Abstract The trends of miniaturization, lightweight, and high integration in electronics have brought serious issues in heat dissipation and electromagnetic compatibility and also limited the simultaneous use of thermally conductive and microwave absorption materials. Therefore, it is imperative to design materials that possess those dual functions. In this work, one‐pot method is used to anchor zeolitic imidazolate framework ZIF‐67 coated with polydopamine (PDA) on boron nitride (BN) to obtain BN@ZIF‐67@PDA. The pyrolysis product BN@Co‐C@C is used as heterostructured thermally conductive/microwave absorption fillers and blended with polyethylene terephthalate (PET) to prepare BN@Co‐C@C/PET composites. When the mass ratio of BN to ZIF‐67@PDA is 7.5:1 and the mass fraction of BN 7.5 @Co‐C@C is 45 wt%, the BN 7.5 @Co‐C@C/PET composites exhibit excellent thermal conductivities and microwave absorption performances. The thermal conductivity coefficient is 5.37 W m −1 K −1 , which is 35.8 times higher than that of PET (0.15 W m −1 K −1 ), and also higher than that of 45 wt% (BN 7.5 /Co‐C@C)/PET composites (4.03 W m −1 K −1 ) prepared by directly mixing. The minimum reflection loss of 45 wt% BN 7.5 @Co‐C@C/PET composites are −63.1 dB at 4.72 GHz, and the corresponding effective absorption bandwidth is 1.28 GHz (4.08–5.36 GHz), achieving excellent microwave absorption performance at C band.
BACKGROUND: Although effective role models are important in medical education, little is known about the characteristics of physicians who serve as excellent clinical role models. We therefore conducted a case-control study to identify attributes that distinguish such physicians from their colleagues. METHODS: We asked members of the internal-medicine house staff at four teaching hospitals to name physicians whom they considered to be excellent role models. A total of 165 physicians named by one or more house-staff members were classified as excellent role models (these served as the case physicians in our study). A questionnaire was sent to them as well as to 246 physicians who had residency-level teaching responsibilities but who were not named (controls). Of these 411 physicians, 341 (83 percent) completed questionnaires while unaware of their case-control status. RESULTS: Of the 341 attending physicians who responded, 144 (42 percent) had been identified as excellent role models. Having greater assigned teaching responsibilities was strongly associated with being identified as an excellent role model. In the multivariate analysis, five attributes were independently associated with being named as an excellent role model: spending more than 25 percent of one's time teaching (odds ratio, 5.12; 95 percent confidence interval, 1.81 to 14.47), spending 25 or more hours per week teaching and conducting rounds when serving as an attending physician (odds ratio, 2.48; 95 percent confidence interval, 1.15 to 5.37), stressing the importance of the doctor-patient relationship in one's teaching (odds ratio, 2.58; 95 percent confidence interval, 1.03 to 6.43), teaching the psychosocial aspects of medicine (odds ratio, 2.31; 95 percent confidence interval, 1.23 to 4.35), and having served as a chief resident (odds ratio, 2.07; 95 percent confidence interval, 1.07 to 3.98). CONCLUSIONS: These data suggest that many of the attributes associated with being an excellent role model are related to skills that can be acquired and to modifiable behavior.
As the portable device hardware has been increasing at a noticeable rate, ultrathin thermal conducting materials (TCMs) with the combination of high thermal conductivity and excellent electromagnetic interface (EMI) shielding performance, which are used to efficiently dissipate heat and minimize EMI problems generated from electronic components (such as high speed processors), are urgently needed. In this work, graphene oxide (GO) films are fabricated by direct evaporation of GO suspension under mild heating, and ultrathin graphite‐like graphene films are produced by graphitizing GO films. Further investigation demonstrates that the resulting graphene film with only ≈8.4 μm in thickness not only possesses excellent EMI shielding effectiveness of ≈20 dB and high in‐plane thermal conductivity of ≈1100 W m ‐1 K ‐1 , but also shows excellent mechanical flexibility and structure integrity during bending, indicating that the graphitization of GO film could be considered as a new alternative way to produce excellent TCMs with efficient EMI shielding.
