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C: Physical Processes in Nanomaterials and Nanostructures

A Shear Driven Chemical Decomposition of Boron Carbide Yang Gao, and Yanzhang Ma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b03599 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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The Journal of Physical Chemistry

A Shear Driven Chemical Decomposition of Boron Carbide Yang Gao and Yanzhang Ma* Department of Mechanical Engineering, Texas Tech University, 2500 Broadway St. Lubbock, Texas 79409 Abstract We report for the first time the observation of a shear induced decomposition of boron carbide into B50C2 and nano-crystalline graphite at pressures from 1.0 to 3.5 GPa. It is proved that shear under modest compression provides finer controllability and more effective initiation capability than either compression alone or compression under high temperature. Most importantly, shear, as a driving force, is proved capable of overriding materials’ energy surfaces and thus realizing new chemical reactions as well as structural transformations that have not been discovered. Consequently, shear is of great significance shedding light on both new technologies for material synthesis and advance in material sciences.

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Most solid materials have sophisticated energy surface, manifesting the stable state and metastable states

1-5.

While intrinsically a stable state is to be reached because of its energy favorability

especially in cases such as energy overflows to pass all the intermediate phases. An intermediate metastable state may nevertheless be attained mostly due to a transformation pathway where transformation to a stable phase is inhibited/prohibited even when sufficient or overwhelming driving force is provided. Even decomposition may occur in a solid to reconcile exotic conditions. Pressure, temperature and their combination have been known for its capability of altering the energy surface and establish energy to overcome energy barrier, thus has been the regulatory factors, in a transformation or reaction 6-10. Under hydrostatic compression, the atoms, molecules and their spatial distribution are commonly homogenously deformed (compressional straining) that alter the energy surface and formulate a transformation routine. Under a uniaxial compression along with an extensive torsion (shear), these particles and structures are subjected to an exotic geometrical distortion in addition to compressional straining, causing much sophisticated manipulation of energy surface along with alternative course of material transformation 11-14. Here, we report a shear-induced chemical reaction (decomposition) of B4C, a new route of transformation driven by shear other than that under quasi-hydrostatic compression15-16. By application of large plastic shear initiated at pressures as low as 1.0 GPa, B4C decomposed to the boron-very-rich compound B50C2 and nano-crystalline graphite. The result provides evidence that shear at pressure drives atoms in the molecular crystal system along the energy surface and is capable of detaining of metastable phases as well as override the compressional straining defined energy surface. The discovery brings about new chemistry of materials at shear and also brings new thoughts in both synthesis of novel materials and comprehension of the mechanism of shear effects on materials. Boron-rich materials have exceptional properties such as extreme hardness, low density, high mechanical, chemical and thermal stability, and low wear coefficient

17-18.

Boron carbide (B4C),

as a typical boron-rich material, shows, besides easy synthesis, both extreme hardness and low density 19-22. The crystal structure of B4C is a rhombohedron with a space group of 𝑅3𝑚. Eight 12boron-atom icosahedra are allocated at the vertex while a three-carbon-atom chain is allocated on the diagonal [111] direction of the rhombohedron between the interstices of the icosahedra 23. Such structure, along with the strong atomic bonding, makes B4C the third hardest material after 2


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diamond and cubic boron nitride. What more interesting is that B4C suffers a sudden reduction of its shear strength at pressure elevation to above 23 GPa 24-25, which in turn significantly limited its application potentials 26-27. Recent research suggests that this softening phenomenon is associated with pressure-induced amorphization, whereas the non-hydrostatic stress is believed to have played a critical role in the localized amorphization at high-rate impacts

28.

