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Atomic scale friction properties of confined water layers
No full text© 2021 Author(s).The atomic scale friction properties of water layers are a highly important subject for understanding the lubricating mechanism of ice. Recently, two-dimensional (2D) materials gave rise to an excellent platform for investigating the tribological properties of water, since water layers can be easily intercalated between 2D layers and hydrophilic substrates. In this Review, we highlight recent research on the friction properties of the confined water between hydrophilic (mica and silica) substrates and 2D materials. Recent friction force microscopy work has revealed the nanotribological properties of water intercalated between 2D materials (e.g., graphene and MoS2), and a hydrophilic substrate increases the friction force. Moreover, the friction on both graphene and MoS2 increased as the number of stacking water layers increased. On the other hand, the magnitude of friction increase went down as the number of covering 2D layers above the intercalated water layer increased; the friction is eventually indistinguishable from the multilayer stack excluded water adsorption. The isotope effect of frictional enhancement has been addressed, and it was shown that the intercalation of deuterium oxide (D2O) leads to the decrease of friction at H2O intercalated graphene on mica due to the lower vibrational frequency of D2O adsorbate, which is associated with the low rate of frictional energy dissipation at the interface. Water exfoliated by 2D layers prepared with mechanical exfoliation and chemical vapor deposition exhibits similar frictional enhancement, indicating the universal tendency of friction by intercalated water between 2D atomic layers and hydrophilic surfaces.11Nsciescopu
sj-pdf-1-ajs-10.1177_03635465221130759 – Supplemental material for Effect of Quality of Repair on Clinical and Structural Outcomes of Rotator Cuff Repair
No full textSupplemental material, sj-pdf-1-ajs-10.1177_03635465221130759 for Effect of Quality of Repair on Clinical and Structural Outcomes of Rotator Cuff Repair by Tae Won Go, Ji Eun Park, Sohee Oh, Minjoon Cho and Chris Hyunchul Jo in The American Journal of Sports Medicine</p
Revealing Pt-seed-induced structural effects to tribological/electrical/thermoelectric modulations in two-dimensional PtSe2 using scanning probe microscopy
No full text© 2021 The AuthorsTwo-dimensional (2D) transition metal dichalcogenides (TMDCs) provide an opportunity to investigate diverse scientific phenomena at an atomic level and have effective technological potential. In recent times, unlike conventional TMDCs, platinum diselenide (PtSe2) has garnered considerable attention owing to its i) fascinating electrical and optical features coupled with mechanical strain and ii) scalable synthesis method. However, its local electrical/tribological/mechanical properties are yet to be examined in detail. In this study, we prepared two different types of PtSe2 layers by controlling the thickness of the Pt-seed film: i) one with horizontal alignments, which are made from ultrathin Pt films and ii) one with vertical and horizontal alignments, which are made from a relatively thick Pt film. Then, the local tribological, mechanical, and electrical properties of PtSe2 were evaluated mainly with scanning probe microscopy (SPM). The local friction study unveiled the marked friction variation depending on the layer directions. An additional study on the local mechanical property indicated that the apparent modulus difference between the two growth directions can lead to dissimilar friction behaviors. Furthermore, local current probing using SPM showed that PtSe2 with both layer directions provides ohmic features to both directions, although the magnitude of conductance along the horizontal direction is ~100 times smaller than that along the vertical one. On the contrary, PtSe2 with only in-plane alignments is configured with multiple horizontal PtSe2 layers, densely stitching themselves together one after another with a very narrow trench region at their boundary. Surprisingly, a local current–voltage spectroscopy analysis showed a small current gap on the bridged trench region, in contrast to the ohmic behavior of the grain interior. This indicates that the local metal-to-semiconductor transition is available, owing to thin PtSe2 layers in the trench regions. The following study at macroscale showed how the overall electrical and thermoelectric properties of PtSe2 can be influenced by local structures triggered by the variation in Pt-seed thickness. The results not only provide a new insight on better understanding the intrinsic features of 2D PtSe2, but also offer important characterization guidance on tailoring their functionalities.11Nsciescopu
Synthesis and Properties of Sr M-type Hexaferrite/Magnetite Magnetic Composites
Get PDF학위논문 (석사)-- 서울대학교 대학원 : 공과대학 재료공학부, 2018. 2. 유상임.소형화된 모터에 필요한 고특성 경자성체의 수요가 증대되고 있다. 경자성체의 성능지수인 최대에너지적((BH)max)을 향상시키기 위한 많은 노력이 있어왔고, 그 중에서 연자성체와의 교환 결합 효과를 이용하여 경자성체의 이론적 한계 이상의 (BH)max 값을 달성하기 위한 연구가 활발하게 수행되어 왔다. 한편, 고성능 모터에 주로 쓰이는 희토류 자석은 현재까지 개발된 경자성체 중 가장 뛰어난 성능을 보이지만 희소자원인 희토류를 필요로 하며 정제시 환경오염이 발생하는 문제가 있다.
