Graphene

Tuning a circular p–n junction in graphene from quantum confinement to optical guiding – Rutgers University-New Brunswick

Abstract

The photon-like propagation of the Dirac electrons in graphene, together with its record-high electronic mobility1,2,3, can lead to applications based on ultrafast electronic response and low dissipation4,5,6. However, the chiral nature of the charge carriers that is responsible for the high mobility also makes it difficult to control their motion and prevents electronic switching. Here, we show how to manipulate the charge carriers by using a circular p–n junction whose size can be continuously tuned from the nanometre to the micrometre scale7,8. The junction size is controlled with a dual-gate device consisting of a planar back gate and a point-like top gate made by decorating a scanning tunnelling microscope tip with a gold nanowire. The nanometre-scale junction is defined by a deep potential well created by the tip-induced charge. It traps the Dirac electrons in quantum-confined states, which are the graphene equivalent of the atomic collapse states (ACSs) predicted to occur at supercritically charged nuclei9,10,11,12,13. As the junction size increases, the transition to the optical regime is signalled by the emergence of whispering-gallery modes14,15,16, similar to those observed at the perimeter of acoustic or optical resonators, and by the appearance of a Fabry–Pérot interference pattern17,18,19,20 for junctions close to a boundary.

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Nickel Hexacyanoferrate Electrodes for Continuous Cation Intercalation Desalination of Brackish Water – USA – Illinois

Abstract:

Using porous electrodes containing redox-active nickel hexacyanoferrate (NiHCF) nanoparticles, we construct and test a device for electrochemical water desalination in a two flow-channel device where the electrodes are in direct contact with an anion-exchange membrane. Upon reduction of NiHCF, cations intercalate into it and the water in its vicinity is desalinated; at the same time water in the opposing electrode becomes more saline upon oxidation of NiHCF in that electrode. In a cyclic process of charge and discharge, fresh water is continuously produced, alternating between the two channels in sync with the direction of applied current. Compared to capacitive deionization using porous carbon electrodes, a higher salt adsorption capacity per cycle is achieved, much lower cell voltages are needed, and the energy costs of desalination can be significantly reduced. Electrochemical water desalination with porous electrodes can make use of two fundamentally different mechanisms for salt storage. The first mechanism is capacitive electrosorption, where ions are held in electrical double layers (EDLs) formed in the micropores of porous electrodes comprised
of ideally polarizable material (e.g., carbon) [1]. In the second mechanism, which has recently begun research exploration [2–6], intercalation electrodes are used where ions are stored within the sites of a solid-state host compound. The first mechanism, capacitive electrosorption, is used in Capacitive Deionization (CDI), a  process in which ions are held near the carbon surface in the diffuse part of the EDL. CDI electrodes are made of carbon (carbon nanotubes, graphene, activated carbon powder, etc.) which can be processed into porous, ion- and electron-conducting, thin electrode films, suspensions, or fluidized beds [7]. CDI based on capacitive EDL charging is a promising method, but to reach a certain salt adsorption capacity (SAC; a typical number being of the order of SAC=5-15 mg/g, referring to mass of NaCl removed, per total mass of carbon in a two-electrode cell, measured at a standard cell voltage of Vcell=1.2 V), the energy input is not insignificant [8–10], while the current efficiency  (quantifying the fraction of current input that results in salt adsorption) of CDI cells can be well below unity, implying that in the charging process not only counterions adsorb but also coions desorb from the electrode [11]. In CDI with membranes, or using improved chargingschemes, can be close to unity [10]. Like capacitive carbons, intercalation host compounds (IHCs) can be incorporated into porous electrode films and can adsorb charge, but the ion storage mechanism of IHCs is fundamentally
different from EDL charging. In intercalation electrodes, ions are stored in the crystallographic sites of the IHC as a result of its redox activity. Water desalination using IHCs, which is currently much less developed and utilized than CDI, has the advantage that to reach a certain SAC a much lower voltage and energy is needed than using capacitive electrosorption, because the change in electrode potential with electrode charge can be much lower. Also, IHCs have the potential to selectively remove one ion (e.g., Na+) out of a multi-ion mixture with other ions of the same valence and charge.

