ISSN 1063�4576, Journal of Superhard Materials, 2015, Vol. 37, No. 1, pp. 1–7. © Allerton Press, Inc., 2015.Original Russian Text © N.B. Novikov, L.K. Shvedov, Yu.N. Krivosheya, V.I. Levitas, 2015, published in Sverkhtverdye Materialy, 2015, Vol. 37, No. 1, pp. 3–12.

PRODUCTION, STRUCTURE, PROPERTIES

New Automated Shear Cell with Diamond Anvils for in situ Studies of Materials Using X�ray Diffraction

N. B. Novikova, L. K. Shvedova, *, Yu. N. Krivosheyaa, and V. I. Levitasb, **

aBakul Institute for Superhard Materials, National Academy of Sciences of Ukraine, ul. Avtozavodskaya 2, Kiev, 04074 Ukraine

bIowa State University, 2351 Howe Hall, Ames, Iowa 50011, USA

*e�mail: shvedov@kv.chereda.net

**e�mail: vlevitas@iastate.eduReceived May 7, 2014

Abstract—A new design of an automated shear cell with diamond anvils for X�ray in situ studies has been developed and manufactured. As compared with a previous design, the dimensions and weight of the new cell have been reduced almost by one half on retention of the pressure range up to 100 GPa. To demon�strate the efficiency of the apparatus, the investigations of the possibility to use polycrystalline NaCl as a sensor of pressure at diffraction studies of samples in nonhydrostatic compression conditions have been carried out.

DOI: 10.3103/S1063457615010013

Keywords: high pressures, shear deformations, phase transformations, diamond anvils, shear cell.

1. INTRODUCTION

For the last 50 years a cell with diamond anvils (DAC) is widely used to study the effect of high pressures and temperatures on structural and phase variations in materials [1, 2].

Lawson and Tang [3] were the first to use diamonds to produce high pressures for X�ray diffraction. Since then DAC is virtually the only device for creation superhigh statical pressures up to 300 GPa, which is com�parable with pressures existent in the Earth centre, in a laboratory. The main advantages of this method are the possibility to generate a superhigh pressure due to the use of diamond as the strongest one of the known mate�rials and having a wide spectrum of transparency for the radiation from X�ray to IR beams. This material has the highest thermal conductivity, wear� and chemical resistance, which may be important for studying various chemical substances. This made it possible to conduct in situ investigations of materials at high pressures and temperatures with the help of numerous experimental methods applied in combination with DAC like spec�troscopic methods using IR and Raman spectrometers; optical methods; X�ray diffraction methods with the use of X�ray diffractometers and synchronous radiation sources [4, 5]. New fundamental knowledge in phys�ics, chemistry, material science, and other sciences were gained [6–8].

The principle of the apparatus operation is very simple and does not essentially change from the time of its invention. The scheme of a diamond cell is given in Fig. 1. A sample is placed between parallel faces (culets) of a pair of high�quality diamond anvils with a brilliant faceting and of 0.14–0.5 ct in weight. The parallel faces shrink to increase the pressure in a sample. Different types of DAC were developed and are used. The main differences between them are devices of adjustment of diamond anvils and mechanisms of creating load [9]. These apparatuses allow the development of the free compression of a sample with gradient of pressure and shear strain, which appears in the displacement of the sample material from the center of the diamond anvil to its periphery or quasi�hydrostatic compression with the use of a gasket, which on the one hand is a cell to place sample and generate a required pressure in it, on the other hand the gasket supports the diamond anvil and prevents the anvil from a destruction at high pressures.

The gasket was prepared by preliminary compression of a metal foil between diamond anvils and drilling a hole in the center of the indentation made by diamonds. Usually the diameter of the hole for a sample is 60–80% of the culet size. The most widely used material for a gasket in DACs is metal rhenium. It has high yield strength and allows a generation of plastic strain in the course of a sample compression and maintain a suffi�ciently high thickness of the gasket up to high pressures. A hexagonal close packed crystalline structure of the

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material retains up to 215 GPa [10]. In recent years instead of pure rhenium a rhenium–tungsten alloy is used. This alloy has a higher strength than a pure rhenium, particularly on heating to high temperatures.

