Thomas Laude's pages - Science de la matière et de l'énergie

Boron Nitride Nanotubes Grown by Non-Ablative Laser Heating: Synthesis, Characterisation and Growth Process

Boron Nitride Nanotubes Grown by Non-Ablative Laser Heating, by T. Laude PhD (pdf 182 pages)

Thesis abstract

The beam of a CO₂ laser, both continuous and low power (40-80 W), focused on a hexagonal boron nitride (h-BN) target (hot pressed powders), induces no ablation, but a stable temperature gradient, radial along target surface. Such a heating, in low nitrogen pressure, produces a macroscopic growth of BN nano-tubes. Tubes grow on a ring around impact, forming a crown of entangled tubes, perpendicular to target surface.

This method is efficient to synthesise BN nano-tubes and other nano-spherical BN particles, often rich in boron. Tubes are extremely long (measured up to 120 microns), mostly thin (typically 2 to 4 layers) and self-assembled in ropes. In a tube, BN is stoichiometric and well crystallised. Spherical particles are faceted BN onions, often containing a boron nano-crystal inside their cavity. The synthesis method is simple and low cost. Quantity produced may be extended for commercial purposes, by scanning the laser beam (or the target), by using a higher laser power, or by collecting the material dropped from the target,・

Growth occurs at high temperature but not directly from h-BN platelets. After dissociation and evaporation, boron condenses in nitrogen atmosphere, forming spherical particles, rich in boron, which spread around impact. Then, boron recombines with gaseous nitrogen if and only if boron is liquid, and hence, growth occurs on a ring of specific temperatures. While forming BN shells, some spherical particles evolve toward tubular extrusions. The evolution of a spherical particle toward a tube can be driven by its temperature decrease. A temperature gradient forms along the tube, essentially because of thermal radiation. The gradient is exponentially decreasing with tube length, for an order of 200 K over a few tens of microns. Growth speed also decreases quickly with tube length. It is of an order of 10 m m/s in the beginning of the growth.

Résumé de thèse:

Le faisceau d'un laser CO₂, continu et de basse puissance (40-80 W), focalisé sur une cible de nitrure de bore (BN) hexagonal (poudres pressées à chaud), n'induit pas d'ablation, mais un gradient de température stable, radial le long de la surface. Un tel chauffage, sous basse pression d'azote, produit une croissance macroscopique de nano-tubes de BN. Les tubes croissent sur un anneau autour de l'impact, formant une couronne de tubes enchevêtrés perpendiculaire à la surface de la cible.

Cette méthode est efficace pour synthétiser des nano-tubes de BN ainsi que des nano-particules sphériques de BN, souvent riches en bore. Les tubes sont extrêmement longs (mesurés jusqu'à 120 microns), fins (typiquement 2 a 4 couches) et souvent assemblés en cordes. Dans les tubes, le BN est stoichiométrique, et bien cristallisé. Les particules sphériques sont des oignons facettés de nitrure de bore, contenant souvent un nano-cristal de bore à l'intérieur de leur cavité. La méthode de synthèse est simple et peu coûteuse. La quantité produite peut être augmentée, en balayant le rayon laser (ou la cible), en utilisant une puissance laser plus élevée, ou en collectant le matériel détaché de la cible.

La croissance se produit à température élevée, mais pas directement depuis les plaquettes de h-BN. Après dissociation puis évaporation, le bore condense dans l'atmosphère d'azote, en formant les particules sphériques riches en bore, qui se déposent autour de l'impact. Le bore se recombine ensuite avec l'azote gazeux si et seulement si le bore est liquide; d'ou une croissance sur un anneau de température déterminée. En formant des coquilles de BN, certaines particules sphériques évoluent vers des extrusions tubulaires. L'évolution d'une particule sphérique vers un tube peut être entraînée par la chute de sa température. Un gradient de température se forme le long du tube, essentiellement à cause du rayonnement thermique. Le gradient décroit exponentiellement avec la longueur du tube, de l'ordre de 200 K, sur une distance de quelques dizaines de microns. La vitesse de croissance diminue aussi rapidement avec la longueur de tube. Elle est de l'ordre de 10 micron/s en début de croissance.

