Nanostructured materials under high-pressure
TiO2 is a widely studied and used material. Nanostructured TiO2 has numerous technological applications, such as photocatalysis, optoelectronic devices, chemical sensors, dielectric materials or dye-sensitised solar cells. Titania possesses eleven known phases, five of which are high pressure forms. However, only few studies got interested in the high pressure behaviour of nanostructured TiO2.
For instance, Olsen and co-workers studied nanostructured rutile under high pressure.37 In this study, rutile was used as 10 nm particles (confirmed by XRD and TEM measurements) and compressed in a DAC, up to 47 GPa, at room temperature. The evolution of the material was followed by in situ XRD. It shows (Figure I-14) that the baddeleyite phase starts to crystallise around 20 GPa, until the phase transition is completed at about 30 GPa. Upon further compression, the XRD pattern is not modified further than pressure-induced cell shrinking. The baddeleyite phase is conserved along decompression, until 4 GPa, where the α-PbO2-type phase crystallises. It is the sole crystalline phase present under 2 GPa.
Top-down approach: high pressure as grain size reducer
In metallurgy, severe plastic deformation (SPD) is used to strengthen metals or alloys.64 SPD techniques allow grain size reduction by introducing extensive strains within materials. Among these techniques, High-Pressure Torsion (HPT), for instance, consists in compression of the sample in the GPa range and rotation of one of the anvils to introduce plastic torsional strains, while maintaining the sample’s original shape (Figure I-22). SPD can reach nanomaterials, for instance 2024 Al alloy, obtained as 50-60 nm particles after HPT treatment, with a strong increase in the mechanical properties.65 However, SPD is usually performed at room temperature or below, and mostly reaches > 100 nm grain size.
Also at room temperature, an original route toward grain size reduction has been mentioned by Gonzales et al., and Endo and co-workers.66,67 As first order phase transitions are accompanied with strong unit cell volume change, such phase transitions in bulk materials are accompanied with crystallite fragmentation (at defects or fractures). Therefore, in the case of pressure-induced phase transitions, applying pressure cycles (i.e. go over and below the phase transition pressure, thus corresponding to a phase transition cycle), the crystallite phase is reduced at each pressure cycle. This method has been applied to several materials, including CdSe, CdTe and Cd0.5Mn0.5Te, reaching sub-50 nm particles.
The studies combining nanostructure and high pressure show that HPHT approach is mainly limited to light elements based materials aiming at producing new or improved functional materials for high performance cutting tools or abrasive. Other nanostructured materials such as oxides or metals have been studied under high pressure but mainly kept a room temperature (except for MgAl2O4 transparent ceramics).
Boron: elemental forms and properties of the fifth element
As argued by Oganov and Solozhenko, “boron is arguably the most complex element in the periodic table”.
The discovery of boron dates back to the experiments of Thénard and Gay-Lussac in 1808, by reduction of boric acid with potassium.70 However, it is estimated that the products obtained by Gay-Lussac, Thénard and Davy (by electrolysis, in 1808) had less than 50 % of boron. Reduction of boric acid with magnesium was conducted by Moissan later on (1897) and reached purer amorphous boron by reducing B2O3 with magnesium (mixed with MgB2).70,71 Pure boron was obtained in 1909 by Weintraub, more than a century after the discovery of the element. Boron is extremely sensitive to impurities, as boron-rich compounds exhibits structures different from that of doped boron allotropes, even with small amounts of impurities: among these, one can cite PuB100 or YB66 (which has more than 1500 atoms in its unit cell, 2.34 nm long, see Figure I-23). Concerning pure boron, the literature reports no less than 16 modifications subject to discussion (in 2009).
Stability of boron phases and phase diagram
Along with controversies concerning the existence of several boron allotropes, the nature of the most stable phase has been subject to long debates. At room pressure, liquid boron crystallises into β-B, making this allotrope the stable phase at high temperature.71 Furthermore, at room pressure, no phase transition of β-B into other allotropes has been reported. Therefore, early reports made β-B the thermodynamically stable phase at all temperatures below the melting point at ambient pressure.71,110 However, first computational studies established that α-B12 has a lower energy than β-B, but these calculations were made without taking into account the macroscopic amounts of defects present in β-B, which exhibits partial occupation of numerous crystallographic sites (POS), as established by Slack et al.110,111 Five sites are reported to exhibit POS, for instance the B13 position in the B28 triply fused iscosahedra (Figure I-33).
Nanostructured amorphous boron: nano Bam
The colloidal synthesis of metal borides relies on the reaction between a metal chloride and sodium borohydride. The process patented by G. Gouget and co-workers4 uses the same experimental set-up, except for the metal chloride: the reaction consists in the decomposition of sodium borohydride in salt melts. NaBH4 is synthesised in the LiI/KI eutectic mixture, heated at 800 °C with a 10 °C/min heating rate and a 1 h dwell time. The washing steps are performed in methanol, after which the powder is dried under vacuum at 60 °C for 2 h. After drying, the powder is transferred to an argon-filled glovebox to avoid contact with air. Characterisations such as XRD and NMR spectroscopy are conducted with very limited exposure to air. The powder XRD pattern (Figure II-15) shows that the material is amorphous. Commercial amorphous boron is a brown powder, while the SMS-derived material is black (Figure II-16).