Abstract In order to ensure the operational reliability and information security of sophisticated electronic components and to protect human health, efficient electromagnetic interference (EMI) shielding materials are required to attenuate electromagnetic wave energy. In this work, the cellulose solution is obtained by dissolving cotton through hydrogen bond driving self-assembly using sodium hydroxide (NaOH)/urea solution, and cellulose aerogels (CA) are prepared by gelation and freeze-drying. Then, the cellulose carbon aerogel@reduced graphene oxide aerogels (CCA@rGO) are prepared by vacuum impregnation, freeze-drying followed by thermal annealing, and finally, the CCA@rGO/polydimethylsiloxane (PDMS) EMI shielding composites are prepared by backfilling with PDMS. Owing to skin-core structure of CCA@rGO, the complete three-dimensional (3D) double-layer conductive network can be successfully constructed. When the loading of CCA@rGO is 3.05 wt%, CCA@rGO/PDMS EMI shielding composites have an excellent EMI shielding effectiveness (EMI SE) of 51 dB, which is 3.9 times higher than that of the co-blended CCA/rGO/PDMS EMI shielding composites (13 dB) with the same loading of fillers. At this time, the CCA@rGO/PDMS EMI shielding composites have excellent thermal stability ( T HRI of 178.3 °C) and good thermal conductivity coefficient ( λ of 0.65 W m -1 K -1 ). Excellent comprehensive performance makes CCA@rGO/PDMS EMI shielding composites great prospect for applications in lightweight, flexible EMI shielding composites. Graphic abstract
Abstract Lead halide perovskite quantum dots (QDs) possess color‐tunable and narrow‐band emissions and are very promising for lighting and display applications, but they suffer from lead toxicity and instability. Although lead‐free Bi‐based and Sn‐based perovskite QDs (CsSnX 3 , Cs 2 SnX 6 , and (CH 3 NH 3 ) 3 Bi 2 X 9 ) are reported, they all show low photoluminescence quantum yield (PLQY) and poor stability. Here, the synthesis of Cs 3 Bi 2 Br 9 perovskite QDs with high PLQY and excellent stability is reported. Via a green and facile process using ethanol as the antisolvent, as‐synthesized Cs 3 Bi 2 Br 9 QDs show a blue emission at 410 nm with a PLQY up to 19.4%. The whole series of Cs 3 Bi 2 X 9 (X = Cl, Br, and I) QDs by mixing precursors can cover the photoluminescence emission range from 393 to 545 nm. Furthermore, Cs 3 Bi 2 Br 9 QDs show excellent photostability and moisture stability due to the all‐inorganic nature and the surface passivation by BiOBr, which enables the one‐pot synthesis of Cs 3 Bi 2 Br 9 QD/silica composite. A lead‐free perovskite white light‐emitting diode is fabricated by simply combining the composite of Cs 3 Bi 2 Br 9 QD/silica with Y 3 Al 5 O 12 phosphor. As a new member of lead‐free perovskite QDs, Cs 3 Bi 2 Br 9 QDs open up a new route for the fabrication of optoelectronic devices due to their excellent stability and photophysical characteristics.
A facile process was developed to synthesize layered MoS(2)/graphene (MoS(2)/G) composites by an l-cysteine-assisted solution-phase method, in which sodium molybdate, as-prepared graphene oxide (GO), and l-cysteine were used as starting materials. As-prepared MoS(2)/G was then fabricated into layered MoS(2)/G composites after annealing in a H(2)/N(2) atmosphere at 800 °C for 2 h. The samples were systematically investigated by X-ray diffraction, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, and high-resolution transmission electron microscopy. Electrochemical performances were evaluated in two-electrode cells versus metallic lithium. It is demonstrated that the obtained MoS(2)/G composites show three-dimensional architecture and excellent electrochemical performances as anode materials for Li-ion batteries. The MoS(2)/G composite with a Mo:C molar ratio of 1:2 exhibits the highest specific capacity of ∼1100 mAh/g at a current of 100 mA/g, as well as excellent cycling stability and high-rate capability. The superior electrochemical performances of MoS(2)/G composites as Li-ion battery anodes are attributed to their robust composite structure and the synergistic effects between layered MoS(2) and graphene.