이에 반해 Sr M–type 육방정 페라이트(SrM)는 희토류 자석에 비해 가성비와 열적, 화학적 안정성이 우수하여 SrM의 (BH)max 향상에 대한 관심이 증가되고 있다. SrM은 Fe2O3와 안정적으로 상이 공존하고, Fe2O3는 연자성체인 Fe3O4로 환원될 수 있다는 것은 잘 알려져 있다. 이를 이용해 수소 후열처리 방법을 사용하여 SrM/Fe3O4의 분말 복합체를 제조해 교환 결합 효과로 SrM의 (BH)max 를 향상시킨 선행 연구가 존재한다. 그러나 영구 자석에 주로 쓰이는 벌크 형태의 SrM/Fe3O4 복합체에 관한 보고는 거의 없다. 따라서 본 연구에서는 선행 연구를 참고해서 SrM/Fe3O4의 벌크 복합체를 제조하고 SrM 입자의 크기와 소결온도 등의 변수가 경자성체와 연자성체 간의 교환 결합 효과에 미치는 영향을 규명하여 SrM의 (BH)max 값을 향상시키고자 하였다.
본 연구를 위해 서로 다른 크기의 SrM 분말을 졸겔 연소법, 고상 반응법, 용융염 합성법으로 제조하였다. SrM 와 Fe2O3의 혼합 분말을 볼밀링하고 성형한 후, 고온에서 수소와 순철을 환원제로 각각 활용하는 수소 분위기 후열처리와 순철 분말 환원 반응의 두 가지 방법으로 SrM/Fe3O4 복합체를 제조하였다. 제조된 시편의 상, 미세구조, 소결밀도, 자기적 특성을 분석하여 다음의 결과를 얻을 수 있었다.
첫째, 후열처리 전 소결체를 만들기 위해 공기중에서 소결하는 경우, SrM-Fe2O3 복합체는 순수한 SrM에 비해 보자력(Hc)이 증가하였으나 포화자화(Ms)와 (BH)max 가 감소하였다. Ms 값의 감소는 비자성을 띠는 Fe2O3의 첨가에 의한 것이 확실하고, (BH)max 값의 감소는 다공성 미세구조로 인한 낮은 소결밀도에 의한 것으로 사료된다. 소결온도가 증가하면 소결밀도는 증가하나 SrM결정립이 성장하여 Hc 가 감소하였다. 고상 반응법으로 SrM이 합성된 경우, 다른 분말 합성법에 비해 소결밀도와 (BH)max 가 높게 나타났다.
둘째, 소결체를 후열처리 하였을 때, Fe2O3 첨가 여부와 소결 온도에 관계없이 수소에 의해 SrM이 분해되어 Ms 와 Hc 및 (BH)max 가 감소하였다. (BH)max 의 감소폭은 기공이 많은 미세구조를 가진 소결체일수록 크게 나타났다.
셋째, 순철 분말 환원 반응의 경우, 순철 분말의 양이 증가할수록 Fe3O4가 더 많이 생성돼서 Ms 값은 증가하였으나 Hc 와 (BH)max 값은 감소하였다. 1000 ℃ 소결온도에서는 소결밀도가 낮아 다공성 미세구조가 나타났으며, 1100 ℃ 소결온도에서는 이차상인 Fe2O3와Sr2Fe2O5가 생성되었으나 소결밀도와 (BH)max 가 증가하였다. 용융염 합성법으로 합성된 SrM의 경우, M–H 곡선이 교환 결합 효과를 나타내는 단일 루프에 가까운 형태로 나타났다. 그러나 큰 결정립으로 인한 다자구화와 낮은 소결밀도로 인해 교환 결합 효과에 의한 (BH)max 값의 증가는 미비하였다.