Different types of diamond–anvil cells for experiments with laser and resistive heating, cooling to low tem�peratures, studies of magnetic properties, and NMR of samples with the use of monochrome X�ray and syn�chrotron radiations have been developed [11–14]. In the majority of cases the studies are conducted at quasi�hydrostatic conditions of samples compression without shear strains.

However, numerous experiments show that plastic strains at high pressure lead to a decrease of the pressure of the onset of phase transformations; formation of new phases (materials), which is impossible to produce without a strain; replacement of the reversible phase transformation by the irreversible one; appearance of the so�called pressure selfmultiplication effect, when at the shear strain and constant pressing force the pressure in the transformation region increases despite a decrease of this region because of the occurred phase trans�formation; formation of amorphous and nanostructural phases of materials at different methods of plastic deformation, for example, like rotating under pressure, equal channel angular compression or processing in ball mills [15–18].

As early as in the middle of the past century Bridgman started the full�scale investigations of this phenom�enon [19]. He was the first to develop a cell with anvils of a hardened steel or tungsten carbide with flat working surfaces when the rotation of one movable anvil about the other immovable one generate additional shear strains in a sample due to an intensive plastic deformation. This cell was given the title Bridgman anvils. How�ever, he did not used diamond as the anvil. That’s why the pressure in his cell did not exceed 12 GPa and the resulting samples should be studied only after recovery them from the cell.

Only in the mid�1980s Blank with co�workers improved the Bridgman anvils using diamond anvils instead of hard�alloy ones. The researchers developed and fabricated shear diamond anvil cell (SDAC) [20]. This made it possible to study in situ materials at the joint action of static pressure and shear strains using numerous experimental methods together with DAC.

In the early 2000s we have further improved the SDAC [21]. We were the first to apply the automation of measurements in the SDAC and determination of parameters like the sample thickness, load, and rotation angle of the movable diamond anvil in the course of the studies. In 2006 we obtained the patent for a new design of the SDAC [22].

A change of the pressure in a sample under study, which is between the diamond anvils, as a rule, is made manually by screwing the forcing screw through a lever using flat springs [14]. In this case the position of the cell relative to optical axis of devices, with which it is conjugated, e.g., a diffractometer or a source of the syn�chrotron radiation, is disturbed. It is necessary to overlap the X�ray radiation and additionally adjust the cell position. This operation should be performed at each increase of the pressure in a sample, which results in a considerable increase of the time of a measurement for the given pressure range and in a decrease of the mea�surements number. In the use of the automation of the loading process and generation of the shear strain the

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Fig. 1. Scheme of the diamond cell: 1—diamond anvil, 2—culet, 3—gasket, 4—sample.

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measurements may be made at a distantce at a small step according to the computer commands without a dis�placement of the cell about the optical axis. This will make it possible to considerably increase a self�descrip�tiveness of the measurements, decrease their time, and with the higher accuracy to determine the pressure of the onset of structural and phase variations in a sample. Later a simpler automated DAC without the possibility of the generation of a shear strain was also manufactured in the USA [23].

2. SUBSTANTIATION OF THE NECESSITY FOR THE DEVELOPMENT OF A NEW SDAC

The old design of the SDAC allows us to load a sample under study by a force of 1 ton and generate in it static pressure up to 100 GPa and shear strains due to a turn of the movable diamond anvil about the axis of the loading through an angle of 360° and more. The apparatus operates in an automated mode according to the computer commands using two step motors. In the course of the research the load force, thickness of the sample, and the turning angle of the movable piston with diamond anvils are measured. The design and oper�ation principle of the SDAC are described in greater detail in [13, 21, 22].