Abstract Black TiO 2 attracts enormous attention due to its large solar absorption and induced excellent photocatalytic activity. Herein, a new approach assisted by hydrogen plasma to synthesize unique H‐doped black titania with a core/shell structure (TiO 2 @TiO 2‐ x H x ) is presented, superior to the high H 2 ‐pressure process (under 20 bar for five days). The black titania possesses the largest solar absorption (≈83%), far more than any other reported black titania (the record (high‐pressure): ≈30%). H doping is favorable to eliminate the recombination centers of light‐induced electrons and holes. High absorption and low recombination ensure the excellent photocatalytic activity for the black titania in the photo‐oxidation of organic molecules in water and the production of hydrogen. The H‐doped amorphous shell is proposed to play the same role as Ag or Pt loading on TiO 2 nanocrystals, which induces the localized surface plasma resonance and black coloration. Photocatalytic water splitting and cleaning using TiO 2‐ x H x is believed to have a bright future for sustainable energy sources and cleaning environment.
Nitrogen-doped activated porous carbon fibres (ACFs) were prepared as anode materials for Na-ion batteries. They exhibit excellent electrochemical performance, especially rate performance. The excellent rate performance is ascribed to the fibre-like morphology and the facilitated charge transfer. The influence of nitrogen functionalities on charge transfer and electrochemical performance of N-doped carbon anodes for Na ion batteries is discussed.
Shape memory hydrogels have promising applications in a wide variety of fields. Here we report the facile fabrication of a novel type of shape memory hydrogels physically cross-linked with both stronger and weaker hydrogen bonding (H-bonding). Strong multiple H-bonding formed between poly(vinyl alcohol) (PVA) and tannic acid (TA) leads to their coagulation when they are physically mixed at an elevated temperature and easy gelation at room temperature. The amorphous structure and strong H-bonding endow the PVA-TA hydrogels with excellent mechanical properties, as indicated by their high tensile strengths (up to 2.88 MPa) and high elongations (up to 1100%). The stronger H-bonding between PVA and TA functions as the "permanent" cross-link and the weaker H-bonding between PVA chains as the "temporary" cross-link. The reversible breakage and formation of the weaker H-bonding imparts the PVA-TA hydrogels with excellent temperature-responsive shape memory. Wet and dried hydrogel samples with a deformed or elongated shape can recover to their original shapes when immersed in 60 °C water in a few seconds or at 125 °C in about 2.5 min, respectively.
Abstract With the rapid development and popularization of smart, portable, and wearable flexible electronic devices, urgent demands have been raised for flexible electromagnetic interference (EMI) shielding films to solve related electromagnetic pollution problems. With polyvinyl alcohol (PVA) as polymer matrix, the sandwich‐structured EMI shielding nanocomposite films are prepared via electrospinning‐laying‐hot pressing technology, where Fe 3 O 4 /PVA composite electrospun nanofibers in the top and bottom layers and Ti 3 C 2 T x /PVA composite electrospun nanofibers in the middle layer. Owing to the electrospinning process and the successful construction of the sandwich structure, when the amounts of Ti 3 C 2 T x and Fe 3 O 4 are respectively only 13.3 and 26.7 wt%, the EMI shielding effectiveness (EMI SE) of the sandwich‐structured EMI shielding nanocomposite films reach 40 dB with the thickness of 75 µm, higher than that of (Fe 3 O 4 /Ti 3 C 2 T x )/PVA EMI shielding nanocomposite films (21 dB) prepared based on blending‐electrospinning‐hot pressing process under the same amounts of fillers. Furthermore, the prepared sandwich‐structured EMI shielding nanocomposite films possess excellent thermal conductivities and mechanical properties. This novel kind of flexible sandwich‐structured EMI shielding nanocomposite films with excellent EMI shielding performances, thermal conductivities, and mechanical properties presents broad application prospects in the fields of EMI shielding and protection for high‐power, portable, and wearable flexible electronic devices.