Consequential

investigation had been performed both theoretically 29 and experimentally 30-31. Materials and methods A rotational anvil cell with diamond composite anvils 32 was utilized in the experiments to generate combination of uniaxial compression and shear on materials. A cubic boron nitride (c-BN) sheet of around 100 μm in thickness was introduced as gasket. A 400–μm hole was drilled at the center of the gasket as the sample chamber. The sample (B4C powder from Alfa Aesar Co.) was then loaded in the chamber without pressure-transmitting media. A small segment of gold wire of diameter of 50 μm was positioned in the center of the sample chamber as pressure calibrate 33. In the experiment, the sample was first compressed to 0.2 GPa under uniaxial loading before successive anvil rotation (termed shear loading). In-situ X-ray diffraction (XRD), ex-situ Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) measurements were performed to probe and characterize the sample. In-situ XRD measurements were performed along the radial direction of the sample after each uniaxial loading, shear loading and quenching operations using a 0.4849 Å monochromatic X-ray beam at B1 station, Cornell High Energy Synchrotron Source (CHESS), Cornell University. Raman spectroscopy measurements was conducted using a homemade Raman system with a 532-nm NdYAG laser at 300 mW for excitation and recorded by a Princeton Instruments Acton SP2500 spectrometer accompanied with a Pixis100 CCD detector. The XPS measurement was performed using a Thermo Scientific ESCALAB 250 High Performance Imaging XPS apparatus with monochromatic Al Kα X-ray (ℎ𝑣 = 1486.6 𝑒𝑉), and the spectrum was acquired at a nominal photoelectron take-off angle of 55°. The morphology and energy dispersive X-ray spectroscopy (EDS) of the sample were measured using a Hitachi H-8100 IV TEM operating at 200 kV accelerating voltage.

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Results and Discussion

Fig. 1. Diffraction patterns of the starting material, the material under selected conditions, and the quenched material. The vertical bar, diamond, and cross represent the diffraction peaks originating from B4C, B50C2, and anvil/gasket/pressure-calibration materials, respectively. The numbers above the patterns represent the loading and rotation angles in degrees, as well as twodigit pressure in GPa calculated using gold’s equation of state, respectively. In the experiments, we measured X-ray diffraction in-situ after each compression and shear operation. In the pattern taken after anvil rotation of 120° (Fig. 1), (1.0 GPa), new diffraction spots were observed emerging and after which intensified as the diffractions from the initial phases started diminishing, marking the initiation of a transformation of the sample to another crystalline phase. After anvil rotation of 660°, (3.5 GPa), the axial load was completely removed, and the sample was screened. The diffraction pattern was discovered to be from a new phase in addition 4


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to phases from the residual B4C, the gasket material cBN, and materials that composed the rotation anvils. The d-spacings of the new phase can be indexed to the tetragonal B50C2 phase (Table I), except two at 6.373 and 3.906 Å, which may originate from the synchrotron diffraction system. The refined cell parameters are a = 8.717 Å and c = 5.076 Å, which are consistent with those from literature within the experimental error range 34-35. Table I. Index of experimental d-spacings with comparison to literature. dexp/Å 6.373 6.184 4.39 T 3.94 T 3.906 2.763/2.77 T 2.573 2.51 T 2.46 T 2.256 2.195 2.184 2.127 1.952

dref/Å 34, 36

Index

ddiff/ Å*

6.189 4.402 3.932

(1 1 0) (1 0 1) (1 1 1)

-0.005 0.012 0.012

2.768 2.570 b 2.532 2.432 2.251 b 2.192 2.188 2.135 1.957

(3 1 0) (1 0 4) b (3 0 1) (3 1 1) (2 0 2) b (2 0 2) (3 2 1) (2 1 2) (4 2 0)

-0.005/0.002 0.003 -0.022 -0.012 0.005 0.003 -0.004 -0.008 -0.005

Refined cell parameters for B50C2, 𝑎 = 8.717 Å, 𝑐 = 5.076 Å. T

d-spacings observed in TEM measurements.

b

d-spacings also from starting material, B4C.

*

𝑑𝑑𝑖𝑓𝑓 = 𝑑𝑒𝑥𝑝 ― 𝑑𝑟𝑒𝑓

In the TEM screening of the quenched sample, the interplanar distances determined in the various grains, e.g. 0.439, 0.394, 0.277, and 0.246 nm, can be indexed into the planes of B50C2, e.g., the (101), (111), (310), and (311) (Fig. 2 (a), Table I), and are consistent with XRD observations. In addition, compared to the large grain in the B4C sample before processing (Fig. 2 a), the quenched sample shows grain sizes being reduced to the order of 10 nm, which is also consistent with the 5



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observed size reduction in XRD observations. Hence, we determine that B4C transforms to B50C2 after the compression and shearing process.

