a connected
sheet
of product. Then you have the workstations pass the blocks "hand
over
hand" to the edge of the workstation sheet. In a primitive
nanofactory
design, much of the operational complexity would be included
in the
incoming control information rather than the nanofactory's
hardware.
This implies that each workstation would have a general-purpose
robot
arm or other manipulator capable of passing blocks to the
next workstation.
After the blocks get to the edge of the sheet, they are
added to the
product. Instead of the product being built incrementally
at the surface
of V-folded sheets, the sheets are stacked fully parallel,
just like a
ream of paper, and the product is built at the edge of the
ream.
Three things will limit the product ‘extrusion’ speed:
1) The block delivery speed. This would be about 1 meter
per second, a
typical speed for mechanisms at all scales. This is not a
significant
limitation.
2) The speed of fastening a block in place. Even a 100-nanometer
block
has plenty of room for nanoscale mechanical fasteners that
can basically
just snap together as fast as the blocks can be placed. Fasteners
that
work by molecular reactions could also be fast.
3) The width (or depth, depending on your point of view)
of the sheet:
how many workstations are supplying blocks to each workstation-width
edge-of-sheet. The width of the sheet stack is limited by
the ability to
circulate cooling fluid, but it turns out that even micron-wide
channels
can circulate fluid for several centimeters at moderate pressure.
So you
can stack the sheets quite close together, making a centimeter-thick
slab. With 100-nanometer workstations, that will have several
thousand
workstations supplying each 100-nanometer-square edge-of-stack
area. If
a workstation takes an hour to make a 100-nanometer block,
then you're
depositing several millimeters per hour. That's if you build
the product
solid; if you provide a way to shuffle blocks around at the
product-deposition face, you can include voids in the product,
and
'extrude' much faster; perhaps a mm per second.
Tom pointed out that a nanofactory that built products by
block
deposition would require extra engineering in several areas,
such as
block handling mechanisms, block fasteners, and software
to control it
all. All this is true, but it is the type of problem we have
already
learned to solve. In some ways, working with nanoblocks will
be easier
than working with today's industrial robots; surface forces
will be very
convenient, and gravity will be too weak to cause problems.
On the same blog post, Jamais Cascio [asked] why I keep
saying that a
nanofactory will take about an hour to make its weight of
product. The
answer is simple: If the underlying technology is much slower
than that,
it won't be able to build a kilogram-scale nanofactory in
any reasonable
time. And although advanced nanofactories might be somewhat
faster, a
one-hour nanofactory would be revolutionary enough.
http://crnano.typepad.com/crnblog/2005/11/what_does_extre.html#comment-11299571
A one-kilogram one-hour nanofactory could, if supplied with
enough
feedstock and energy, make thousands of tons of nanofactories
or
products in a single day. It doesn't much matter if nanofactories
are
faster than one hour (3600 seconds). Numbers a lot faster
than that
start to sound implausible. Some bacteria can reproduce in
15 minutes
(900 seconds). Scaling laws suggest that a 100-nm scanning
probe
microscope can build its mass in 100 seconds. (The non-manufacturing
overhead of a nanofactory--walls, computers, and so on--would
probably
weigh less than the manufacturing systems, imposing a significant
but
not extreme delay on duplicating the whole factory.) More
advanced
molecule-processing systems could, in theory, process their
mass even
more quickly, but with reduced flexibility.
On the slower side, the first nanofactory can't very well
take much
longer than an hour to make its mass, because if it did,
it would be
obsoleted before it could be built. It goes like this: A
nanofactory can
only be built by a smaller nanofactory. The smallest nanofactory
will
have to be built by very difficult lab work. So you'll be
starting from
maybe a 100-nm manufacturing system (10^-15 grams) and doubling
sixty
times to build a 10^3 gram nanofactory. Each doubling takes
twice the
make-your-own-mass time. So a one-hour nanofactory would
take 120 hours,
or five days. A one-day nanofactory would take 120 days,
or four months.
If you could double the speed of your 24-hour process in
two months
(which gives you sixty day-long "compile times" to
build increasingly
better hardware using the hardware you have), then the half-day
nanofactory would be ready before the one-day nanofactory
would.
Tom Craver pointed out that if the smaller nanofactory can
be
incorporated into the larger nanofactory that it's building,
then
doubling the nanofactory mass would take only half as long.
So, a
one-day nanofactory might take only two months, and a one-hour
nanofactory less than three days. Tom also pointed out that
if a one-day
tiny-nanofactory is developed at some point, and its size
is slowly
increased, then when the technology for a one-hour nanofactory
is
developed, a medium-sized one-hour nanofactory could be built
directly
by the largest existing one-day nanofactory, saving part
of the growing
time.
In my "primitive nanofactory" paper, which used
a somewhat inefficient
physical architecture in which the fabricators were a fraction
of the
total mass, I computed that a nanofactory on that plan could
build its
own mass in a few hours. This was using the Merkle pressure-controlled
fabricator, [see "Casing an Assembler"], with a
single order of
magnitude speedup to go from pressure to direct drive.
http://www.foresight.org/Conferences/MNT6/Papers/Merkle/
In summary, the one-hour estimate for nanofactory productivity
is
probably within an order of magnitude of being right.
The question about cooling a nanofactory was asked at a
talk I gave a
few weeks ago, and I don't remember who asked it. To build
a kilogram
per hour of diamond requires rearranging on the order of
10^26 covalent
bonds in an hour. The bond energy of carbon is approximately
350 kJ/mol,
or 60 MJ/kg. Spread over an hour, that much energy would
release 16
kilowatts, about as much as a plug-in electric heater.
Of course, you don't want a nanofactory to glow red-hot.
And the
built-in computers that control the nanofactory will also
generate quite
a bit of heat--perhaps even more than the covalent reactions
themselves.
So, fluid cooling looks like a good idea. It turns out that,
although
the inner features of a nanofactory will be very small--on
the order of
one micron--cooling fluid can be sent for several centimeters
down a
one-micron channel with only a modest pressure drop. This
means that the
physical architecture of the nanofactory will not need to
be adjusted to
accommodate variable-sized tree-structured cooling pipes.
In the years I have spent thinking about nanofactory design,
I have not
encountered any problem that could not be addressed with
standard
engineering. Of course, engineering in a new domain will
present
substantial challenges and require a lot of work. However,
it is not
safe to assume that some unexpected problem will arise to
delay
nanofactory design and development. As work on enabling technologies
progresses, it is becoming increasingly apparent that nanofactories
can
be addressed as an integration problem rather than a fundamental
research problem. Although their capabilities seem futuristic,
their
technology may be available before most people expect it.