결론적으로, 순철 분말 환원 반응으로 SrM/Fe3O4 복합체를 제조할 수 있었으나 일반적인 소결 공정으로는 높은 소결밀도를 어려워 교환결합 효과에 의한 Ms 값의 향상은 확인했으나, (BH)max 값의 향상을 달성할 수 없었다. 만일 방전 플라즈마 소결법과 같은 다른 소결 공정으로 두 상의 공존을 유지하며 치밀화를 이룬다면 복합체에서의 교환 결합 효과로 SrM의 (BH)max 를 향상시킬 수 있을 것이라 사료된다.목차
I 서론 1
II 문헌연구 5
II.1 페라이트의 결정구조 5
Ⅱ.1.1 Mtype 육방정 페라이트 5
Ⅱ.1.2 Spinel 페라이트 8
II.2 자성체의 MH 곡선 11
II.3 자성체의 성능 향상을 위한 방법 13
Ⅱ.3.1 단자구화 13
Ⅱ.3.2 교환 결합 효과 16
II.4 페라이트의 합성법 19
Ⅱ.4.1 졸겔 반응법 19
Ⅱ.4.2 고상 연소법 20
Ⅱ.4.3 용융염 합성법 21
II.5 산화철의 수소 환원법 22
III 실험방법 25
Ⅲ.1 시료의 준비 25
Ⅲ.1.1 SrM 분말의 합성 25
Ⅲ.1.1.1 졸겔 연소법을 통한 SrM 분말 합성 25
Ⅲ.1.1.2 고상 반응법을 통한 SrM 분말 합성 27
Ⅲ.1.1.3 용융염 합성법을 통한 SrM 분말 합성 28
Ⅲ.1.2 수소 분위기 후열처리된 SrM/Fe3O4 복합체 제조 30
Ⅲ.1.2.1 SrM과 Fe2O3 혼합물의 소결 30
Ⅲ.1.2.2 SrM과 Fe2O3 pellet의 후열처리 32
Ⅲ.1.3 순철 분말 환원 반응된 SrM/Fe3O4 복합체 제조 32
Ⅲ.2 시료의 상, 미세구조, 소결밀도 및 자기적 특성 분석 33
IV 실험 결과 및 고찰 34
Ⅳ.1 수소 분위기 후열처리된 SrM/Fe3O4 복합체의 특성 34
Ⅳ.1.1 SrM의 크기와 Fe2O3의 양에 따른 소결 효과 34
Ⅳ.1.2 수소 분위기 후열처리의 효과 50
Ⅳ.2 순철 분말 환원 반응된 SrM/Fe3O4 복합체의 특성 66
V 결론. 90
VI 참고문헌 93
VII Abstract 97Maste
Direct Observation of Atomic-Scale Gliding on Hydrophilic Surfaces
No full textNanoscale friction behavior on hydrophilic surfaces (HS), influenced by a probe gliding on a confined water layer, has been investigated with friction force microscopy under various relative humidity (RH) conditions. The topographical and frictional responses of the mechanically exfoliated single-layer graphene (SLG) on native-oxide-covered silicon (SiO2/Si) and mica were both influenced by RH conditions. The ordinary phenomena at ambient conditions (i.e., higher friction on a HS than on a SLG due to different hydrophilicity), nondistinguishable height, friction of SLG with SiO2/Si at high RH (> 98%), and the superlubricating behavior of friction on a HS were observed. Furthermore, the subdomain within SLG, consisting of an ice-like water layer intercalated between SLG and SiO2/Si, showed friction enhancement. These results suggest that the abundant water molecules at the interface of the probe and a HS can make a slippery surface that overcomes capillary and viscosity effects through the gliding motion of the probe.11Nsciescopu
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Skin-attachable and biofriendly chitosan-diatom triboelectric nanogenerator
No full text© 2020 Published by Elsevier Ltd. Skin-attachable wearable devices including triboelectric nanogerators (TENGs) and self-powered sensors will have biofriendly and biocompatible issues, because they can directly interact with the human body. Based on the diatom frustule and chitosan, known as biocompatible ocean biomaterials, we developed a chitosan-diatom TENG for biofriendly and skin-attachable wearable devices. The highly porous diatom frustule, which is naturally abundant and mass-producible from mega-scale ocean environments, can be used as a biocompatible additive to greatly change electro-positivity and surface properties of chitosan films, showing bio-affinity and much higher output power. The time averaged power density of the chitosan-diatom triboelectric nanogenerator is measured as 15.7 mW/m2, which is 3.7 times higher than that of the pure chitosan TENG. For practical applications, the electrode-encapsulated chitosan-diatom film is successfully applied for a self-powered watch and a skin-attachable motion sensor. We believe that the diatom bio-silica can be used to make a synergy with other biomaterials for achieving the high power of TENGs for the skin-attachable wearable devices11sci
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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