(a)

(b)

(c)

(d)

Fig. 2: HRTEM image of (a) original and (b) quenched sample. (c) Some of the selected positions, a, b, and c (marked by circles), where EDS was measured and (d) the B:C ratio at corresponding positions. In the original B4C sample, the B to C ratio measured by EDS is consistently 4.0. However, the ratio in the quenched sample (Fig. 2d) ranges from as low as 1.0 to as high as 9.8. This discrete B:C ratio suggests that the transformation of B4C to B50C2 is a decomposition mechanism during the compression and shearing processing, and the products of the decomposition have formed alternative distributed domains in the sample. Considering that the B:C ratio in a B50C2 crystal is 25, we believe that regions that have higher B:C ratio (>4) are the regions dominated by B50C2 6


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nano-crystals that overlap with minimal substance of large carbon concentration, and those with lower ratio are dominated by a substance of high carbon concentration that overlaps with minimal B50C2 nano-crystals. The B50C2 crystal and the substance of high carbon concentration should be separated from the B4C crystal during its decomposition. The composition change from B4C to B50C2 requires part of the carbon atoms to disintegrate from the initial B4C structural framework during the decomposition process. It is plausible that the other product of the decomposition is carbon. The assumption is evidenced by the Raman spectrum from the quenched sample (Fig. 3), where a number of typical B4C Raman peaks and broad bands below 1200 cm-1 and two extra broad bands centered at 1346 and 1605 cm-1 are observed. The former indicates that the majority of the framework in B4C crystal is retained in the transformation, which is consistent with the XRD observation that most of the icosahedral structure and their bonding over each other remains after the transformation. While the latter demonstrates clear dislocation from the typical amorphous B4C bands at 1325 and 1520 cm-1 37. Combined with the lacking band at ~1800 cm-1, we thus determined that these extra bands are not from amorphous B4C. Instead, due to their comparable positions to the D- and G- bands of disordered graphite, we attribute them to the formation of carbon clusters during the transformation. Correlating the structure characteristics of carbon clusters with the intensity ratio of the D- and G-bands (ID/IG, equals 1 in this experiment) and the band width of G-band (FWHM 70 cm-1 in this experiment) 38, the size of the clusters is determined to be around 4 nm. The cluster size, peak positions, and all the scattering features of the extra bands are closely comparable to those of graphite nanocrystals

38-39.

The

minimal drift of the G-band (from 1600 to 1605 cm-1) 38 is attributed to the interaction of carbon atoms with the neighboring boron atoms due to the history of its formation (also see bonding discussion below). Hence, the disintegrated carbon atoms are in the form of nanocrystalline graphite (NC-graphite). Based on the composition distribution from EDS measurement, we expect that the graphite nanocrystals are uniformly distributed in, and thus well separated by, the crystals of B50C2. The low concentration, good isolation, and weak scattering coefficient made the diffraction below the detection limit, which explains why we did not observe the graphitic phase in our XRD measurements.

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Fig. 3: Raman spectra of the original B4C and quenched material. The B (1s) and C (1s) X-ray photoelectron spectroscopies of B4C and sheared sample were shown in Fig. 4 and Table II to investigate the bonding status of boron and carbon atoms. The B (1s) spectrum of the original B4C consists of an intense peak at 187.28 eV and a weak peak at 188.99 eV that respectively correspond to the B-B and B-C bonds in B4C

40-43.

Yet that of the sheared

sample shows a much broader band consisting of three components, 187.83, 189.72, and 191.60 eV, corresponding to the B-B, B-C, and B-O bonds, respectively. Analysis shows that the original B-B and B-C bond peaks become broader and shift to higher energy after shearing. The broadening and the shift mainly reflect the disordering of atoms in crystals after shearing, or the so-called residual strain build up by compression and shear, particularly for the B-B bond. Furthermore, the peak of the B-C bond shows so much width expansion that we can consider it the combination of broadening of two comparable broadened peaks. That is, there are two different types of bonds formed in the solid, one of which is the B-C bond formed within the B50C2 crystalline, and the other is the bond between the B50C2 and the NC-graphite. There is an observable intensity increase of the peak of the B-C bond, which indicates that the overall interaction between boron and carbon 8


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in the so-formed solid has been enhanced even though the carbon atoms from the B4C crystals have disintegrated during the decomposition process. Since large numbers of carbon atoms are in the isolated graphite, this in turn reflects the intensive interaction between the graphite domain and B50C2 crystal. The minimal sign of a peak at 191.60 eV might be due to the interaction of boron with the oxygen atoms absorbed in the sample during shear processing.

