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Effect of composition of SiAlON ceramics on microwave dielectric properties
No full text학위논문(석사) - 한국과학기술원 : 신소재공학과, 2005.2, [ [iv], 81 p. ]SiAlON은 에서 Si와 N이 Al과 O로 각각 치환된 고용체(solid solution) 형태의 세라믹 재료로서 와 동일한 결정구조를 가지고 있으며 치환된 Al-O의 결합에 의한 결정 격자의 변형량(lattice strain)이 적어 치환 고용체 형성이 용이하다. 상온에서의 SiAlON의 기계적 특성은 와 유사하지만, 보다 우수한 소결성, 내 부식성, 내 산화성 등의 높은 화학적 안정성을 나타낸다.
SiAlON에는 α, β 의 두 종류의 상이 존재하며, β상은 β - 의 결정 구조에서 Si와 N이 Al과 O로 치환된 형태인 것에 비해, α 상은 Si와 N이 Al과 O로 치환됨과 동시에 α - 결정 구조 내부의 격자간 위치(interstitial site)에 금속 이온이 침투해 있는 구조를 나타낸다. α 상은 상대적으로 경도가 높으며, β 상은 강도 및 파괴인성이 높은 특성을 보인다. SiAlON의 경우 α 상과 β 상 모두 넓은 고용범위를 가지고 있어 조성변화를 통하거나 , 등의 소결조제들을 첨가하여 기계적 특성과 화학적 특성을 조절할 수 있다. 소결조제를 첨가한 경우 소결체의 조직 내부에 elongated grain을 형성하여 파괴 인성이나 강도를 향상 시킬 수 있으며, 고용되는 Al, O 및 금속 이온의 양이 조절되면 밀도, 강도, 경도, 영률, 열팽창계수, 열전도도, 내 산화성, 내 부식성 등의 다양한 특성 조절이 가능하여, 고온에서도 강도 및 경도 유지가 가능하고 내 산화성이 뛰어난 특성을 보인다.
SiAlON의 이러한 고온에서의 우수한 특성으로 인해 고온용 금속 성형틀, 금속 절단 공구 등의 구조용 재료로 널리 연구되어 왔으며, 상온에서 고온에 이르기까지 다양한 기계적 특성 및 화학적 특성에 대한 연구가 이루어지고 있다. 또한, 고온용 마이크로파 투과창인 질화물계 고온용 레이돔으로의 적용 가능성도 검토되고 있으나, 아직까지 SiAlON의 유전 특성, 특히 고온 유전특성에 관한 연구는 거의 이루어져 있지 않다.
본 연구에서는 조성이 다른 다양한 SiAlON을 제조하고, SiAlON의 조성변화가 마이크로파 유전 특성에 미치는 영향을 고찰하였다. 소결조제와 기공의 효과를 최소화 하기 위해 고온가압소결 방법을 통해 이론밀도에 가까운 β -SiAlON을 조성별로 제조하여 상온 및 고온에서의 유전 특성을 분석하였으며, 이트륨이 포함된 α -SiAlON 에서 소결조제가 유전특성에 미치는 영향에 대해 평가하였다.한국과학기술원 : 신소재공학과
평면 충격파를 이용한 나노결정화 물질의 압축 성형 및 특성 평가
No full text학위논문(박사) - 한국과학기술원 : 신소재공학과, 2010.2 , [ xii, 127 p. ]Nanocrystalline materials have been widely focused for the interesting subject of research for a few of decades, and their understanding have been advanced significantly in the last few years. Nanocrystalline materials are structurally characterized by a large volume fraction of grain boundaries, which may significantly alter their physical, mechanical, and chemical properties in comparison with conventional coarse-grained polycrystalline material because of the change of the volume fraction of inter-crystal regions and triple-junctions. Thus, nanocrystalline materials provide industrial or commercial applications an attractive potential with their novel properties mentioned above. A number of techniques have suggested and explored for couple of decades to produce nanostructured materials, but most of them takes a multiple processes and long time to achieve bulk specimen. Moreover, the inevitable heat-treatment to bond particles with each other promotes the grains to grow.
