Configurational, electronic entropies and the thermoelectric properties of nanocarbon ensembles

Gruen, D. M.; Bruno, P.; Xie, Ming

doi:10.1063/1.2909150

Cited by 10 publications

(7 citation statements)

References 14 publications

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“…In the trend of |α| ( Figure 2 b), a broad maximum can be seen slightly below to the equiatomic composition, close to the composition of the highest chemical disorder, a similar situation to that of other entropic parameters of such alloys [ 31 ], but a shoulder at a composition of about 70 at.%-Ni is also evident. This observation of high |α|for a high chemical disorder reflects the general finding that high configurational entropy is a prerequisite for the observation of large |α| [ 32 ]. Because of the close relationship between large |α| and high configurational entropy, it was recently suggested to even use configurational entropy as a gene-like performance indicator for the computational search of new thermoelectric materials [ 33 ].…”

Section: Resultssupporting

confidence: 60%

Entropy of Conduction Electrons from Transport Experiments

Pérez

Wolf

Kunzmann

et al. 2020

Entropy

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The entropy of conduction electrons was evaluated utilizing the thermodynamic definition of the Seebeck coefficient as a tool. This analysis was applied to two different kinds of scientific questions that can—if at all—be only partially addressed by other methods. These are the field-dependence of meta-magnetic phase transitions and the electronic structure in strongly disordered materials, such as alloys. We showed that the electronic entropy change in meta-magnetic transitions is not constant with the applied magnetic field, as is usually assumed. Furthermore, we traced the evolution of the electronic entropy with respect to the chemical composition of an alloy series. Insights about the strength and kind of interactions appearing in the exemplary materials can be identified in the experiments.

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Section: Resultssupporting

confidence: 60%

Entropy of Conduction Electrons from Transport Experiments

Pérez

Wolf

Kunzmann

et al. 2020

Entropy

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“…The Seebeck coefficient exceeds 100 lV K À1 in all foils at high temperature, leading to decent power factors exceeding 0.1 mW m À1 K À2 . Unlike other carbon-based materials, such as graphite [68], nanocarbon ensembles [69] or graphitic boron carbon nitride (BC 3 N) [70], diamond achieves a considerable Seebeck coefficient. From its temperature dependence a metal-like transport is concluded for our foils, whereas the dependence of the electrical conductivity on temperature exhibits a thermally activated transport, which may be explained with grain boundary scattering.…”

Section: Resultsmentioning

confidence: 99%

Thermoelectric transport properties of boron-doped nanocrystalline diamond foils

Engenhorst

Fecher

Notthoff

et al. 2015

Carbon

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A B S T R A C TNatural diamond is known for its outstanding thermal conductivity and electrical insulation. However, synthetic production allows for doping and tailoring microstructural and transport properties. Despite some motivation in the literature and the ongoing search for abundant and non-toxic thermoelectric materials, the first experimental study on a set of eight substrate-free boron-doped nanocrystalline diamond foils is presented herein.All transport coefficients were determined in the same direction within the same foils over a broad temperature range up to 900°C. It is found that nanostructuring reduces the thermal conductivity by two orders of magnitude, but the mobility decreases significantly to around 1 cm 2 V À1 s À1 , too. Although degenerate transport can be concluded from the temperature dependence of the Seebeck coefficient, charge carriers notably scatter at grain boundaries where sp 2 -carbon modifications and amorphous boron-rich phases form during synthesis. A detailed analysis of doping efficiency yields an acceptor fraction of only 8-18 at%, meaning that during synthesis excess boron thermodynamically prefers electrically inactive sites. Decent power factors above 10 À4 W m À1 K À2 at 900°C are found despite the low mobility, and a Jonker-type analysis grants a deeper insight into this issue.Together with the high thermal conductivity, the thermoelectric figure of merit zT does not exceed 0.01 at 900°C.