Fig. 4. The XPS spectra of original and quenched samples. Squares, data points; black line, fitted curve; blue line, fitted peak. The C (1s) XPS spectrum of the original material consists of two intense peaks at 284.67 and 281.59 eV and two weak peaks at 286.84 and 288.48 eV, which respectively correspond to 𝐶 ― 𝐶, 𝐵 ― 𝐶, 𝐶 ― 𝑂, and 𝐻𝑂 ― 𝐵 = 𝑂 bonds 40-43. In the C (1s) spectrum of the sheared sample, the peak of C-C bond is located at 284.74 eV, showing a very limited shift from the original (284.67 eV). Thus, in the disintegrated carbon cluster, carbon remains in an sp2 hybridization states. Thus, the so-formed carbon domain would be graphite-like, which is consistent with other observations. The peak is slightly broadened, reflecting a small degree of residual strain established, which suggests that the carbon domain is a part that is softer than the other part of the solid. The peak of B-C bond is located at 282.13 eV. It shifts 0.54 eV higher in energy compared to that in B4C. This again is due to the residual strain established in the processing. The intensity of this peak is weak but comparable to that of B4C, indicating that the quantity of these bonds does not change much, if any, during the decomposition. The peak’s width also does not seem to change as significantly as 9



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it does in the B (1s) spectrum, indicating minimal B-C bond disordering due to the decomposition. Over all, the XPS results are consistent with the XRD, TEM, and Raman spectroscopy results that the quenched sample consists of B50C2 and NC-graphite. Table II. XPS analysis of sheared B4C 𝑩―𝑩

𝑩―𝑪

𝑩―𝑶

B.E./eV

Norm. Int.

B.E./eV

Rel. Int.

B.E./eV

Rel. Int.

B4C

187.28

100

188.99

18.6

-

-

Processed

187.83

100

189.72

39.0

191.60

19.1

𝑪―𝑪

𝑩―𝑪

𝑪―𝑶

𝑯𝑶 ― 𝑪 = 𝑶

B.E./eV

Norm. Int.

B.E.

Rel. Int./%

B.E./eV

Rel. Int./%

B.E./eV

Rel. Int./%

B4C

284.67

100

281.59

13.2

286.84

3.1

288.48

4.3

Processed

284.74

100

282.13

13.5

286.68

25.5

-

-

The B50C2 crystal, a so-called B48B2C2 structure (Fig. 4b), is a high temperature phase of boron carbide. It has a tetragonal system of space group 𝑃42/𝑛𝑛𝑚. The four 12-element boron icosahedra are located in the 4(e) position and the two single boron atoms statistically occupy the 8(i) and 8(h) positions. The two single carbon atoms occupy the 2(b) position. There are 6 atoms in an icosahedron that form bonds with neighboring icosahedra, 2 to carbon atoms, and 4 to single boron atoms 44. In the B4C crystal (Fig. 4a), on the other hand, the icosahedra are at the vertex of a rhombohedral lattice (space group 𝑅3𝑚) and a 3-carbon-atom chain along the three-fold axis. Each of the boron atoms is on an icosahedron, and an icosahedron forms bonds with 6 boron atoms on neighboring icosahedra and 6 carbon atoms 34. There are two main differences between the B4C and B50C2 crystalline. One is that all boron atoms in B4C belong to an icosahedron, while some of the boron atoms B50C2 are in single atom form. The other is that carbon is chained in B4C and isolated in B50C2. Hence, part of the boron icosahedron, along with some of the carbon chain, breaks apart to form single atoms; these atoms redistribute, as single isolated atoms, in the void positions of the restructured icosahedra framework introduced by shear processing. Such cluster (boron icosahedron and carbon chain) destruction and redistribution reflects a dynamical preferred compensation of the molecular and structural instability resulting from non-homogeneous stress 10


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and strain. In the other words it is favorable in terms of stability for isolated single atoms, rather than a group of atoms, to occupy the void position and form a spatial framework under nonuniform external stress attack.

