다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

The stability of two-dimensional (2D) layers and membranes is subject of a long standing theoretical debate. According to the so called Mermin-Wagner theorem, long wavelength fluctuations destroy the long-range order for 2D crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be crumpled. These dangerous fluctuations can, however, be suppressed by anharmonic coupling between bending and stretching modes making that a two-dimensional membrane can exist but should present strong height fluctuations. The discovery of graphene, the first truly 2D crystal and the recent experimental observation of ripples in freely hanging graphene makes these issues especially important. Beside the academic interest, understanding the mechanisms of stability of graphene is crucial for understanding electronic transport in this material that is attracting so much interest for its unusual Dirac spectrum and electronic properties. Here we address the nature of these height fluctuations by means of straightforward atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon. We find that ripples spontaneously appear due to thermal fluctuations with a size distribution peaked around 70 \AA which is compatible with experimental findings (50-100 \AA) but not with the current understanding of stability of flexible membranes. This unexpected result seems to be due to the multiplicity of chemical bonding in carbon.

Intrinsic ripples in graphene

Because of their wide abundance, their renewable and environmentally benign nature, and their outstanding mechanical properties, a great deal of attention has been paid recently to cellulosic nanofibrillar structures as components in nanocomposites. A first major challenge has been to find efficient ways to liberate cellulosic fibrils from different source materials, including wood, agricultural residues, or bacterial cellulose. A second major challenge has involved the lack of compatibility of cellulosic surfaces with a variety of plastic materials. The water-swellable nature of cellulose, especially in its non-crystalline regions, also can be a concern in various composite materials. This review of recent work shows that considerable progress has been achieved in addressing these issues and that there is potential to use cellulosic nano-components in a wide range of high-tech applications.

Cellulosic nanocomposites: a review

Torrefaction of biomass can be described as a mild form of pyrolysis at temperatures typically ranging between 200 and 300°C in an inert and reduced environment. Common biomass reactions during torrefaction include devolatilization, depolymerization, and carbonization of hemicellulose, lignin, and cellulose. The torrefaction process produces a brown to black uniform solid product, as well as condensable (water, organics, and lipids) and Noncondensable gases (CO2, CO, and CH4). Typically during torrefaction, 70% of the mass is retained as a solid product, containing 90% of the initial energy content, while 30% of the lost mass is converted into condensable and noncondensable products. The system’s energy efficiency can be improved by reintroducing the material lost during torrefaction as a source of heat. Torrefaction of biomass improves its physical properties like grindability; particle shape, size, and distribution; pelletability; and proximate and ultimate composition like moisture, carbon and...

REVIEW: A review on biomass torrefaction process and product properties for energy applications

Graphene is a new and exciting material that has attracted much attention in the last decade and is being extensively explored because of its properties, which have been described with so many superlatives. Production of graphene for large scale application is still a major challenge. Top-down graphene exfoliation methods from graphite, such as liquid-phase exfoliation which is promising because of low cost and high scalability potential will be briefly discussed. We also analyze the challenges and possibilities of using graphene as a nanofiller in polymer composites which has resulted in enhanced electrical, mechanical and thermal properties. In this review, we take a panoramic approach to give insight on the different aspects of graphene such as properties, graphite-based production methods and also examples of graphene application in polymer composites and which will be beneficial to both novice and experts.

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General overview of graphene: Production, properties and application in polymer composites

The state of water in wet pulps was studied over a wide moisture range, reflecting the conditions on the paper machine from the press section to the reel. Two methods were used: water retention value (WRV) and differential scanning calorimetry (DSC). To increase the measuring accuracy, a modification to the WRV procedure was introduced. The results were directly compared by plotting the values over the observed moisture range. Different types of water were defined from DSC data: free water, freezing bound and non-freezing bound water. The water types were plotted in water distribution charts against the solids content. These charts were compared with the WRV data. For all pulps, the three water fractions were found to be interdependent while the free water removal rate decreased. Water removal did not take place as an independent water fraction removal sequence. The presence of fines and pulp recycling were found to influence the amount of freezing bound water, but not non-freezing bound water. The results showed an irreversible decrease in WRV (hornification) for all investigated pulps in the range of 20–75% solids content. This pattern corresponded to the relative amount of free water present. Freezing bound water disappeared at a solids content of over 80–85%. With the absence of free water, no further hornification occurred. It was concluded that the WRV appears to be reduced by a closure of large voids by adhesion of the cellulosic pore walls. The softening effect of water is required to cause ‘wet hornification’ of the structure.