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“…16,17 However, the crucial role in determining thermoelectric properties played by the unique nature of the graphite lattice in providing a multitude of boron substitutional configurations each with its distinctive energy signature appears to have been explicitly recognized for the first time in our earlier work. 4 This recognition led us to examine in detail the nature of the acceptorlike feature using density functional calculations on boron substituted polyaromatics that serve as molecular models for finite as well as stacked graphene sheets. 18 These calculations provide insight into the relative stabilities of high spin versus low spin states as a function of graphene sheet size, Kekule versus non-Kekule structures, as well as the effects on orbital energetics of various boron substitutional configurations.…”

Section: Discussionmentioning

confidence: 99%

“…It is for this reason that carbon and, in particular, boron doped nanocarbons were chosen as model systems for studies of their thermoelectric performance characteristics. 4 In that earlier work it was shown that reaction of disperse ultrananocrystalline diamond ͑UNCD͒ and mixtures of UNCD with 10%-20% nano-boron-carbide ͑B 4 C͒ with methane gas at temperatures near 1200 K results in mechanically rigid compacts called nanocarbon ensembles and boron doped nanocarbon ensembles, respectively. It needs to be understood that the amount of boron that enters into a substitutional solid solution in the nanocarbon ensembles as a dopant is only a small fraction of the quantity added initially.…”

Section: Thermoelectric Power Factors Of Nanocarbon Ensembles As a Fumentioning

confidence: 99%

“…5 Strongly temperature dependent power factors ͑PF= ␣ 2 , where ␣ is the Seebeck coefficient and is the electrical conductivity͒ that increase 30-40-fold between ambient and 1000 K were found for boron doped versus undoped material. 4 Here we present recent results aimed at enhancing the thermoelectric performance of these ensembles including materials prepared by reacting methane with boron doped nanographite ͑NG͒ starting materials. In the current paper, measurements of PFs have been made up to 1200 K on samples annealed at temperatures up to 2500 K. Anticipating the detailed discussion of the results we note that the PFs continue to increase even at 1200 K, the current limit of our measuring apparatus.…”

Section: Thermoelectric Power Factors Of Nanocarbon Ensembles As a Fumentioning

confidence: 99%

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Thermoelectric power factors of nanocarbon ensembles as a function of temperature

Gruen

Bruno

Arenal

et al. 2009

Journal of Applied Physics

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Thermoelectric power factors of nanocarbon ensembles have been determined as a function of temperature from 400 to 1200 K. The ensembles, composed of mixtures of nanographite or disperse ultrananocrystalline diamond with B4C, are formed into mechanically rigid compacts by reaction at 1200 K with methane gas and subsequently annealed in an argon atmosphere at temperatures up to 2500 K. The ensembles were characterized using scanning electron microscopy, Raman, x-ray diffraction, and high resolution transmission electron microscopy techniques and found to undergo profound nanostructural changes as a function of temperature while largely preserving their nanometer sizes. The power factors increase strongly both as a function of annealing temperature and of the temperature at which the measurements are carried out reaching 1 μW/K2 cm at 1200 K without showing evidence of a plateau. Density functional “molecular analog” calculations on systems based on stacked graphene sheets show that boron substitutional doping results in a lowering of the Fermi level and the creation of a large number of hole states within thermal energies of the Fermi level [P. C. Redfern, D. M. Greun, and L. A. Curtiss, Chem. Phys. Lett. 471, 264 (2009)]. We propose that enhancement of electronic configurational entropy due to the large number of boron configurations in the graphite lattice contributes to the observed thermoelectric properties of the ensembles.

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Configurational, electronic entropies and the thermoelectric properties of nanocarbon ensembles

Cited by 10 publications

References 14 publications

Entropy of Conduction Electrons from Transport Experiments

Entropy of Conduction Electrons from Transport Experiments

Thermoelectric transport properties of boron-doped nanocrystalline diamond foils

Thermoelectric power factors of nanocarbon ensembles as a function of temperature

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