Available from: https://www.researchgate.net/publication/311925778_Nickel_Hexacyanoferrate_Electrodes_for_Continuous_Cation_Intercalation_Desalination_of_Brackish_Water [accessed Oct 14 2017].

Entrance Desalination technology

Desalination Technology is developing globally very fast these days. More and more desalination technologies getting applied into the conventional water treatment business today. DME does follow and support this development on an international basis starting from the first idea up to commodity products.

In order to give some direction what technology is available today this area will give a basic direction.

Following the navigation of this side you will find fundamentals such as on the right listed.

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Graphene Membrane Withstands Ultrahigh Pressure, Shows Desalination Potential

Rohit Karnik, an associate professor at the Massachusetts Institute of Technology’s Department of Engineering, said the discovery could open graphene to a number of new applications, including desalination, where filtration membranes that can withstand high-pressure flows  can more efficiently remove salt from seawater.

“This is a fundamental study at this point, and what it shows is the possibility that one can design graphene membranes that can withstand high pressures,” Karnik said during an exclusive interview with R&D Magazine. . “This in itself doesn’t immediately lead to any application but it is basically a demonstration that you can design graphene membranes to withstand high pressures.”

According to Karnik, the research indicates which substrate designs are better to support graphene under higher pressures and that when graphene was placed over substrates with larger pores it failed to withstand even low pressure.

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Classification of Desalination Technologies

Desalination technology is used by humans since thousands of years. Being in the desert surrounded by sand and rocks or being on the sea sailing through salty waters, always fresh water is needed every day to keep the human body in a balanced hydrate mode.

Nature started using desalination processes from the very first moment. Different concentration levels drive everything alive on our planet. Humans started using first evaporation and condensation processes to purify unusable water. In the last century the number of processes to desalinate water and other liquids has gone through a very big development. New processes have been found, existing processes have been improved.

Overviewing all known desalination processes of today we may find a large number in a lot of science areas. In order to give this convolute of technology a systematic structure a larger group of desalination experts and scientists developed a Classification of Desalinaiton Technology (CDT) system.

This Classification of Desalinaiton Technoogy system is based on the natural working principles of chemistry, physics and biology. It starts as low as electrochemistry and nuclear physics. From here on it is building up its structure. Under the five main science fields 20 subordinated science areas have been found describing all natural working principles in desalination systems.

In order to structure all known desalination technologies a general structure was build up to the Classification of Desalinaiton Technology system (CDT):

The first classification level does divide in transient phase and stable phase technologies.
The second level classes represent the separation processes  of desalination technologies.
The third level classes show the main technology, in some cases without further structure.
The fourth level classes are split in advanced sub-desalination technologies.
In total there were 51 desalination technologies found and classified inside CDT.

As of today the used technologies mainly are Multi-Stage Flash and Reverse Osmosis they can be found within the structure at the third level.

Everybody is free to make use of this CDT but is asked to make a reference to DME GmbH on behalf of all people being involved, please.

We will keep developing CDT and you are free to participate in this discussion!

(more details LINK)

Desalination Technology in use

Today three technologies are dominating the international market. These are Multi-Stage Flash (MSF), Multiple-Effect Distillation (also Multi Effect Distillation or MED) and Reverse Osmosis (RO).

Looking into a global installed capacity in 2016 of about 90 mio m³/d app. 90% of this water is produced by these three technologies.

Multi-Stage Flash (MSF) takes app 20%, Multiple-Effect Distillation (also Multi Effect Distillation or MED) app. 5% and Reverse Osmosis (RO) app 65% of this.

The number of installed plants differs very much because a single MSF plant today easily does  produce 60.000 m³/d versus a RO plant very often produces 5.000 m³/d or less. Today more then 50 different desalination technologies are classified.

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Future Desalination Technology

Saving material and energy per ton of water produced is the main target of all developments in desalination. Due to this basic attempt every of the today known technology has to prove its benefits against the established ones. Not pointing out every new technology three of them are selected and briefly described at this stage (2017).

Obviously an idea for a technology needs to develop. On the right hand side you can see an overview of different technologies. This sketch does illustrate the state of science knowledge and in the same way the state of Development, Technology and Art.