The electronic and chemical properties of graphene can be modulated by chemical doping foreign atoms and functional moieties. The general approach to the synthesis of nitrogen-doped graphene (NG), such as chemical vapor deposition (CVD) performed in gas phases, requires transitional metal catalysts which could contaminate the resultant products and thus affect their properties. In this paper, we propose a facile, catalyst-free thermal annealing approach for large-scale synthesis of NG using low-cost industrial material melamine as the nitrogen source. This approach can completely avoid the contamination of transition metal catalysts, and thus the intrinsic catalytic performance of pure NGs can be investigated. Detailed X-ray photoelectron spectrum analysis of the resultant products shows that the atomic percentage of nitrogen in doped graphene samples can be adjusted up to 10.1%. Such a high doping level has not been reported previously. High-resolution N1s spectra reveal that the as-made NG mainly contains pyridine-like nitrogen atoms. Electrochemical characterizations clearly demonstrate excellent electrocatalytic activity of NG toward the oxygen reduction reaction (ORR) in alkaline electrolytes, which is independent of nitrogen doping level. The present catalyst-free approach opens up the possibility for the synthesis of NG in gram-scale for electronic devices and cathodic materials for fuel cells and biosensors.
An advanced supercapacitor material based on nitrogen-doped porous graphitic carbon (NPGC) with high a surface area was synthesized by means of a simple coordination-pyrolysis combination process, in which tetraethyl orthosilicate (TEOS), nickel nitrate, and glucose were adopted as porogent, graphitic catalyst precursor, and carbon source, respectively. In addition, melamine was selected as a nitrogen source owing to its nitrogen-enriched structure and the strong interaction between the amine groups and the glucose unit. A low-temperature treatment resulted in the formation of a NPGC precursor by combination of the catalytic precursor, hydrolyzed TEOS, and the melamine-glucose unit. Following pyrolysis and removal of the catalyst and porogent, the NPGC material showed excellent electrical conductivity owing to its high crystallinity, a large Brunauer-Emmett-Teller surface area (SBET =1027 m(2) g(-1) ), and a high nitrogen level (7.72 wt %). The unusual microstructure of NPGC materials could provide electrochemical energy storage. The NPGC material, without the need for any conductive additives, showed excellent capacitive behavior (293 F g(-1) at 1 A g(-1) ), long-term cycling stability, and high coulombic efficiency (>99.9 % over 5000 cycles) in KOH when used as an electrode. Notably, in a two-electrode symmetric supercapacitor, NPGC energy densities as high as 8.1 and 47.5 Wh kg(-1) , at a high power density (10.5 kW kg(-1) ), were achieved in 6 M KOH and 1 M Et4 NBF4 -PC electrolytes, respectively. Thus, the synthesized NPGC material could be a highly promising electrode material for advanced supercapacitors and other conversion devices.
A variety of biomass-based carbon materials with two-level porous structure have been successfully prepared by one-step carbonization process. The first level of microscale pores templates from the inherent porous tissues, while the second one of nanopores is produced by the in situ etching by the embedded alkaline metal elements. The superimposed effect of nano and microscale pores endows the hierarchically porous carbons (HPCs) with excellent microwave absorption (MA) performance. Among them, the spinach-derived HPC exhibits a maximum reflection loss of -62.2 dB and a broad effective absorption bandwidth of 7.3 GHz. Particularly, this excellent MA performance can be reproduced using the biomass materials belonging to different families, harvested seasons, and origins, indicating a green and sustainable process. These encouraging findings shed the insights on the preparation of biomass-derived microwave absorbents with promising practical applications.