However, its design developed by us more than 10 years ago has the following essential disadvantages: – relatively large dimensions and weight, respectively 120 × 150 × 60 mm and 4.5 kg. Such dimensions and

weight present certain difficulties in conducting experiments, for example, with a Raman spectrometer or a source of the synchrotron radiation;

– to control the SDAC operation two step engines are used: one for turning a movable piston with a dia�mond anvil and generating shear strains, the other for producing a loading force up to 1 ton with the help of a hydraulic booster. It requires a rather often technical service for replacement of ring gaskets and hydraulic liq�uid;

– for an electric contact with the electronic block of foil�clad textolite plates of the capacitance sensor of the thickness and loading sensor, which are on the movable piston a ring current collector is used, which offers a possibility to turn a movable piston through 360° and more. Such a design decreases the reliability of electric contact of these sensors with an electronic block and considerably increases dimensions of the movable piston;

– the electronic block of control is of rather large dimensions 260 × 230 × 70 mm and communicates with a computer via an additional interface card, which is installed inside of a stationary computer, which is incon�venient in transportation;

– the computer control program was developed using the Paskal language as a file with an .exe extension. To make changes in the program, if necessary, may only the professional programmer, who developed this pro�gram.

3. FEATURES OF THE NEW SDAC DESIGN

The scheme and appearance of the new SDAC are shown in Figs. 2, 3 and SDAC with a loading device is given in Figs. 4, 5. The new SDAC has the following advantages as compared with its previous design:

1. Dimensions and weight are almost halved on retention of the possibility of a pressure generation in the sample under study to 100 GPa. The SDAC dimensions are 85 × 60 × 60 mm, its weight is 700 g.

2. To control the operation of a new SDAC, two step engines are also used. One is the force engine with a moment on the shaft of about 6 kgcm and the other is a small�size control one with a moment on the shaft of about 0.4 kgm. It commutates the torque strength of a force step engine to generate a loading force or turn the movable piston. A reducer of a mechanical type with a worm�and�worm pair without a hydraulic booster is used. This considerably improves operating characteristics of the apparatus on retention a compression force of about 1 ton. At the loading the force engine rotates, through a reducer, a force screw, which compresses 6 pairs of disk springs. Through a class two lever the force from the disk springs is transmitted to the movable piston to compress diamond anvils. In generation of shear strains the torsional moment of the force step engine is transmitted after reducer via a gear pair on a gear wheel, which is rigidly fixed on a movable piston causing it to turn through a given angle.

3. The design of the capacitance sensor of the thickness was considerably simplified. It consists of one plate of a foil�clad textolite, which is fixed on a plate of fastening the unmovable diamond anvil and connects with an electronic block and computer, the second plate of the sensor is a plate of fastening the diamond anvil on a movable piston. As a sensor of the loading force four tensometric sensors fixed on 2 class two levers that trans�mit the force to the movable piston. These levers deform proportionally to the loading force, which is regis�tered by these sensors, connected to the electronic block and computer that calculates the load value by a cer�tain algorithm.

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Fig. 2. Scheme of a SDAC: movable piston (1), case (2), bottom (3), screw (4), hemispheric upper support (5), tooth gear (6), limb (7), flange (8), upper support (9), bottom support (10), hemispheric bottom support (11), hemispheric support (12), textolite plate (13), screw (14), steel plate (15), screws (16–20), movable diamond anvil (21), unmovable diamond anvil (22).

Fig. 3. Appearance of a SDAC.

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Fig. 4. Scheme of a SDAC with the loading device: SDAC (1), levers (2), disk springs (3), axis (4), switching device (5), case (6), reducer (7).

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Such a design made it possible to avoid the use of cables on the movable piston and remove the ring current collector, which considerably simplified the cell design and reduced the SDAC dimensions.

4. Dimensions and weight of the control electronic device are considerably reduced. In a new form it con�sists of two small electronic blocks located directly on the case of the apparatus. It is connected to computer via the USB connector without the use of the additional conjugation plates. Therefore, one may use any laptop with a program to control the SDAC operation.

5. The computer program of control is developed in the Lab View medium, which allows, if necessary, the executer to make changes in the program operation without calling professional programmers.

To demonstrate the efficiency of our SDAC, we studied in situ the possibility to use polycrystalline NaCl as a pressure sensor at the diffraction investigations of a nonhydrostatically compressed sample in the SDAC.

