Opening the x-ray water window
Electron microscopes provide ultrahighresolution images of cells,
but the technique requires considerable sample preparation, which
takes time and can damage the cells’ structures. To achieve
comparable resolutions optically requires coherent X-rays with energies
above 284 eV, an energy level absorbed by carbon but not by water.
Using X-rays in this water window allows researchers to view cells
by quick-freezing them, which preserves more of their internal structure
than is possible when preparing samples for electron microscopy.
|By focusing a high-power femtosecond
laser on a gas-filled modulated filament (left) and developing
higher-harmonic radiation, X-rays with energies above 284 eV
can be generated in this tabletop device (right).
(University of Colorado)
Until now, the principal source of coherent X-rays has been synchrotron
radiation produced by an accelerator. But such accelerator facilities
typically cost around $20 million, and for cost-effective operation,
many researchers must share them.
A research collaboration between the University of Colorado at
Boulder, the University of California at Berkeley, Sofia University
(Bulgaria), and the Lawrence Berkeley National Laboratory has now
developed a way to generate water-window coherent X-rays with a
tabletop device that will cost about $250,000 (Science
2003, 302, 95). The device focuses a laser on a gas-filled filament,
which converts a fraction of the light to X-rays using higher-harmonic
generation (HHG). In HHG, intense laser light strips electrons from
atoms and as the field oscillates, the electrons are then slammed
back into the ions, releasing higher-energy photons. The light is
created over several cycles of the laser field, resulting in higher
harmonics of the laser light. The harmonic light frequencies increase
with the intensity of the laser and can be more than 100 times the
frequency of the original light. However, a basic limitation of
the process has prevented HHG from efficiently generating X-ray
energies as high as the water window. As the ionization fraction
increases, the index of refraction starts to decrease, due to the
freed electrons. This effect is dependent on the square of the wavelength
of the radiation involved, so it affects light far more than it
does X-rays. The difference in the index of refraction creates a
relative velocity between the light and X-rays, causing them to
go out of phase and undergo destructive interference before the
harmonic signal can build up in the fiber. This greatly decreases
the amount of light that can be generated in the highest harmonics.
To overcome this limitation, the researchers put the gas to be
ionized into a waveguide whose diameter is modulated periodically
every 250 µm. “Because the process is very
sensitive to the laser light intensity, modulating the filament
diameter limits the HHG process to a series of discrete, small regions,”
explains Henry C. Kapteyn of the University of Colorado. The discrete
regions are small, and thus, there is not enough space for the relative
velocity differences between the X-rays and the laser light to destroy
the phase matching. Emissions from all of the regions add together
constructively, which allows the high-energy harmonics in the water
window to build up. The team’s experiments showed that a flux
between 106 and 108 photons/s is produced in the water-window region,
which may be sufficient for biological-imaging applications.
The next steps, says K apteyn, are to lengthen the waveguide from
its present 2.5 cm to as long as 40 cm and increase laser intensity
to about 5 × 1015 W/cm2. These advances
should greatly increase the intensity of the water-window X-rays
and make possible significant X-ray production at energies approaching
Zero thermal expansion
The vast majority of materials expand when heated, a phenomenon
that often creates engineering headaches. If a material is subjected
to swift heating or cooling, as occurs in space applications or
with rapid pulses of electric power, thermal expansion or contraction
may exceed the strength of the material. This causes it to plastically
deform and eventually fracture. A material with a tiny coefficient
of expansion, or, ideally, zero thermal expansion (ZTE), would be
useful for demanding applications, especially if it were an electrical
|Near-zero thermal expansion
is exhibited by the electrically conductive intermetallic compound
YbGaGe (red atoms are Ga, blue Ge, and yellow Yb) because cell
constants a and b increase with falling temperature, but the
c axis contracts.
(Michigan State University, East Lansing, Michigan/Ian
Researchers at Michigan State University in East Lansing have
developed an intermetallic compound, ytterbium-gallium-germanium
(YbGaGe), that exhibits near-ZTE over a temperature range of 100
to 400 K (Nature 2003, 425, 702). The phenomenon that creates this
ZTE, called electronic valence transition, could prove useful in
developing other such ZTE materials.
YbGaGe drew the researchers’ attention because of several
peculiarities in its crystal structure. For one, the Yb atoms seemed
to remain in an intermediate valence state—in other words,
the atoms did not appear to have given up two or three electrons,
but somewhere in between.
X-ray crystallographic studies showed that when cooled, one of
the bonds in the crystal cell between the Yb and Ge atoms actually
gets longer. As a result, the cell stretches in one direction and
slightly contracts in another direction, yet it maintains almost
exactly the same volume. The actual change in volume implies a thermal
expansion coefficient of only 3% that of tungsten and less than
1% that of copper.
“What is happening to cause this effect is that as the compound
cools, electrons drop out of the conduction band and become valence
electrons of the Yb atoms,” explains James R. Salvador, a
member of the research team, which was led by Mercouri G. Kanatzidis.
“The addition of the electrons causes the Yb atoms to grow
and thus the bond length to get longer. In the meantime, the other
bonds are shrinking slightly, as would normally be expected with
decreasing temperature. So the net result is virtually no change
Although the compound’s volume changes little with temperature,
there is a substantial coefficient of thermal expansion along the
axes of the cr ystal, which could still cause some strain in a rapidly
heated or cooled part. However, along certain directions at an angle
to the crystal axes, expansion or contraction would be negligible,
and parts could be fabricated to take advantage of these directions
of least expansion. Another limitation of YbGaGe is that its electrical
conductivity is only fair, with a value at 300 K of 2.3 ×
105 S/m, only 0.4% that of copper.
The researchers believe that the valence-transition phenomenon
observed in this compound can probably be applied in designing other
ZTE metallic compounds with similar properties, including ones with
less expansion along each crystal axis and higher conductivity.
