压缩科学Putting the squeeze on materialsWord下载.docx

1、 However, DAC studies of such properties as electrical conductivity and magnetic susceptibility are extremely difficult to perform. The 1-microgram samples have a diameter of about 75 micrometers, and diagnostic instruments cannot be placed close enough to them to make the required measurements. Pro

2、blems especially arise when researchers try to obtain information about materials at static pressures above 1 million atmospheres, or 100 gigapascals （GPa）. （For comparison, the atmospheric pressure at sea level is about 1/10,000th of 1 GPa, and the pressure at the center of Earth is about 3.6 milli

3、on atmospheres.） To overcome the problems posed by standard diamond anvils, Livermore researchers have taken advantage of recent improvements in diamond synthesis technology to fabricate microcircuits within the diamond anvils themselves. The tungsten microcircuits serve as tiny diagnostic instrumen

4、ts that measure data about materials fundamental physical and mechanical properties under high pressures. The researchers call this modified tool a designer diamond anvil because the microcircuits can be altered to suit the needs of the experimenter. Scanning electron micrographs show a completed de

5、signer diamond anvil for measuring electrical conductivity. （a） Tungsten microcircuits lead from the sides of the diamond, where they form electrical contact pads with instruments, to the tip of the diamond face, or culet, where they monitor various properties of the sample. （b, c） Progressive magni

6、fication of the diamond tip with a light microscope shows the termination of the tungsten wires. Click for a high resolution photograph. Pressuring Materials to Change Materials behave quite differently under extreme pressures than they do at normal atmospheric pressure. Oxygen, for example, becomes

7、 a shiny metal under ultrahigh pressure. In support of the National Nuclear Security Administrations Stockpile Stewardship Program, Livermore researchers are particularly interested in better understanding how nuclear weapon materials, such as plutonium and uranium, behave under high pressures. Expe

8、riments with DACs provide stockpile stewardship data that complement data from shock experiments and tests driven by high explosives. All of these data improve the precision of computer codes that scientists use to model weapon performance and thus, help to ensure the safety and reliability of the n

9、ations aging nuclear weapons stockpile. In particular, experimental data are used to refine a materials pressureolume emperature relationship （its equation of state, or EOS） and the resulting structural changes （its phase diagram）. With DACs, researchers can measure material properties directly unde

10、r static pressure, and they can vary pressures and temperatures slowly over the course of many hours. Livermore scientists are using designer DACs to learn how high pressures cause materials to change their magnetic properties, switch from insulators to metals, and alter their molecular structures.

11、It is difficult to learn about electrical conductivity and magnetic properties with standard diamond anvils at high pressures, says Livermore physicist and designer anvil inventor Sam Weir. Until recently, we were limited to trying to maneuver wires into place with tweezers, but these wires deform,

12、break, and short-circuit. Our approach now is to build tiny tungsten wires inside the diamonds so they survive the high pressures. We lithographically fabricate thin-film wires on top of the anvil and then grow a layer of diamond on top of the wires to protect them.A designer diamond anvil uses a on

13、e-third-carat diamond. Tungsten metal microcircuits are fabricated on the diamond抯 300-micrometer-wide polished tip. These microcircuits are covered with a thin film of diamond and then polished to reveal the tips of the microcircuits on the top of the diamond face. Designer Diamonds Hand-Fashioned

14、Every designer diamond anvil is custom-fabricated by researchers from Livermore and the University of Alabama at Birmingham. The production team makes three types of designer diamond anvils: one for high-pressure electrical conductivity experiments, another for magnetic susceptibility experiments, a

15、nd a third for electrically heating high-pressure samples to high temperatures. Each type features a unique pattern of microcircuits, usually made of tungsten, which are fabricated on the diamond tip and then encapsulated within a diamond film. These microcircuits terminate on the diamonds sides, wh

16、ere they can be connected to instruments that collect data with high accuracy and sensitivity. Electrical conductivity experiments use four to eight tungsten wires, magnetic susceptibility experiments require a microloop of about ten turns of wire, and high-temperature experiments use eight wires. T

17、he designer diamond anvil is placed in a beryllium朿opper cell about 6 centimeters tall and 3 centimeters in diameter. The cell, in turn, is placed in a small device consisting of a gear-driven piston and cylinder mechanism that can push diamond tips together with a controlled force great enough to g

18、enerate ultrahigh pressures between the tips. Turning the knob on this mechanism pushes the designer diamond anvil （usually located on the bottom） against a stationary, standard diamond anvil, increasing the pressure and maintaining it indefinitely. Because diamonds are transparent, scientists can u

19、se DACs to make optical and x-ray measurements. Livermore researchers use a light microscope to monitor an experiment. In addition, they place a tiny chip of ruby next to the sample to measure pressure. When green or blue visible laser light shines on the ruby, the ruby emits red light at a waveleng

20、th of about 694 nanometers. As the pressure increases, the wavelength increases. For some experiments, the researchers transport the DAC to a source of very bright, highly collimated x rays, such as the National Synchrotron Light Source at Brookhaven National Laboratory in New York. The scientists p

21、ass a beam of x rays through the sample and both diamonds and record the resulting diffraction pattern on an x-ray film or detector. Changes in the diffraction pattern reveal how a materials structure responds to pressure. Each type of designer diamond anvil features a unique pattern of microcircuit

22、s that are fabricated on the diamond tip. A light microscope shows the tip for （a） an electrical conductivity experiment and （b） a magnetic susceptibility experiment. Focus on Two Element Groups Many designer DAC experiments focus on two groups of elements-the lanthanides and the actinides-which inc

23、lude the nuclear weapon metals uranium and plutonium. The experiments provide data about lanthanides and actinides that standard DAC techniques and dynamic experiments cannot supply. Most of the pressure-driven changes the researchers see can be explained by the behavior of a materials electrons. We

24、ir explains that under extreme pressures, certain electrons, which are normally tightly held within an atoms inner electron bands or shells, can move about, resulting in changes in material properties and molecular structures. In lanthanides and actinides, these electrons belong to an atoms 4f and 5

25、f bands. Most experiments dont give insight about the cause of volume changes, says Weir. Our experiments do because we can explain the changes by the delocalization of electrons from specific bands they normally occupy.Livermore scientist Chantel Aracne monitors a high-pressure experiment using a d

26、esigner diamond anvil cell. Click here for a high resolution photograph. How Insulators Become Metals Postdoctoral researcher Reed Patterson performed one of the first experiments with a designer DAC to determine why compounds such as manganese oxide （MnO） are insulators-that is, why they resist the

27、 movement of electrons. Electrical conductivity experiments, which probe materials insulating nature, can only be accomplished at ultrahigh pressures using DACs equipped with designer diamond anvils. Patterson performed several high-pressure electrical conductivity experiments on a MnO sample. The e

28、xperiments used a designer diamond anvil with eight tungsten probes measuring 10 micrometers wide and 0.5 micrometer thick. The probes were covered with diamond film and exposed only at the surface near the center of the diamond anvils culet, where they make contact with the MnO sample. Electrical c

29、onductivity was determined by passing a direct current through the wires to the sample and measuring the electrical resistance as a function of pressure. The researchers noted that the samples electrical resistance rapidly decreased by a factor of 100,000 between 85 and 106 GPa, signaling the transformation of MnO