压缩科学Putting the squeeze on materials.docx

1、压缩科学Putting the squeeze on materials压缩科学：Putting the squeeze on materialsFew gemstones are as mesmerizing as diamonds. Livermore physicists also find diamonds attractive but for reasons other than their beauty. The researchers use flawless, polished diamonds in opposing pairs, or anvils, to slowly c

2、ompress samples of materials at extreme pressures. This device, called a diamond anvil cell (DAC), forces materials to reveal new information about how their structure and electrical and magnetic properties change-sometimes drastically-in response to increasing pressure.A DAC is a small mechanical p

3、ress that forces together the small, flat tips (called culets) of two brilliant-cut diamond anvils. The diamond tips press on a microgram sample of a material, held within a metal gasket, to create extremely high pressures. Diamonds are used because they are the hardest known solid and so can withst

4、and ultrahigh pressures. They also permit diagnostic radiation, such as x rays and visible light, to pass unhampered through their crystalline structure. However, DAC studies of such properties as electrical conductivity and magnetic susceptibility are extremely difficult to perform. The 1-microgram

5、 samples have a diameter of about 75 micrometers, and diagnostic instruments cannot be placed close enough to them to make the required measurements. Problems especially arise when researchers try to obtain information about materials at static pressures above 1 million atmospheres, or 100 gigapasca

6、ls (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 million atmospheres.) To overcome the problems posed by standard diamond anvils, Livermore researchers have taken advantage of recent improvements in di

7、amond synthesis technology to fabricate microcircuits within the diamond anvils themselves. The tungsten microcircuits serve as tiny diagnostic instruments that measure data about materials fundamental physical and mechanical properties under high pressures. The researchers call this modified tool a

8、 designer diamond anvil because the microcircuits can be altered to suit the needs of the experimenter. Scanning electron micrographs show a completed designer diamond anvil for measuring electrical conductivity. (a) Tungsten microcircuits lead from the sides of the diamond, where they form electric

9、al contact pads with instruments, to the tip of the diamond face, or culet, where they monitor various properties of the sample. (b, c) Progressive magnification of the diamond tip with a light microscope shows the termination of the tungsten wires. Click for a high resolution photograph. Pressuring

10、 Materials to Change Materials behave quite differently under extreme pressures than they do at normal atmospheric pressure. Oxygen, for example, becomes a shiny metal under ultrahigh pressure. In support of the National Nuclear Security Administrations Stockpile Stewardship Program, Livermore resea

11、rchers are particularly interested in better understanding how nuclear weapon materials, such as plutonium and uranium, behave under high pressures. Experiments with DACs provide stockpile stewardship data that complement data from shock experiments and tests driven by high explosives. All of these

12、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 nations aging nuclear weapons stockpile. In particular, experimental data are used to refine a materials pressureolume emperature relationship (its

13、equation of state, or EOS) and the resulting structural changes (its phase diagram). With DACs, researchers can measure material properties directly under static pressure, and they can vary pressures and temperatures slowly over the course of many hours. Livermore scientists are using designer DACs

14、to learn how high pressures cause materials to change their magnetic properties, switch from insulators to metals, and alter their molecular structures. It is difficult to learn about electrical conductivity and magnetic properties with standard diamond anvils at high pressures, says Livermore physi

15、cist and designer anvil inventor Sam Weir. Until recently, we were limited to trying to maneuver wires into place with tweezers, but these wires deform, break, and short-circuit. Our approach now is to build tiny tungsten wires inside the diamonds so they survive the high pressures. We lithographica

16、lly 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 one-third-carat diamond. Tungsten metal microcircuits are fabricated on the diamond抯 300-micrometer-wide polished tip. These microcircuits are cover

17、ed with a thin film of diamond and then polished to reveal the tips of the microcircuits on the top of the diamond face. Click for a high resolution photograph. Designer Diamonds Hand-Fashioned Every designer diamond anvil is custom-fabricated by researchers from Livermore and the University of Alab

18、ama at Birmingham. The production team makes three types of designer diamond anvils: one for high-pressure electrical conductivity experiments, another for magnetic susceptibility experiments, and a third for electrically heating high-pressure samples to high temperatures. Each type features a uniqu

19、e 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, where they can be connected to instruments that collect data with high accuracy and sensitivity. Electrical

20、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. The designer diamond anvil is placed in a beryllium朿opper cell about 6 centimeters tall and 3 centimeters i

21、n 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 generate ultrahigh pressures between the tips. Turning the knob on this mechanism pushes the designer diamo

22、nd 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 use DACs to make optical and x-ray measurements. Livermore researchers use a light microscope to monitor an

23、 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 wavelength of about 694 nanometers. As the pressure increases, the wavelength increases. For some experiments, the

24、 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 pass a beam of x rays through the sample and both diamonds and record the resulting diffraction pattern on

25、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 microcircuits that are fabricated on the diamond tip. A light microscope shows the tip for (a) an electrical conductiv

26、ity experiment and (b) a magnetic susceptibility experiment. Click for a high resolution photograph. Focus on Two Element Groups Many designer DAC experiments focus on two groups of elements-the lanthanides and the actinides-which include the nuclear weapon metals uranium and plutonium. The experime

27、nts 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. Weir explains that under extreme pressures, certain electrons, which

28、 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 5f bands. Most experiments dont give insight about the cause of vol

29、ume 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 designer diamond anvil cell. Click here for a high resolution phot

30、ograph. 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 movement of electrons. Electrical conductivity experiments, whic

31、h 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 experiments used a designer diamond anvil with eight tungsten prob

32、es 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 conductivity 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