Synthesis of metal borides-boron nanoparticles in molten salts
The synthesis of the nanoscaled precursors was performed by precipitation in the inorganic molten eutectic mixture LiCl:KCl with high thermal stability, which enables reaching temperatures up to 900 °C within the liquid medium at room pressure.35,42 After the synthesis and to ensure subsequent evolution under HPHT of the amorphous matrix toward borates and not boron polymorphs, the powders were exposed to air for 7 days to allow oxidation of the boron matrix and incorporation of B-O bonds. The presence of B-O bonds characteristic vibrations is evidenced by bands at 1350 cm-1 on the infrared spectra (Supplementary information Figure III-S1). XRD shows that hafnium diboride is obtained at 900 °C as a pure crystalline phase (Figure II-3). The crystallite size is 7.5(6) nm according to the Scherrer’s formula.43 It is consistent with the particle diameter observed by TEM (Figure III-3a), which exhibits sizes ranging from 5 to 10 nm range in agreement with our previous report.35 The particles are single-crystals and isotropic. To our knowledge, this molten salt method yields the smallest particle size reported for crystalline metal borides. As previously reported,35,36 these particles are embedded in an amorphous matrix which was shown to consist mainly in slightly oxidised boron, where oxygen arises from the washing step when the powders are exposed to water and air. The nature of the matrix was also assessed by the mean of Scanning Transmission Microscopy-High Angle Annular Dark Field (STEM-HAADF), more sensitive to difference in atomic numbers than bright field TEM. For the Hf-based system, the comparison of bright and dark field observations (Figure III-4) shows that the matrix around the particles does contain small clusters (ca. 2 nm) contrasting with the rest of the matrix. This shows the presence of hafnium atoms within the matrix. The thickness of this amorphous phase is about 2 nm. Consequently, between two adjacent nanocrystals, a 4 nm gap is filled with amorphous boron doped with Hf atoms and clusters. As two adjacent particle shells are fused together, then the system is best described as “nano-cookie dough”: the boride particles are inclusions within the amorphous boron nanostructured three-dimensional matrix. Therefore, both components are nanostructured (Figure III-3a): the crystalline phase as nanoscale inclusions and the amorphous phase consisting in nanoscale walls between the crystalline inclusions. The nanostructure is illustrated in the scheme in Figure III-3b.
Table of contents :
Chapter I: Bibliography
I-1- Nanomaterials & high pressure
I-1-1- Nanostructured high-pressure phases
I-1-1-a- Nanostructured diamond
I-1-1-b- Other nanostructured superhard materials: B-C-N system
I-1-2- Nanostructured materials under high-pressure
I-1-2-a- Oxides and related materials
I-1-2-b- Other materials
I-2- Boron: elemental forms and properties of the fifth element
I-2-1- Allotropes of boron
I-2-2- Stability of boron phases and phase diagram
I-3- Boron-metal alloys: structure, properties and synthesis
I-3-1- Properties and structures
I-3-2- Synthesis and nanostructures
Chapter II: Synthesis of nanostructured precursors in molten salts
II-1- Experimental set-up
II-2- Molten salts colloidal synthesis
II-2-1- Hafnium diboride
II-2-1-a- Synthesis in LiCl/KCl
II-2-1-b- Synthesis in LiI/KI
II-2-2- Calcium hexaboride
II-2-2-a- Synthesis in LiCl/KCl
II-2-2-b- Synthesis in LiI/KI
II-2-3- Nanostructured amorphous boron: nano Bam
Chapter III: HPHT treatments on oxygen-containing nanocomposites
III-3- Results and discussion
III-3-1- Synthesis of metal borides-boron nanoparticles in molten salts
III-3-2- High Pressure-High Temperature (HPHT) treatments
III-5- Associated content
Chapter IV: HPHT formation of non-oxidised nanocomposites
IV-1- Experimental set-ups
IV-2- Calcium hexaboride
IV-2-1- In situ XRD in Paris-Edinburgh cell
IV-2-1-a- Experiment at ID27/ESRF
IV-2-1-b- Experiment at PSICHE/SOLEIL Synchrotron
IV-2-2- Ex situ experiments
IV-2-2-a- Heating at 1350 °C
IV-2-2-b- Heating at 1550 °C
IV-2-2-c- Heating at 1750 °C
IV-3- Hafnium diboride
IV-3-1- In situ XRD in Paris-Edinburgh cell
IV-3-2- Ex situ experiments
IV-4- Conclusions & Prospects
Chapter V: HPHT treatments of nanostructured amorphous boron
V-1- In situ XRD in Paris-Edinburgh cell
V-2- Ex situ experiments
V-2-1- 5 GPa experiments
V-2-2- 14 GPa experiments
V-3- Conclusion and prospects
1- Synthesis in molten salts
2- HPHT experiments
3- Characterisation techniques