Legend: State of science knowledge values

0 Very high, 1 High, 2 Medium, 3 Low, 4 Very low

In this context DME does illustrate some of the know how build up and some of the support given so far in this field of activities preforming. In order to classify the development of a technology DME did introduce the

– Desalination Technology Development Benchmark –

also called “DesTeDeBe”. You will find more details “here (LINK)”

Some of the latest developments in technology are now benchmarked here. In order you want to know more, please get in contact with DME.

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Membrane Distillation (MD)

Membrane Distillation (MD) is a thermal desalination process in which water vapour is transported through a hydrophobic porous membrane. Due to a partial pressure difference of vapour between a liquid surface and an air or gas stream, water is vaporised although the liquid phase is much below boiling temperature. A hydrophobic membrane can separate the two phases – liquid water and either vapour or vapour-gas mixture – in order to avoid the transfer of liquid or allow compact stream arrangements. Thus, MD is a non-isothermal membrane separation process with combined heat and mass transfer over the membrane. Parallel arrangement of the membranes in flat or spiral wound configuration allows generating multi-effect setups. The steam, generated over a first membrane layer can be condensed in the permeate channel using the latent heat of condensation for heating an adjacent feed channel. This way, a diversity of possible stream arrangements and system configurations arises. The main distinguishing feature is the configuration of the permeate channel. In order to minimize sensible heat transfer from the hot feed channel to the next feed channel, as it is the main drawback of direct contact configurations (Direct Contact MD (DCMD), DCMD with Liquid Gap (LGDCMD),s. Fig. (a)), either air or any other sweep gas can be filled in the permeate channel. The Air Gap (AGMD) or Sweep Gas Membrane Distillation (SGMD, s. Fig.(c)) minimize the heat transfer over the permeate channel by dividing the condensate from the membrane. The main drawback of these configurations is the higher diffusion resistance over the gas and the worse condensation due to the presence of non-condensable gases. This problem can be handled by running the system under vacuum for removing the non-condensable gases (Vacuum Membrane distillation (VMD), s. Fig. (b)). (Khayet Souhaimi & Matsuura, 2011) Besides the application of flat membranes, systems based on hollow fibre membranes are researched. The main advantage of Membrane Distillation systems is the low temperature heat (below 100 °C) at low pressure for running the system. For that cheap construction materials (plastics) can be used. As described before, the usage of membranes allows compact system setups which reduces the footprint of the plant.

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Capacitive Deionization (CDI)

In Capacitive Deionization (CDI), charged ionic species are removed from aqueous solutions. The ions are adsorbed onto the surface of a pair of electrically charged electrodes, usually composed of highly porous carbon materials, upon applying an electrical voltage difference. Upon charging the electrodes with a voltage difference of typically 1-1.4 V, the salt ions present in the feed migrate to the electrode of opposite charge, cations to the cathode and anions to the anode, and form electrical double layers (EDLs) along the pore surface. Thus, the water flowing through the CDI cell is partially desalinated. In a discharging step, where either the applied voltage is shorted or the polarity reversed, the salt ions are released in a brine stream. The system architecture can be in flow-by or flow-through mode with the feed either streaming past the electrodes in parallel direction or streaming vertically through the electrode. New developments propose floating electrodes suspended in the aqueous solution that enable continuous operation. Various porous carbon materials have been suggested as electrode material, such as carbon aerogels, activated carbon and carbon nanotubes. An important factor is the sorption performance. The technology has been previously applied to brackish and seawater desalination, wastewater remediation and water softening, but has proven to be highly effective for solutions with low molar concentration such as brackish water. CDI does not require high pressure or temperature, as in membrane or thermal desalination, making the technology more energy efficient in comparison. It also has a higher accuracy in removing only particular salts and ions that enables the recovery of valuable compounds such as lithium among others. However, problems may arise in the regeneration phase as during reversed-polarity, repelled ions might be attracted to the oppositely charged electrode and by electrical shorting the only driving force is diffusion, which is slow and inefficient. The special applicability to brackish water offers a great potential for development, as the demand for desalination of brackish water is increasing due to salt intrusions into the groundwater in many regions worldwide. Nonetheless, many basic settings have not been uniquely defined until now and have to be further optimized.

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