Fig. 5: The crystalline structure of (a) B4C and (b) B50C2. The red and green spheres represent respectively carbon and boron atoms. The bi-color spheres represent the atomic positions of minimal SOF. The high symmetrical boron cluster of 12-element icosahedron has been proved to be a stable structure in boron and boron rich compound 45-47. Compression of these materials turned out only to be able to affect the other structures made of single elements or other clusters. The decomposition of B4C to B50C2 and graphite involves break down of boron icosahedra. It indicates 11



that application of shear provide a greater impact on structure framework and thus significantly altered the energy surface of a material. It also indicates that application of shear is capable of forcing the displacement of molecule as a whole as well as within the relative displacement between small scale particles, molecules and atoms. Such an effect has been considered thermal equivalent 13, 48. What’s more than the so called thermal effect are its exceptional characteristics. One is that the shear produced displacement is one dimensional, meaning that the motion of atom and molecule is only along the geometrical direction that shear applied, which lead to the distortion of atom, molecule and microstructures of materials. The second is that the controllability of the displacement; displacement starts when shear is applied, it retains with no further expansion due to the surrounding pressure when shear stops. The third is that the existence of surrounding pressure in some degree inhibit the overflow phenomenon in thermal induced displacement, making it possible to access as many metastable state as the minimal degree of shear increment allows. It thus makes it possible for the molecular system as well as its imbedded spatial structure to surf the entire energy surface of the molecular system, and makes possible the production of as many metastable phases as the energy surface defines. Above all, our results show that shear at pressure may lead to break down of molecular and crystalline structure of materials and to new energy surfaces. Conclusion B4C decomposes into B50C2 and NC-graphite under large plastic shear with modest compression. The pressure range for this decomposition is determined to be from 1.0 to 3.5 GPa according to the XRD experiments. It has proved that shear can not only serve as an effective driving force for chemical reactions as well as structural transformation, but most importantly distort the energy surface and consequently brings new chemical reaction and transformation. Based on present result, we may highly anticipate further exploration of new reactions that might fundamentally inhibited without shear and discover their significance beyond.

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Acknowledgement This work is supported by National Science Foundation (Grant No. DMR1431570, program manager, John Schlueter). Synchrotron X-ray experiment was performed at Cornell High Energy Synchrotron Source. The authors thank Dr. Zhongwu Wang for experimental technical support.

