Flame Synthesis of Single-Walled Carbon Nanotubes

Height, Murray J., Jack B. Howard, Jefferson W. Tester, John B. Vander Sande. Flame Synthesis of Single-Walled Carbon Nanotubes. Carbon. 42 2295-2307(2004).

Purpose of the Study

The purpose of the study was to synthesize and analyze single-walled carbon nanotubes using a combustion flame methodology. A premixed flame synthesis of CNTs is much lower in cost and higher in yield than other production methods. Conditions for nanotube formation were examined and the carbon distribution characteristics were analyzed after synthesis.

Methods

A fuel rich flame was used as the requisite fuel-rich, high temperature environment necessary for nanotube formation. Although both diffusion flames and premixed flames are observed to produce CNTs, a premixed 1-dimensional flame was chosen to further understand nanotube formation zone and nanotube structure over a range of operating conditions.

A premixed acetylene/oxygen/argon flame with argon dilution at a pressure of 6.7 kPa was used. Iron pentacarbonate was chosen as the metallic precursor. The burner consisted of a copper plate with uniformly spaced holes filled with steel wool to facilitate uniform flow of the reactant gases. The burner was mounted on a vertical translation stage that allowed various heights above burner (HAB) to be observed. Thermophoretic sampling onto TEM grids allowed the characterization of nanotubes at different heights and lengths of time in flame.

Key Findings

1.     A zone of less than 40 mm burner height was dominated by discrete metal particles with a distribution of particle sizes.

2.     Nanotubes formed between 30 mm and 50 mm above the burner at a rate of 10 to 100 microns per second.

3.     The upper region of the flame (50 to 70 mm HAB) was dominated by tangled webs of nanotubes formed by the coalescence of nanotubes formed earlier in the flame.

4.     The nanotube formation window lies in a fuel equivalence ratio region between 1.5 and 1.9, with lower values dominated by discrete particles and higher values dominated by soot-like structures.

5.     Operating at a fuel equivalence ratio of 1.6 with a higher proportion of iron-oxide particles in flame produces the highest quality nanotube material relative to condensed material.

Definitions

Fuel Equivalence Ratio: ratio of the fuel to oxidizer present to the fuel to oxidizer in the stoichiometric case

Fabrication and Properties of Carbon Nanotube Reinforced Fe/Hydroxyapatite Composites by In-Situ Chemical Vapor Deposition

Li, Haipeng, Naiqin Zhao, Yuan Liu, Chunyong Liang, Chunsheng Shi, Xiwen Du, Jiajun Li. Fabrication and Properties of Carbon Nanotube Reinforced Fe/Hydroxyapatite Composites by In-Situ Chemical Vapor Deposition. Composites. 39 1128-1132(2008).

Purpose of the Study

The purpose of the study was to synthesize a homogeneously dispersed compound of carbon nanotubes and hydroxyapatite. Hydroxyapatite is a naturally occurring component of bone and is being developed as a bone implant material, but as-synthesized lacks ductility and strength under load. Carbon nanotubes (owing to their excellent mechanical properties) may be implanted as a secondary material to increase ductility and strength. Interfacial properties between hydroxyapatite matrix and CNTs mixed by standard mechanical mixing have failed to yield the homogeneous properties necessary for the increase of strength of the hydroxyapatite matrix.

Methods

Fe(NO₃)₃*9H₂O and hydroxyapatite powder were mixed and NaOH was dripped into the mixture with constant stirring. After aging a colloid was attained and calcined in nitrogren to yield an iron oxide/hydroxyapatite composite powder. This powder was placed into a quartz tube and mounted into an electrical tube furnace and heated to reduction temperature under nitrogen. The powder was deoxidized and a chemical vapor deposition methane/nitrogen environment was introduced. After cooling, a powder metallurgy process was employed at 500 MPa, followed by vacuum sintering and pressing at 1.2 GPa to yield the CNT(Fe)/HA composite.

