Hydrodynamics and local mass transfer characterization under gas–liquid–liquid slug flow in a rectangular microchannel

Gas-liquid-liquid three-phase slug flow was generated in a glass microreactor with rectangular microchannel, where aqueous slugs were distinguished by relative positions to air bubbles and organic droplets. Oxygen from bubbles reacted with resazurin in slugs, leading to prominent color changes, which was used to quantify mass transfer performance. The development of slug length indicated a film flow through the corner between bubbles and the channel wall, where the aqueous phase was saturated with oxygen transferred from bubble body. This film flow results in the highest equivalent oxygen concentration within the slug led by a bubble and followed by a droplet. The three-phase slug flow subregime with alternate bubble and droplet was found to benefit the overall mass transfer performance most. These results provide insights into a precise manipulation of gas-liquid-liquid slug flow in microreactors and the relevant mass transfer behavior thereof

Bubble splitting under gas–liquid–liquid three-phase flow in a double T-junction microchannel

Gas-aqueous liquid-oil three-phase flow was generated in a microchannel with a double T-junction. Under the squeezing of the dispersed aqueous phase at the second T-junction (T2), the splitting of bubbles generated from the first T-junction (T1) was investigated. During the bubble splitting process, the upstream gas-oil two-phase flow and the aqueous phase flow at T2 fluctuate in opposite phases, resulting in either independent or synchronous relationship between the instantaneous downstream and upstream bubble velocities depending on the operating conditions. Compared with two-phase flow, the modified capillary number and the ratio of the upstream velocity to the aqueous phase velocity were introduced to predict the bubble breakup time. The critical bubble breakup length and size laws of daughter bubbles/slugs were thereby proposed. These results provide an important guideline for designing microchannel structures for a precise manipulation of gas-liquid-liquid three-phase flow which finds potential applications among others in chemical synthesis. (c) 2017 American Institute of Chemical Engineers AIChE J, 63: 376-388, 201

Observation of room-temperature ferroelectricity in elemental Te nanowires

Ferroelectrics are essential in low-dimensional memory devices for multi-bit storage and high-density integration. A polar structure is a necessary premise for ferroelectricity, mainly existing in compounds. However, it is usually rare in elemental materials, causing a lack of spontaneous electric polarization. Here, we report an unexpected room-temperature ferroelectricity in few-chain Te nanowires. Out-of-plane ferroelectric loops and domain reversal are observed by piezoresponse force microscopy. Through density functional theory, we attribute the ferroelectricity to the ion-displacement created by the interlayer interaction between lone pair electrons. Ferroelectric polarization can induce a strong field effect on the transport along the Te chain, supporting a self-gated field-effect transistor. It enables a nonvolatile memory with high in-plane mobility, zero supply voltage, multilevel resistive states, and a high on/off ratio. Our work provides new opportunities for elemental ferroelectrics with polar structures and paves a way towards applications such as low-power dissipation electronics and computing-in-memory devices

Theoretical Studies of Diamond for Electronic Applications

Diamond has since many years been applied in electronic fields due to its extraordinary properties. Substitutional dopants and surface functionalization have also been introduced in order to improve the electrochemical properties. However, the basic mechanism at an atomic level, regarding the effects of dopants and terminations, is still under debate. In addition, theoretical modelling has during the last decades been widely used for the interpretation of experimental results, prediction of material properties, and for the guidance of future materials. Therefore, the purpose of this research project has been to theoretically investigate the influence of dopants and adsorbates on electronic and geometrical structures by using density functional theory (DFT) under periodic boundary conditions. Both the global and local effects of dopants (boron and phosphorous) and terminations have been studied. The models have included H-, OH-, F-, Oontop-, Obridge- and NH2-terminations on the diamond surfaces. For all terminating species studied, both boron and phosphorous have been found to show a local impact, instead of a global one, on diamond structural geometry and electronic properties. Therefore, the terminating species only affect the DOS of the surface carbon layers. In addition, Oontop-terminated (111) diamond surfaces present reactive surface properties and display metallic conductivity. Moreover, the conductivity of the diamond surface can be dramatically increased by the introduction of a phosphorous dopant in the lattice. The work function of a diamond surface has also been found to be influenced to a large extent by the various adsorbates and the dopant levels. Diamond can also be used as a promising substrate for an epitaxial graphene adlayer. The effects of dopants and terminations on the graphene and diamond (111) interfacial systems have been investigated theoretically in great detail. The interfacial interaction is of the Van der Waal type with an interfacial distance around 3 Å. The interactions between graphene and a terminated diamond substrate were found to be relatively weaker than those for a non-terminated diamond substrate (even with dopants). For all interface systems between graphene and diamond, a diamond-supported graphene adlayer without induced defects can still keep its intrinsic high carrier mobility. A minor charge transfer was observed to take place from the graphene adlayer to a non-terminated diamond substrate (with or without dopants) and to Oontop-, OH- or Obridge-terminated diamond substrates. However, for the situation with an H-terminated diamond surface, the electron transfer took place from the diamond surface to graphene. On the contrary, an interfacial system with a non-terminated diamond surface offers a more pronounced charge transfer than that of the terminated diamond substrates. A small finite band gap at the Dirac point was also observed for the Oontop-terminated diamond-supporting graphene adlayer.    

