Nanostructures and energy matters - Science des nanostructures 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.

Table of contents

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

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