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19. Lipp, A., Boron Carbide: Production, Properties, and Applications; Battelle Northwest Laboratories, 1970. 20. Thévenot, F., Boron Carbide—a Comprehensive Review. J. Eur. Ceram. Soc. 1990, 6, 205225. 21. Kisly, S.; Kuzenkova, M.; Bodnaruk, N.; Grabchuk, B., Karbid Bora (Boron Carbide). Kiev: Naukova Dumka 1988, 189. 22. Suri, A.; Subramanian, C.; Sonber, J.; Murthy, T. C., Synthesis and Consolidation of Boron Carbide: A Review. Int. Mater. Rev. 2010, 55, 4-40. 23. Morosin, B.; Mullendore, A.; Emin, D.; Slack, G. In Rhombohedral Crystal Structure of Compounds Containing Boron‐Rich Icosahedra, AIP Conference Proceedings, AIP: 1986; pp 7086. 24. Grady, D., Dynamic Properties of Ceramic Materials: Sandia National Laboratories Report, Sand 94-3266. Sendia National Laboratories, Albuquerque, NM 1995. 25. Dandekar, D. P. Shock Response of Boron Carbide; ARMY RESEARCH LAB ABERDEEN PROVING GROUND MD: 2001. 26. Gooch, W.; Burkins, M., Paper Presented at the 13th Annual Ground Vehicle Survivability Symposium. Monterey, CA 2002, 8. 27. Orphal, D.; Franzen, R.; Charters, A.; Menna, T.; Piekutowski, A., Penetration of Confined Boron Carbide Targets by Tungsten Long Rods at Impact Velocities from 1.5 to 5.0 Km/S. Int. J. Impact Eng. 1997, 19, 15-29. 28. Chen, M.; McCauley, J. W.; Hemker, K. J., Shock-Induced Localized Amorphization in Boron Carbide. Science 2003, 299, 1563-1566. 29. An, Q.; Goddard III, W. A.; Cheng, T., Atomistic Explanation of Shear-Induced Amorphous Band Formation in Boron Carbide. Phys. Rev. Lett. 2014, 113, 095501. 30. Domnich, V.; Gogotsi, Y.; Trenary, M.; Tanaka, T., Nanoindentation and Raman Spectroscopy Studies of Boron Carbide Single Crystals. Appl. Phys. Lett. 2002, 81, 3783-3785. 31. Somayazulu, M.; Akella, J.; Weir, S.; Hauserman, D.; Shen, G., X-Ray Diffraction Measurements on B4c Boron Carbide at High Pressures and High Temperatures. Advanced Photon Source Activity Reports 2001. 32. Hall, H., Method of Making a Unitary Polycrystalline Diamond Composite and Diamond Composite Produced Thereby. Google Patents: 1974. 33. Anderson, O. L.; Isaak, D. G.; Yamamoto, S., Anharmonicity and the Equation of State for Gold. J. Appl. Phys. 1989, 65, 1534-1543. 34. Clark, H.; Hoard, J., The Crystal Structure of Boron Carbide. J. Am. Chem. Soc. 1943, 65, 2115-2119. 35. Hoard, J.-L.; Hughes, R.; Sands, D., The Structure of Tetragonal Boron1. J. Am. Chem. Soc. 1958, 80, 4507-4515. 36. Will, G.; Kossobutzki, K., X-Ray Diffraction Analysis of B50c2 and B50n2 Crystal-Lizing in the “Tetragonal” Boron Lattice. J. Less-Common Met. 1976, 47, 33-38. 37. Proctor, J.; Bhakhri, V.; Hao, R.; Prior, T.; Scheler, T.; Gregoryanz, E.; Chhowalla, M.; Giulani, F., Stabilization of Boron Carbide Via Silicon Doping. J. Phys. : Condens. Matter 2014, 27, 015401. 38. Ferrari, A. C.; Robertson, J., Interpretation of Raman Spectra of Disordered and Amorphous Carbon. Phys. Rev. B 2000, 61, 14095. 39. Compagnini, G.; Puglisi, O.; Foti, G., Raman Spectra of Virgin and Damaged Graphite Edge Planes. Carbon 1997, 35, 1793-1797. 15



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Table of Contents Graphic

Boron Carbide (B4C) decomposes into B50C2 and nano-crystalline graphite under the influence of shear combined with moderate uniaxial compression.

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Fig. 1. Diffraction patterns of the starting material, the material under selected conditions, and the quenched material. The vertical bar, diamond, and cross represent the diffraction peaks originating from B4C, B50C2, and anvil/gasket/pressure-calibration materials, respectively. The numbers above the patterns represent the loading and rotation angles in degrees, as well as two-digit pressure in GPa calculated using gold’s equation of state, respectively. 38x62mm (300 x 300 DPI)


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Fig. 2: HRTEM image of (a) original and (b) quenched sample. (c) Some of the selected positions, a, b, and c (marked by circles), where EDS was measured and (d) the B:C ratio at corresponding positions. 63x64mm (300 x 300 DPI)



Fig. 3: Raman spectra of the original B4C and quenched material. 57x52mm (300 x 300 DPI)


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Fig. 4. The XPS spectra of original and quenched samples. Squares, data points; black line, fitted curve; blue line, fitted peak. 82x36mm (300 x 300 DPI)



Fig. 5: The crystalline structure of (a) B4C and (b) B50C2. The red and green spheres represent respectively carbon and boron atoms. The bi-color spheres represent the atomic positions of minimal SOF. 57x66mm (300 x 300 DPI)


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A Shear Driven Chemical Decomposition of Boron Carbide | The

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