As anyone who has waited for a computer to boot or load a program
knows, the limiting factor on a computer’s real speed is not
its processor but how long it takes to transfer information in and
out of memory. This problem affects not just the long-term memory
on the hard drive but also the short-term memory in the random access
memory (RAM). As a result, researchers have long worked on schemes
to place memory devices and logic processors closer and closer together—on
the same chip or even in the same device. However, one problem has
been that the physical processes used to build memory devices and
transistors are incompatible.
|This magnetoresistive element consists
of two magnetic layers separated by a nonmagnetic spacer and
three independent input lines (a). Setting the magnetic layers
into one of the four parallel (logic 1) or antiparallel (logic
0) configurations produces logic tables corresponding to AND,
OR, NAND, and NOR (b).
( Paul-Drude-Institut fuer Festkoerperelektronik, Berlin, Germany)
Reinhold Koch and his colleagues at Paul-Drude-Institut fuer Festkoerperelektronik
(Berlin) have developed another approach based on magnetoresistors
(Nature 2003, 425, 485). They combined the functions of
memory and logic processing into a single device that does without
Their device is related to magnetic RAMs (MRAMs), which will soon
enter the commercial market. The core of the new programmable spin-logic
device—as with MRAMs—is a giant magnetoresistive element
consisting of two magnetic layers separated by a nonmagnetic barrier.
When the magnetization in the top and bottom layers is parallel,
the resistance of the sandwich is much lower than when the two elements
are magnetized antiparallel.
If the device is used as a memory, a current carried by an input
layer can create a magnetic field to flip the magnetization of one
of the layers, changing the resistance of the device. Because the
magnetization is stable, this creates a nonvolatile memory, which
does not require power to maintain itself. The new device uses three
input lines to convert it to either a logic or memor y unit, depending
on the inputs. The strengths of the currents are set so that a single
input current cannot switch either magnetic layer, but two inputs
together will switch the top layer, and three together will switch
By using the current to preset the device into one of its four
either parallel or antiparallel configurations representing the
four logic functions—AND, OR, NAND, and NOR—the device
can be programmed to carry out all the logical operations that transistors
can do. For example, for the logical operation AND, the output will
be positive only if both inputs are positive. Any device that can
carry out all four logical operations can carry out any programmable
computation, which is made up of a string of such logical operations.
Although laboratory experiments indicate that the new device has
maximum switching speeds of only a few gigahertz, which is much
slower than that of the fastest complementary- metal-oxide-semiconductor
processors, Koch believes that spin-logic devices have strong compensating
advantages. “Because the memories are nonvolatile, you can
do parallel processing without worrying about keeping everything
synchronous,” he points out, and the time saved in transferring
information in and out of memory can increase the overall speed
by 100 to 1,000 times compared with existing architectures.
A pressure-driven battery
For more than a century, converting the kinetic energy of water
or gas to electricity, generally through a turbine, has produced
by far the greatest amounts of power. Turbines, however, are not
suited to smallscale electric applications, so portable operations
rely on batteries, which produce energy directly from chemical reactions.
But batteries have some drawbacks, most significantly in their contribution
to heavy-metal pollution when they are discarded.
A Canadian engineering team at the University of Alberta (Edmonton)
has invented a new method for generating electricity on a small
scale from kinetic energy without using turbines or other moving
parts (J. Micromech. Microeng. 2003, 13, 963). The new
approach, termed an electrokinetic microchannel battery, makes use
of electric double layers, a phenomenon that occurs in a conducting
liquid (such as saltwater) very near a solid surface.
|A research team at the University of Alberta
(Edmonton) has demonstrated a nonpolluting battery without
moving parts that uses the small potential between the upstream
and downstream ends when water is forced through the microchannels
of porous glass.
An electric double layer forms because many insulators, such as
glasses and ceramics, have an excess of electrons on their surface.
This excess attracts positively charged ions in the water, creating
a thin, positively charged layer. When the water moves under pressure
through a channel, the ions tend to pile up at the downstream end
of the channel, while the electrons, trapped in an insulator, cannot
follow them. The result is a small positive potential between the
downstream and upstream ends of the channel. If these ends are connected
by a conductor, a small current flows, which converts the kinetic
energy of the water flow into electricity, without moving parts.
(Magnetohydrodynamic conversion can do the same thing, but it requires
an imposed magnetic field.)
Because the ions are confined to a thin layer—generally
on the order of 1 µm—significant conversion
of kinetic to electric energy occurs only when there is a large
ratio of the surface area of the flow to its volume. This occurs
when the fluid is forced through the microchannels of a porous material.
Using a commercial porousglass filter 20 mm in diameter, and with
a pore size from 10 to 16 µm, the team produced a
1.5-µA current using tap water and a 30-cm pressure
Although the current produced and the energy-conversion efficiency
were tiny in the initial experiments, the team calculated that it
could greatly increase efficiency by using water with a higher salinity
and by optimizing other factors, such as the external load resistance.
“In more recent experiments, we have achieved a 1% conversion
efficiency,” explains team leader Daniel Y. Kwok of the university’s
department of mechanical engineering. “We are not trying hard
yet to maximize efficiency. We are still in the proof-of-principle
phase.” Kwok says that the battery’s main advantages
are its complete lack of environmentally polluting materials and
A major disadvantage is that the battery stores energy in the
form of pressurized water, so applications could not require very
energy-storage densities. Even at 100 atm, water would have only
one-sixteenth the energy density of a nickel–cadmium battery.