Key Findings

1.   CNT yield increases with an increase in the Fe content of the catalyst

2.   The purity of the synthesized CNTs is around 96%

3.   Flexural strength of HA alone is 53 MPa, while the CNT(Fe)/HA is 79 MPa

4.   The CNT composite increases fracture toughness 226% from 0.72 MPam^1/2 to 2.35 MPam^1/2

5.   Fractography studies show evidence of a bridge phenomena between the CNT and the hydroxyapatite (stress is distributed between the 2 compounds), which may explain the increase in mechanical strength

Definitions

Vacuum Sintering: the process of bonding adjacent surfaces of a powder by heating at subatmospheric pressure.

High-Resolution TEM and the Application of Direct and Indirect Aberration Correction

Hetherington, Chang, Haigh, Nellist, Gontard, Dunin-Borkowski, Kirkland. High-Resolution TEM and the Application of Direct and Indirect Aberration Correction. Microscopy and Microanalysis. 14 60-67(2008).

Purpose of the Study

The purpose of the study was to apply a combination of both direct and indirect aberration correction to the high-resolution mode of a transmission electron microscope and to study the resulting images.

Methods

Delocalization in images has traditionally led to a difficulty in attaining high resolution micrographs. The development of direct and indirect aberration correction has led to an improvement in the resolution of HRTEM images. Direct aberration correction involves the insertion of elements below the objective lens to correct for spherical aberration. Indirect correction involves taking a series of tilt images and post-processing to remove aberrations.

The instrument used was a JEOL 200 kV FEG-STEM with both probe and imaging aberration correctors. The correctors are hexapoles which enhance resolution from 0.19 nm to less than 0.12 nm. Resolution is then limited by chromatic aberration and energy spread. Diffractograms tilted through a range of angles spaced 18 mrads apart are collected to create a wave aberration correction function.

Indirect and direct methods may be combined. Optical correction is taken to the third order with post-processing aberration coefficients giving a 5^th order correction.

Key Findings

1. Restoration of the exit wavefunction reveals more structural detail

2. <111> Strontium Titanate showed a resolution improvement from 0.12 nm to 0.08 nm

3. Correction of pre- and post-field allows for large condenser and aperture sizes, which allows the instrument to be run as a scanning confocal electron microscope to produce depth of field measurements

Definitions

Confocal Imaging: An imaging technique where pinholes are used to eliminate out-of-focus light

Exit Wavefunction: Used to simulate images of the focal series and compare with experimental results in order to minimize delocalization

Preparation and Photocatalytic Activity of hollow ZnSe microspheres via Ostwald Ripening

Zhang,Yang, Xie, Zhang, Li. Preparation and photocatalytic activity of hollow ZnSe microspheres via Ostwald ripening. Journal of Alloys and Compounds. In Press (2008).

 

Purpose of the Study

Hollow Zinc selenide microspheres were formed via an Ostwald ripening process following an initial ZnSe nanoparticle synthesis. The ZnSe synthesis was performed using a facile hydrothermal reaction. The reaction products were characterized using different techniques (TEM, SEM, XRD) and then subjected to photoluminscence studies to determine band emission.

 

Methods

ZnSe was synthesized after heating and cooling in an autoclave for 12 hours at 180 degrees Celsius. The products were examined using XRD, SEM, TEM, Raman spectroscopy and PL spectroscopy. Photocatalytic properties were determined by measuring the decoloration of methyl orange solution with suspended ZnSe microspheres. Solid microspheres are translated to hollow microspheres by Ostwald ripening over a reaction time of 2 hours.

 

Key Findings

1. Microspheres are hollow of diameter around 200 nm.

2. XRD peaks show that the products are high crystalline.

3. Ostwald ripening is the primary mechanism for the formation of hollow ZnSe.

4. ZnSe crystallites in the inner cores have higher surface energies than those in the outer cores of the aggregate regions.

5. The band emission for the strongest band was 479 nm, with a weak defect related band emission at 556 nm.

6. The size of the hollow interiors of the microspheres can be tuned by tuning the reaction time (increasing reaction time gives microspheres of increasing diameter).

Definitions

Photocatalytic: Acceleration of a photoreaction in the presence of a catalyst