Theoretical Studies of Diamond for Electronic Applications

Diamond has since many years been applied in electronic fields due to its extraordinary properties. Substitutional dopants and surface functionalization have also been introduced in order to improve the electrochemical properties. However, the basic mechanism at an atomic level, regarding the effects of dopants and terminations, is still under debate. In addition, theoretical modelling has during the last decades been widely used for the interpretation of experimental results, prediction of material properties, and for the guidance of future materials. Therefore, the purpose of this research project has been to theoretically investigate the influence of dopants and adsorbates on electronic and geometrical structures by using density functional theory (DFT) under periodic boundary conditions. Both the global and local effects of dopants (boron and phosphorous) and terminations have been studied. The models have included H-, OH-, F-, Oontop-, Obridge- and NH2-terminations on the diamond surfaces. For all terminating species studied, both boron and phosphorous have been found to show a local impact, instead of a global one, on diamond structural geometry and electronic properties. Therefore, the terminating species only affect the DOS of the surface carbon layers. In addition, Oontop-terminated (111) diamond surfaces present reactive surface properties and display metallic conductivity. Moreover, the conductivity of the diamond surface can be dramatically increased by the introduction of a phosphorous dopant in the lattice. The work function of a diamond surface has also been found to be influenced to a large extent by the various adsorbates and the dopant levels. Diamond can also be used as a promising substrate for an epitaxial graphene adlayer. The effects of dopants and terminations on the graphene and diamond (111) interfacial systems have been investigated theoretically in great detail. The interfacial interaction is of the Van der Waal type with an interfacial distance around 3 Å. The interactions between graphene and a terminated diamond substrate were found to be relatively weaker than those for a non-terminated diamond substrate (even with dopants). For all interface systems between graphene and diamond, a diamond-supported graphene adlayer without induced defects can still keep its intrinsic high carrier mobility. A minor charge transfer was observed to take place from the graphene adlayer to a non-terminated diamond substrate (with or without dopants) and to Oontop-, OH- or Obridge-terminated diamond substrates. However, for the situation with an H-terminated diamond surface, the electron transfer took place from the diamond surface to graphene. On the contrary, an interfacial system with a non-terminated diamond surface offers a more pronounced charge transfer than that of the terminated diamond substrates. A small finite band gap at the Dirac point was also observed for the Oontop-terminated diamond-supporting graphene adlayer.    

First Principle Study of the Attachment of Graphene onto Different Terminated Diamond (111) Surfaces

The adhesion of a graphene monolayer onto terminated or 2x1-reconstructed diamond (111) surfaces has in the present study been theoretically investigated by using a Density Functional Theory (DFT) method. H, F, O, and OH species were used for the surface termination. The generalized gradient spin density approximation (GG(S)A) with the semiempirical dispersion corrections were used in the study of the Van der Waals interactions. There is a weaker interfacial bond (only of type Wan-der-Waals interaction) at a distance around 3 angstrom (from 2.68 to 3.36 angstrom ) for the interfacial graphene//diamond systems in the present study. The strongest binding of graphene was obtained for the H-terminated surface, with an adhesion energy of -10.6 eV. In contrast, the weakest binding of graphene was obtained for F-termination (with an adhesion energy of -2.9 eV). For all situations in the present study, the graphene layer was found to retain its aromatic character. In spite of this, a certain degree of electron transfer was observed to take place from graphene to Oontop-, Obridge-, and OH-terminated diamond surface. In addition, graphene attached to Oontop-terminated surface showed a finite band gap

Ethylene/ethane absorption with AgNO3 solutions in ultrasonic microreactors

Ultrasound is an effective method to intensify gas-liquid processes as the oscillation energy is easily focused at the gas-liquid interface, which can induce strong interface oscillation and acoustic streaming. The combination of ultrasound and microreactor provides ideal control over the ultrasound field, making the energy efficiently utilized. In this study, characteristics of chemical absorption of C2H4 from the mixture of C2H6 /C(2)H(4)into AgNO3 solutions under Taylor flow are investigated in a high-power ultrasonic microreactor. The effect of ultrasound on the bubble size reduction, absorption and mass transfer coefficient is presented and discussed. It is demonstrated that very large mass transfer coefficient with k(L)a and k(L) in the range of 7-42 s(-1) and 0.00169-0.0225 m s(-1) respectively are obtained, presenting significant intensification compared to absorption without ultrasound in the same reactor

Ultrasonic Enhancement of CO2 Desorption from MDEA Solution in Microchannels

The enhancement of CO2 desorption from N-methyldiethanolamine (MDEA) rich solution was investigated in ultrasonic microreactors. Under ultrasound irradiation, the rate of bubble growth increased significantly due to rectified diffusion and bubble coalescence. The measured CO2 desorption rate was found to be obviously enhanced by ultrasound, being almost doubled at low temperature. The effects of various parameters on the enhancing effect of ultrasound were also investigated, including desorption temperature, solution flow rate, CO2 loading, MDEA concentration, microchannel length and capillary diameter. The results indicated that ultrasound was more suitable to intensify the CO2 desorption process at low temperature, by virtue of which the regeneration energy consumption and solvent loss could be efficiently reduced

Continuous Synthesis of Ag/AgCl/ZnO Composites Using Flow Chemistry and Photocatalytic Application

Ag/AgCl/ZnO composites were successfully synthesized in a continuous microfluidic system under visible light irradiation, which was employed in situ to reduce a portion of AgCl to metallic Ag. The formation of Ag/AgCl/ZnO composites was confined in small aqueous plugs, which were dispersed by octane as the continuous phase. In this way, enhanced mixing, low risk of channel clogging, and uniform light distribution were achieved. The characterization results revealed that the as-prepared Ag/AgCl/ZnO composites were composed of flowerlike ZnO with Ag/AgCl nanospheres anchored to them. It was found that the synthesis parameters such as water/oil volume flow ratio, total volume flow rate, temperature, and the molar ratio of Zn2+ to Ag+ had effects on the synthesis of Ag/AgCl/ZnO composites. Furthermore, the as-prepared Ag/AgCl/ZnO composites outperformed Ag/ZnO composites and AgCl/ZnO composites in the visible-light-driven degradation of methyl orange