Quantification of water in different states of interaction with wood pulp fibres

In this study, we present willow wood as a new low-cost, renewable, and sustainable biomass source for the production of a highly porous activated carbon for application in energy storage devices. ...

Highly Porous Willow Wood-Derived Activated Carbon for High-Performance Supercapacitor Electrodes.

The influence of swelling on the rheolog- ical and dewatering properties of high consistency nanocellulose based furnishes is considered. Different consistencies of suspensions (1-4 %) and furnishes (5-15 %) were prepared made of two distinctly different grades of nanocellulose containing, micro fibrillated (MFC) and nanofibrillated (NFC) cellulose, and systematic comparison between the rheological and dewatering parameters was conducted. The char- acterization of the rheological and dewatering prop- erties was performed with a stress controlled rheometer combined with an immobilization cell in parallel plate geometry, as well as with an independent gravimetric dewatering device. The surface charge of nanofibrillated cellulose was found to influence the rheological and dewatering properties of the evaluated suspensions and furnishes due to its impact on swelling and effectively bound water. Due to the complex behavior of the novel materials, the immo- bilization times were difficult to determine from the changes in the damping factor, as often used for coating colors. Instead, we propose a modified method for determination of immobilization times based on a rheological analysis adopting the rate of change in viscoelastic loss factor over time, d(tan d = G 00 /G 0 )/dt, describing the critical point(s) in the ratio of the viscous to elastic stress response moduli. With this approach we show that it is possible to characterize immobilization of these materials incorporating the concept of the combined physical interactions of the components and the non-removable bound water, without requiring a direct measure of the nanocellu- lose surface swelling. Based on the results, we hypothesize that fibrillar swelling impacts the dewa- tering of MFC and NFC suspensions, and furnishes containing them, by an interfiber pore connectivity blocking/sealing mechanism, which effectively defines the immobilization of the material matrix at the end point of free water extraction caused by the physical blocking imposed by the remaining bound water.

The role of MFC/NFC swelling in the rheological behavior and dewatering of high consistency furnishes

Dissolving grade pulps are used as raw material for manufacture of regenerated cellulose fibres and their use is constantly growing. Despite intensive research, there is still a need to develop cellulose dissolution-regeneration processes that would be economically viable, fulfil the pre-conditions of sustainability and would be able to meet the strict product quality requirements. The basis for creation of such a process is in deep understanding of the biomass structure and factors affecting the cellulose modification and dissolution. In this paper, the effects of the mechanical and enzymatic pre-treatments on the pore structure and alkaline solubility of dissolving grade pulp are discussed. Formation of micro- and macropores in the pulp fibres during mechanical shredding was found to correlate with the susceptibility of the fibres to enzymatic hydrolysis. The fibre porosity development during the processing was studied by a modified solute exclusion approach, which revealed differences between the effect of mild enzyme or acid hydrolysis on the pore structure of fibres. The dissolution of the modified fibres in NaOH/ZnO was evaluated and found to correlate with overall pore volume and accessible surface area analysed by the modified solute exclusion method.

Thaddeus Maloney

Papers

General overview of graphene: Production, properties and application in polymer composites

Quantification of water in different states of interaction with wood pulp fibres

Highly Porous Willow Wood-Derived Activated Carbon for High-Performance Supercapacitor Electrodes.

The role of MFC/NFC swelling in the rheological behavior and dewatering of high consistency furnishes

Fibre porosity development of dissolving pulp during mechanical and enzymatic processing