Single-step consolidation of powders using high pressure shock-waves generated by planar impact or explosives is considered to be a potentially important method for the synthesis and processing of bulk nanocrystalline materials. The shock-wave compaction (SWC) processes occur during on microsecond timescale and involve the heterogeneous deposition of shock energy, resulting in interparticle bonding and configurational changes in particles, such as the dislocations, subgrains, distorted regions and partially melted regions. Thus, SWC-processed dense monolithic bulks or composites are considerably different from samples densified conventionally via powder processing techniques that involve additional sintering steps at high temperature. Rapid consolidation of powders by SWC can form bulk material that retaining the nanosize of the powder without any substantial grain growth
In this study, nanocrystalline bulk materials were fabricated by planar shock wave compaction technique without addit...한국과학기술원 : 신소재공학과
Shock compaction of Gd-doped ceria ceramics
No full textGd-doped ceria nano powder(similar to 5 nm) was shock-loaded by a plate impact experiment using a single stage light gas gun. A computational model was used to simulate the shock state(pressure and density changes along time) of the sample in two dimensional Eulerian code. The predicted density of compacted sample from the simulation was about 90%. To reveal the effect of shock compaction on sintering behavior, the recovered sample was heat-treated and the microstructure was compared with that of a conventionally compacted and heat-treated sample.This work was supported by Defense Acquisition Program Administration and Agency
for Defense Development under the contract UD060006AD
Dielectric properties of b-SiAlON at high temperature using perturbation method
No full textb-SiAlON with various z-values (z = 0.5~4.0) were produced by hot pressing. The dielectric properties (dielectric constant and tangent loss) of b-SiAlON were characterized by the post-resonator method at room temperature and by the perturbation method from room temperature to 1200 oC at 2.45 GHz, respectively. Effect of z-values and temperatures with b-SiAlON were investigated
ATTITUDES TOWARD AN INFORMATION AND COMMUNICATION TECHNOLOGY (ICT) INSTRUMENT APPLICABILITY IN OLDER ADULTS
Full text linkThesis (Ph.D.)--Michigan State University. Social Work - Doctor of Philosophy, 2025Contemporary society is quickly changing with digital and technology advancements. However, due to a lack of former experience and cognitive and physical challenges, older adults often fall into the digital divide, the gap between technology users and non-users. Such a gap exacerbates the already-existing disparities in health, information, financial, and social arenas older adults may experience. Therefore, understanding the factors promoting older adults to better use and understand technology became an important issue. Among different types of technology, this study focuses on information and communication technology (ICT) that plays a vital role in facilitating communication and interaction for older adults. While existing studies have explored factors influencing older adults' ICT use, including barriers, benefits, and usage patterns, limited research has focused specifically on their attitudes toward ICT. Attitudes are crucial in determining behavioral intentions and technology adoption, yet there is a lack of a standardized measurement for assessing older adults' perceptions of ICT. Most existing instruments focus on general technology or specific devices, such as computers, rather than ICT as a whole, and thereby miss the quickly changing digital landscape. This gap in research highlights the need for a more refined approach to understanding older adults' attitudes toward ICT. Therefore, this study aims to examine the applicability of attitudes toward ICT scale among the older adult population via reliability, confirmatory factor analysis (CFA), convergent validity, and concurrent validity tests. The scale chosen for the reliability and validity testing is 12 attitudinal items from the Media and Technology Usage and Attitudes Scale (MTUAS) developed by Rosen et al. (2013). The scale consist of both affective and cognitive attitudes, was tested for reliability and validity, and was focused on technology, which is broad enough to be modified into ICT. Respondents to the current survey were 231 older adults in Michigan who are 65 years old or older, and able to read and answer the survey in English. MTUAS attitudes toward ICT scale showed good internal reliability. CFA results showed Comparative Fit Index (CFI) and Tucker-Lewis Index (TLI) above and equal to the cut-off line respectively, indicating a good fit, while Standardized Root Mean Square Residual (SRMR) and Root Mean Square Error of Approximation (RMSEA) exceeded the cut-off line, indicating moderate fit. Convergent validity testing between MTUAS attitudes toward ICT scale and attitudes toward ICT semantic questions showed a moderate-to-strong relationship between the two constructs. Concurrent validity testing between MTUAS attitudes toward ICT scale and behavioral intention of using ICT questions showed a moderate relationship between the two constructs. The findings suggest that while the scale demonstrates a moderately good fit, some items showing low correlations in the correlation analysis highlight areas for improvement. Furthermore, convergent and concurrent validity testing suggests that certain items, particularly those measuring negative attitudes, require revision as they exhibited weak associations with the overall factor structure. This study highlights an additional research gap, as few existing questionnaires assess technology dependence in older adults. The results indicate that older adults can relate to technology dependence, not just in terms of technophobia but also in anxiety stemming from not being able to use technology. This demonstrates the need for further research into the affective and cognitive, as well as positive and negative dimensions of technology attitudes among older adult populations. This study contributes significantly to the understanding of older adults' attitudes toward ICT and emphasizes the need for more targeted research, policy improvements, and social work interventions to support older adults' evolving digital experiences.Description based on online resource. Title from PDF t.p. (Michigan State University Fedora Repository, viewed ).Includes bibliographical references
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe 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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