Characterization of carbon aerogels by transport measurements

Carbon aerogels are a special class of low-density microcellular foams. These materials are composed of interconnected carbon particles with diameters of approximately 10 nm. The temperature dependence of the dc electrical resistivity and magnetic susceptibility (χ) from 4 K to room temperature, magnetoresistance (MR) in a magnetic field up to 15 T, and Raman scattering were measured as a function of aerogel density. While Raman scattering measurements are not sensitive to variations in density, the χ data show that there are more free carriers in samples of higher density. Aerogel samples with different densities all show a negative temperature coefficient of resistivity and a positive MR. The less dense samples exhibit a stronger temperature dependence of resistivity and a stronger field dependence of the MR, indicating that with decreasing density and increasing porosity, charge carriers are more localized. Data analysis precludes variable-range hopping in favor of nearest-neighbor hopping and fluctuation-induced tunneling as the most likely conduction mechanisms for carbon aerogels.

Download Full-text

Photoconductivity of carbon aerogels

Journal of Materials Research ◽

10.1557/jmr.1993.0811 ◽

1993 ◽

Vol 8 (4) ◽

pp. 811-819 ◽

Cited By ~ 13

Author(s):

M. Hosoya ◽

G. Reynolds ◽

M.S. Dresselhaus ◽

R.W. Pekala

Keyword(s):

Low Temperatures ◽

Transport Mechanism ◽

Carrier Transport ◽

Relaxation Mechanism ◽

Dark Conductivity ◽

Carbon Aerogels ◽

Low Density ◽

Carbon Particles ◽

Microcellular Foams ◽

Carrier Recombination

Photoconductivity was measured on a series of carbon aerogels to investigate their electronic properties. Carbon aerogels are a special class of low-density microcellular foams, consisting of interconnected carbon particles (∼120 Å diameter) and narrow graphitic ribbons (∼25 Å width) intertwined within each particle. Both the dark- and photoconductivities show drastic changes in the temperature range 5–300 K, which are similar to those in a-Si and chalcogenide photoconductors. At high temperatures, the photoconductivity is dominated by the carrier recombination within each particle. The photoconductivity at low temperatures is dominated by the same carrier transport mechanism as that for the dark conductivity, which is based on hopping and tunneling transport. The activation energy values for transport and recombination identify the electronic structure of the particles among samples of different bulk density. The long decay time of the photoconductivity suggests a relaxation mechanism associated with the dangling bonds.

Download Full-text

Microstructural Characterization of Annealed Samples of Pure-Yttria Using Electron Microscopy

Microscopy and Microanalysis ◽

10.1017/s1431927600010503 ◽

1997 ◽

Vol 3 (S2) ◽

pp. 723-724

Author(s):

S. Banerjee ◽

P. I. Gouma ◽

M. J. Mills

Keyword(s):

Electrical Response ◽

Microstructural Characterization ◽

Negative Temperature ◽

Sensor Material ◽

Type Semiconductor ◽

Electroceramic Material ◽

Temperature Coefficient Of Resistivity ◽

Cold Pressed ◽

Yttria Powder

Yttria is an electroceramic material that has a negative temperature coefficient of resistivity (i.e. it is an NTC type semiconductor). This property, along with its high melting point and good stability, makes yttria a potential sensor material (thermistor) for measuring temperature differences in demanding applications. Recently there has been increasing interesting in studying this ceramic material. How-ever, there is very limited published information on the microstructural characteristics of pure yttria processed under various conditions. Furthermore, reported evidence on the effect of processing conditions on the electrical response of the material is still lacking. Studies within our Center have revealed a correlation between aging time and electrical behaviour in pure yttria samples. The present study examines the evolution of the microstructure of sintered yttria powder during different annealing treatments.Yttria pellets (6mm diameter, 2mm height) were cold pressed and then sintered at 1550°C for 24h in air. The sintered pellets were then annealed at 1100°C for different times. Specimens for SEM examination were prepared by grinding the surface of the pellets (using grit paper up to 400) and subsequently polishing them using l μm alumina powders.

Download Full-text