A nonfullerene-based polymer solar cell (PSC) that significantly outperforms fullerene-based PSCs with respect to the power-conversion efficiency is demonstrated for the first time. An efficiency of >11%, which is among the top values in the PSC field, and excellent thermal stability is obtained using PBDB-T and ITIC as donor and acceptor, respectively. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.
Highly photoluminescent carbon dots with a PL quantum yield of 26% have been prepared in one step by hydrothermal treatment of orange juice. Due to high photostability and low toxicity these carbon dots are demonstrated as excellent probes in cellular imaging.
Bimetallic metal–organic frameworks are rationally synthesized as templates and employed for porous carbons with retained morphology, high graphitization degree, hierarchical porosity, high surface area, CoNx moiety and uniform N/Co dopant by pyrolysis. The optimized carbon with additional phosphorus dopant exhibits excellent electrocatalytic performance for the oxygen reduction reaction, which is much better than the benchmark Pt/C in alkaline media. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.
This article presents a model for quality in qualitative research that is uniquely expansive, yet flexible, in that it makes distinctions among qualitative research’s means (methods and practices) and its ends. The article first provides a contextualization and rationale for the conceptualization. Then the author presents and explores eight key markers of quality in qualitative research including (a) worthy topic, (b) rich rigor, (c) sincerity, (d) credibility, (e) resonance, (f) significant contribution, (g) ethics, and (h) meaningful coherence. This eight-point conceptualization offers a useful pedagogical model and provides a common language of qualitative best practices that can be recognized as integral by a variety of audiences. While making a case for these markers of quality, the article leaves space for dialogue, imagination, growth, and improvisation.
Alloys with composition of AlCoCrFeNiTix (x: molar ratio; x=0,0.5,1,1.5) were designed by using the strategy of equiatomic ratio and high entropy of mixing. The alloy system is composed mainly of body centered cubic solid solution and possesses excellent room-temperature compressive mechanical properties. Particularly for AlCoCrFeNiTi0.5 alloy, the yield stress, fracture strength, and plastic strain are as high as 2.26GPa, 3.14GPa, and 23.3%, respectively, which are superior to most of the high-strength alloys such as bulk metallic glasses.
Thioureas represent the dominant platform for hydrogen bond promoted asymmetric catalysts. A large number of reactions, reported in scores of publications, have been successfully promoted by chiral thioureas. The present paper reports the use of squaramides as a highly effective new scaffold for the development of chiral hydrogen bond donor catalysts. Squaramide catalysts are very simple to prepare. The (-)-cinchonine modified squaramide (5), easily prepared through a two-step process from methyl squarate, was shown to be an effective catalyst, even at catalyst loadings as low as 0.1 mol%, for the conjugate addition reactions of 1,3-dicarbonyl compounds to beta-nitrostyrenes. The addition products were obtained in high yields and excellent enantioselectivities.
Composites incorporating ferromagnetic metal nanopartices into a highly porous carbon matrix are promising as electromagnetic wave absorption materials. Such special composite nanomaterials are potentially prepared by the thermal decomposition of metal-organic framework (MOF) materials under controlled atmospheres. In this study, using Co-based MOFs (Co-MOF, ZIF-67) as an example, the feasibility of this synthetic strategy was demonstrated by the successful fabrication of porous Co/C composite nanomaterials. The atmosphere and temperature for the thermal decomposition of MOF precursors were crucial factors for the formation of the ferromagnetic metal nanopartices and carbon matrix in the porous Co/C composites. Among the three Co/C composites obtained at different temperatures, Co/C-500 obtained at 500 °C exhibited the best performance for electromagnetic wave absorption. In particular, the maximum reflection loss (RL) of Co/C-500 reached -35.3 dB, and the effective absorption bandwidth (RL ≤ -10 dB) was 5.80 GHz (8.40 GHz-14.20 GHz) corresponding to an absorber thickness of 2.5 mm. Such excellent electromagnetic wave absorption properties are ascribed to the synergetic effects between the highly porous structure and multiple components, which significantly improved impedance matching.