In parallel with platinum and gold NaCl is widely used as a pressure sensor during the studies in diamond anvils at hydrostatic compression [24]. This is explained by its simple cubic crystalline lattice and the absence of the phase transitions up to 30 GPa. The pressure is defined by special tables from the relative deformation value of the volume of NaCl crystalline lattice depending on the pressure. The tables are theoretically calcu�lated for various temperatures.

However, at nonhydrostatic compression the crystalline lattice deforms along the axes differently and this causes great errors in the pressure determination by such a method. In addition to define the volume defor�mation, it is necessary to define shears of diffraction lines corresponding to various diffraction planes, responses of which on a diffractometer are very weak in some cases of the in situ measurements in SDAC.

We used fine�grained chemically pure NaCl, which was preliminary tempered at a temperature of about 400°C in a muffle furnace for a half an hour to remove moisture. The resultant powder was placed into a hole about 0.5 mm in diameter of a plastic gasket about 250 μm in thickness and located between diamond anvils with work platforms about 0.6 mm in diameters. The pressure in SDAC was defined in situ on a device for mea�suring pressure on a ruby scale, whose procedure is described in detail in [13]. To assess the pressure in the SDAC in situ by this procedure in the diffraction studies of different materials, a fine�grained NaCl powder is added to a studied substance.

By numerous experiments we revealed almost linear pressure dependence of the shear of the highest power diffraction line (200) of NaCl, which is shown in Fig. 6a. The line is well approximated by the 2 degree poly�nomial Y = 740.29 + 115.66X + 4.49X2 . A fragment of smoothed and normalized diffraction patterns of the (200) line of NaCl taken in situ on a DRON�4 automated X�ray diffractometer in SDAC is given in Fig. 6b. The diffractometer operates by the Dobrovol’skii–Schvedov scheme [25].

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Fig. 5. Appearance of a SDAC with the loading device: force step engine (1), reducer (2), electronic block of the device for measuring thickness (3), electronic block of the control device (4), controlling step engine (5), SDAC (6), case (7), sensor of a rotation angle (8).

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4. CONCLUSIONS

1. A new small�sized SDAC operating in an automatic mode has been developed and manufactured. It allows us to investigate different materials at a static pressure up to 100 GPa and shear strains that are generated by the rotation of a movable piston with a diamond anvil about the axis of loading by 360° and more.

2. In the course of the investigations SDAC provides a possibility to change the loading and shear by distant computer commands and measure the thickness of the studied sample, loading force, and rotation angle of the movable piston.

3. Dimensions and weight of the new SDAC have been almost halved as compared with the SDAC of the previous design on retention of the pressure range to 100 GPa.

4. Dimensions and weight of the electronic control block have been reduced. Its connection with the com�puter is through an USB port without the use of additional interface cards.

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pressure cell, Rev. Sci. Instrum., 1985, vol. 56, issue 7, pp. 1420–1427.2. Ma, Y., Mao, H.K., Hemley, R.J., et al., Two�dimensional energy dispersive X�ray diffraction at high pressures and

temperatures, Ibid., 2001, vol. 72, issue 2, pp. 1302–1305.

13.8 14.0 14.2 14.4 14.6 0

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e, G

Pa

2θ, deg (a)

13.0 13.5 14.0 14.5 15.0 15.5 0

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1

2θ, deg

Inte

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ty,

rel.u

. 6

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Fig. 6. Pressure dependence of a shear of the (200) line of NaCl (a) and a fragment of the diffraction patterns of the shear of the (200) line at different pressures and thicknesses of samples (b): 5 GPa, 90 μm (1); 5.6 GPa, 52 μm (2); 6.9 GPa, 21 μm (3); 8 GPa, 11 μm (4); 8.9 GPa, 6 μm (5), 11.3 GPa, 1 μm (6); unloading (7).

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3. Lawson, A.W. and Tang, T.Y., A diamond bomb for obtaining powder pictures at high pressures, Ibid., 1950, vol. 21, p. 815.

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Translated by G. Kostenchuk

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