270 research outputs found
Enhanced thermoelectric figure of merit in vertical graphene junctions
In this work, we investigate thermoelectric properties of junctions
consisting of two partially overlapped graphene sheets coupled to each other in
the cross-plane direction. It is shown that because of the weak van-der Waals
interactions between graphene layers, the phonon conductance in these junctions
is strongly reduced, compared to that of single graphene layer structures,
while their electrical performance is weakly affected. By exploiting this
effect, we demonstrate that the thermoelectric figure of merit can reach values
higher than 1 at room temperature in junctions made of gapped graphene
materials, for instance, graphene nanoribbons and graphene nanomeshes. The
dependence of thermoelectric properties on the junction length is also
discussed. This theoretical study hence suggests an efficient way to enhance
thermoelectric efficiency of graphene devices.Comment: 6 pages, 4 figures, submitte
A low-cost system for monitoring pH, dissolved oxygen and algal density in continuous culture of microalgae
In a continuous and closed system of culturing microalgae, constantly monitoring and controlling pH, dissolved oxygen (DO) and microalgal density in the cultivation environment are paramount, which ultimately influence on the growth rate and quality of the microalgae products. Apart from the pH and DO parameters, the density of microalgae can be used to contemplate what light condition in the culture chamber is or when nutrients should be supplemented, which both also decide productivity of the cultivation. Moreover, the microalgal density is considered as an indicator indicating when the microalgae can be harvested. Therefore, this work proposes a low-cost monitoring equipment that can be employed to observe pH, DO and microalgal density over time in a culture environment. The measurements obtained by the proposed monitoring device can be utilized for not only real-time observations but also controlling other sub-systems in a continuous culture model including stirring, ventilating, nutrient supplying and harvesting, which leads to more efficiency in the microalgal production. More importantly, it is proposed to utilize the off-the-shelf materials to fabricate the equipment with a total cost of about 513 EUR, which makes it practical as well as widespread. The proposed monitoring apparatus was validated in a real-world closed system of cultivating a microalgae strain of Chlorella vulgaris. The obtained results indicate that the measurement accuracies are 0.3%, 3.8% and 8.6% for pH, DO and microalgae density quantities, respectively. © 2022 The Author(s
Van der Waals Coefficients of Atoms and Molecules from a Simple Approximation for the Polarizability
A simple and computationally efficient scheme to calculate approximate
imaginary-frequency dependent polarizability, hence asymptotic van der Waals
coefficient, within density functional theory is proposed. The dynamical
dipolar polarizabilities of atoms and molecules are calculated starting from
the Thomas-Fermi-von Weizs\"acker (TFvW) approximation for the
independent-electron kinetic energy functional. The van der Waals coefficients
for a number of closed-shell ions and a few molecules are hence calculated and
compared with available values obtained by fully first-principles calculations.
The success in these test cases shows the potential of the proposed TFvW
approximate response function in capturing the essence of long range
correlations and may give useful information for constructing a functional
which naturally includes van der Waals interactions.Comment: 6 pages, 4 figures. To appear in Phys. Rev.
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