Abstracts of Recent Publications

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Structure and Strength of Interfaces in Titanium-Coated Diamond-Glass Composites

Carey T. Williams, Chrysanthe Demetry, and Rounan Li, Ceramic Science and Engineering Proceedings (2000), in press.

Abstract:
This research investigated using titanium (Ti) as a coating on diamond particulate in order to strengthen the interface between diamond and a borosilicate glass matrix. Specific objectives were to determine which interface is weaker, diamond/Ti or Ti/glass, and to determine the presence and extent of carbide and/or oxide interlayers and their effect on interfacial strength. Both physically and chemically deposited titanium (Ti) coatings on diamond particulates were examined, and heat treatments of the coated diamond were also used to produce a variety of interfacial structures. Composites were prepared by hot pressing and were characterized by four point bend tests, fractography, X-ray diffraction, and Auger Electron Spectroscopy. The diamond/Ti interface was found to be weaker than the Ti/glass interface, for both coating processes and for all heat treatments examined. TiC formed at the diamond/Ti interface during the thermal coating process and increased the strength of the composite relative to samples containing uncoated diamond and physically Ti-coated diamond, where there was no carbide at the interface. TiC formed by heat treatment of the physically Ti-coated diamond, however, did not always result in an improvement of interfacial strength since extensive through-thickness cracks formed in the coatings on some diamond particle morphologies.


Design and Tailoring of Ni-Sn-W Composites for Bonded Abrasive Applications

Galina L. Kourtoukova, Chrysanthe Demetry, S. Ramanath, Richard M. Andrews, David S. Jacobs, and Ronald R. Biederman, Materials Science and Engineering A 276[1-2], 58-69 (2000).

Abstract:
The combination of properties ideal for metal bonds in abrasive products can rarely be achieved in a monolithic material. This research demonstrates a successful approach for producing a composite bond with higher elastic modulus without a significant increase in wear resistance, by taking advantage of the reaction between matrix and reinforcement to produce intermetallics. Composites comprised of a Ni-Sn matrix with continuous W fiber and/or W powder dispersoid were prepared by powder metallurgy methods. Composite specimens densified by hot pressing were examined and characterized with a combination of SEM and EDX analyses and measurements of Young's modulus, hardness, and wear resistance. A significant stiffening effect was observed; the elastic modulus of the composites was up to 30% greater than that predicted by a rule of mixtures based on the moduli of the fiber and matrix constituents alone. As desired, the wear resistance of the composite was approximately equal to that of the Ni-Sn matrix. One contribution to this combination of properties is believed to be the high elastic moduli and likely low fracture toughness of the Ni-W and Ni-Sn intermetallics that are formed. Properties of the Ni-Sn-W composites are contrasted with those of a Ni-Sn matrix reinforced with WC particulate, where no reaction occurs at the interface.


Grain size-dependent electrical properties of rutile (TiO2)

C. Demetry and X. Shi, Solid State Ionics 118 (1999) 271-279.

Abstract:
The electrical behavior of Sn-doped nanocrystalline and microcrystalline TiO2 has been studied by DC conductivity measurements and impedance spectroscopy. Specimens were hot pressed from powders synthesized by a solution chemical method, resulting in highly dense, 100% rutile with an average grain size of approximately 50 nm. Results indicate that nanocrystalline TiO2 has enhanced conductivity relative to microcrystalline TiO2, and a lower specific grain boundary resistivity. The activation energy for conduction in air of both nanocrystalline and microcrystalline TiO2 is lower than values obtained from the literature for polycrstalline and single crystal rutile. Furthermore, conductivity measurements over a range of oxygen partial pressures indicate that the conductivity of nanocrystalline and microcrystalline TiO2 exhibits a PO2-independent regime that is unique compared to past studies of polycrystalline and single crystal rutile.


Outcomes Assessment of Educational Approaches: Examples for Materials Science Educators

C. Demetry, Journal of Materials Education, Vol. 20, No. 1-2 (1998) 1-8.

Abstract:
The goal of outcomes assessment is to inform educators about how and what students are learning. It is an iterative process of defining and communicating educational goals and objectives, thinnking about criteria by which we will know when the objectives have been achieved, designing practices to assist students in meeting our expecations, measuring student performance against the criteria, and evaluating results and supplying feedback to appropriate audiences for continuous improvement of the educational process. In this paper, a general model for outcomes assessment is described that can be applied to a wide range of educational goals and activities and for a variety of purposes. Examples of small-scale classroom assessment specific to materials science educators are provided and critiqued.


Alumni-Contributed Materials Selection Projects in an Introductory Materials Science Course

C. Demetry, Journal of Materials Education, Vol. 20, No. 1-2 (1998) 125-133.

Abstract:
Materials selection projects supplied by WPI alumni in industry were introduced to a large enrollment introductory materials science course for all engineering majors. The rationale for introducing the project was multifold: 1) for students to integrate and go beyond course topics by addressing a real, open-ended materials selection problem recently faced by an alumnus in industry; 2) to generate more interest in materials science among students of all majors; and 3) to improve students' communication and teamwork skills. Projects were conducted in assigned teams, with checks for individual accountability. Each team prepared a written report and oral presentation, which together accounted for 25% of the overall course grade. Assessment of this course innovation has indicated that the alumni-contributed project topics did not generate more interest in materials science, in comparison to previous iterations of the course's project component, which included reverse engineering "product dissection" projects and instructor-generated materials selection topics.