3 Related Publications and Patents

4 History of this study and gratitude to the many people who helped

9 Main objectives for this study

10 Constitution of this manuscript

11 Chap. I: Introduction on nano-tubes and related nano-structures

12 I. Nano-structured particles of layered materials
12 1. Materials known to form closed nano-structures
15 2. Spherical and tubular morphologies
18 II. Properties and perspectives of applications
19 1. Some mechanical applications
20 2. Some chemical applications
20 3. Some electronic applications
22 III. Synthesis methods
22 1. For carbon
23 2. For BN
25 IV. Mechanisms of formation usually suggested

29 Chap. II: Continuous CO₂ laser apparatus
30 I. Presentation of the apparatus
30 1. Experimental procedure
31 2. Technical details
33 3. Setting procedures
34 II. Waist of the incident beam
34 1. Optical laws for Gaussian beams
35 2. Size and position of the final waist
36 3. Uncertainty on positioning
37 III. Equations for heat diffusion in the target
37 1. Introducing the formalism
41 2. Setting hypothesises
43 IV. Adapting laser power to target material
43 1. Stabilised temperatures in a large (or cooled) target
46 2. Increase of temperature due to the limited size of the target
47 V. Impurities of commercial h-BN
47 1. Smoke effusing from a raw h-BN target
49 2. Oxide impurities remaining on target despite outgasing

51 Chap. III: Techniques to observe and characterise nano-structures
52 I. Four instances of analysis techniques
52 1. Transmission electron microscopy (TEM) imaging (in a few words)
54 2. Electron diffraction on nano-tubes
56 3. Electron energy loss spectroscopy (EELS)
57 4. X-ray analysis of carbon ropes of SWNT
58 II. Analysis methods used here and their practical difficulties
58 1. TEM imaging
59 2. Electron diffraction inside TEM
60 3. EELS
61 4. Scanning electron microscopy (SEM)

63 Chap. IV: Standard experiment and global conclusions on physical processes
64 I. Standard experimental procedure
66 II. Geography on the heated surface
66 1 Three distinct zones
70 2. Frontiers between zones
73 III. Material from the crown (zone II)
73 1. Global aspect at low magnification
73 2. High resolution imaging of tubes (by TEM)
76 3. Electron diffraction patterns of tubes and ropes
78 4. EELS on ropes
79 5. Nano-polyhedrons (angular onions)
81 6. Tube extremities
81 IV. Global growth processes

87 Chap. V: Temperatures in the h-BN target
88 I. Typical times for target warming
88 1. Time for global warming
89 2. Is the semi-infinite medium hypothesis valid during the rising of temperature?
90 3. Time for first gradient rise
91 4. Temperature drop due to the formation of a liquid boron layer
92 II. Temperatures on front surface during target warming
92 1. Hypothesises
93 2. Modelisation
95 3. Temperatures at the end of a standard experiment
95 4. Is the temperature range constant between experiments?
98 5. Weaknesses of the model

103 Chap. VI: Mechanisms of growth around impact
104 I. Composition of the atmosphere around impact
104 1. Estimations of mean free path
105 2. Comparison of N and B fluxes
106 3. Boron flux versus temperature, from vapour pressure measurements
108 4. Diffusion of boron particles outside cavity
109 5. Particles depositing on target from gas phase
111 II. Evolution of nano-sized particles at high temperatures
111 1. Temperature gradient along a tube
116 2. What determines the evolution toward a tubular morphology?
121 3. Conditions for BN tube growth

129 Chap. VII: Influence of experimental conditions
131 I. Different durations of heating
131 1. Global influence of heating duration
133 2. Description of each samples
139 II. Influence of nitrogen pressure
139 1. Effect of nitrogen pressure on structures (HR imaging)
143 2. Influence of nitrogen pressure on global geography
146 III. Influence of laser power
149 IV. Heating in inert gas
152 V. Surface influence

155 General conclusions of the study

156 Some proposals for further Developments

157 Heating a pure boron target and a graphite target (unfinished study)

159 Are the present conclusions on mechanism extendable to carbon (and other diatomic material), and all synthesis method? (Free and opened discussion)

163 Annex 1: Mathematica language codes for calculation used here

169 Annex 2: Some physical datas: h-BN rod composition, JDPDS cards, C(T), k(T)・

173 Bibliography

182 French and English Abstracts

Thesis erratum

23 August, 2001:

The molecular flux striking a surface which appears p.105,106 and 108 is and not.

Conclusions are not affected but numerical values should be corrected accordingly, in p.105, Fig. VI.3, and Fig. VI.21

To contact me: thomaslaudeABC@uminokai.net (Remove ABC, it is against spam.)
The material present here is copyrighted. Key-words: nanotechnology, science, nanotubes.