Tactile sensing of density uniformity in powder beds after die filling

C. Demetry, F.S. Souto, B.C. Ryden, and J.M. Roy, Powder Technology 99 (1998) 119-124.

Abstract:
The use of tactile sensors has been explored as a novel means of assessing density uniformity of powders in molding dies prior to compaction. Nonuniform mass distribution after the die filling process was detected as gradients in the transmitted pressure by a tactile sensor located at the bottom of the die. The effect of process variables on fill uniformity was determined by comparing the coefficient of variation of transmitted pressure distributions. Direct measurements of density uniformity after compaction were consistent with the trends in pressure distribution measured by the tactile sensor. Thus, this work demonstrates the feasibility of using tactile sensors as a quality assessment, process development, or training tool in dry pressing operations.

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A Comparative Assessment of Students' Experiences in Two Instructional Formats of an Introductory Materials Science Course

C. Demetry and J.E. Groccia, Journal of Engineering Education 86[3], 202-210 (1997).

Abstract:
An educational experiment at Worcester Polytechnic Institute is described in which the instruction of the Introduction to Materials Science course (ES2001) was modified to incorporate active and cooperative learning. The overall goal was simultaneously to enhance educational quality and faculty productivity. Aspects of the course modification included use of "active" rather than traditional lectures, assignment of students to cooperative learning teams, introduction of a "product dissection project," and a "teacher as manager" approach to instruction, in which undergraduate Peer Learning Assistants and the graduate Teaching Assistant took on responsibilities as part of the instructional team. Data was gathered from 382 students in three traditional course offerings and two active/cooperative offerings, using various survey instruments to measure students' learning retention and performance, interest in materials science, their attitudes about learning, and their satisfaction with the course.

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The WPI-Norton Company Project Center: A University-Industry Partnership in Undergraduate Education

C. Demetry, Industry and Higher Education (Invited Contribution), 11[4] 218-223 (1997).

Abstract:
The WPI-Norton Company Project Center, in operation since 1973, is a partnership designed to enhance undergraduate engineering education at WPI and to solve problems of direct relevance to Norton Company. Student teams complete year-long projects on topic s proposed by Norton Company and are advised both by WPI faculty and Norton employees. This arrangement differs from co-operative education in that the projects satisfy a WPI degree requirement, and the students are not paid by the company. This paper des cribes some logistical details of the Center's operation and highlights successes and challenges revealed by a recent evaluation of the partnership.

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Measurements of Excess Enthalpy in Ultrafine-grained Titanium Dioxide

C.D. Terwilliger and Y.-M. Chiang, Journal of the American Ceramic Society 78[8], 2045-55 (1995).

Abstract:
Differential scanning calorimetry has been used to make direct measurements of the excess enthalpy of TiO2 (rutile) with an initial grain size of 30-70 nm. When the heat released during grain growth is normalized to the change in grain boundary area, the specific excess enthalpy at low temperatures and fine grain sizes (600-780 C, 30-200 nm) is found to be 0.5-1 J/m2, while values averaged over a larger temperature and size range (600-1300 C, 30 nm~2um) are 1.3-1.7 J/m2. After exclusion of extraneous cont ributions from other heat-dissipating processes, origins of a specific grain boundary enthalpy that increases with grain size or temperature are considered, including solute segregation, changes in grain boundary structure, and contributions from grain bo undary triple junctions. It is concluded that the most plausible explanation is a size-dependent nonstoichiometry of TiO2 due to the impingement of space charge layers in the temperature and grain size range of the experiments.

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Size-Dependent Solute Segregation and Total Solubility in Ultrafine Polycrystals: Ca in TiO2

C.D. Terwilliger and Y.-M. Chiang, Acta Metallurgica et Materialia 43[1], 319-328 (1995).

Abstract:
Grain boundary segregation at ultrafine grain sizes has been studied. Using a STEM microanalysis technique to quantify the grain boundary coverage of calcium (0.34 mole%) in TiO2 ranging in grain size from 50 nm to 750 nm, it is found that below grain siz es of 150-350 nm, segregation deviates from conventional isotherms, exhibiting a clear size dependence. In this size regime the interfacial area to volume ratio is as important as temperature and composition in determining grain boundary coverage. In the present system, grain boundaries become saturated with calcium when the coverage reaches approximately one half of an equivalent monolayer. The experimental results can be modeled by a statistical thermodynamical treatment of segregation which takes into account the large density of grain boundary sites in this size range. We also find direct evidence of enhanced total solubility at very fine grain sizes due to grain boundary segregation.

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The Effect of Calcium Segregation on Grain Growth in Nanocrystalline TiO2

C.D. Terwilliger and Y.-M. Chiang, NanoStructured Materials 4[6], 651-661 (1994).

Abstract:
Differential scanning calorimetry and transmission electron microscopy are used to monitor grain growth in nanocrystalline TiO2 doped with Sn and Ca. TiO2 powder is prepared by a solution chemical method, and consolidated samples have an initial mean grai n size of 30-50 nm, with a density greater than 95% of theoretical. Calcium is an effective grain growth inhibitor; the present samples show more resistance to grain growth than other nanocrystalline TiO2 reported to date. A Kissinger analysis of grain gr owth exotherms indicates an effective activation energy of 1.3-1.5 eV, and calcium additions do not have a large effect on this value. These observations are discussed in light of the size-dependent segregation behavior that has been observed in these sam ples using STEM microanalysis.

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Characterization of Chemically- and Physically-Derived Nanophase Titanium Dioxide

C.D. Terwilliger and Y.-M. Chiang, NanoStructured Materials 2, 37-45 (1993).

Abstract:
A solution-based processing method has been used to synthesize nanocrystalline TiO2 powder. By doping with a small amount of Sn, the rutile phase is formed with an average grain size <20 nm. These powders can be hot-pressed into pellets of 95% theoreti cal bulk density while maintaining an average grain size between 30-50 nm. Other pellets have been prepared similarly using powder synthesized by the inert gas condensation process. Here we report on the characterization of both types of nanophase TiO2 by x-ray diffraction and transmission electron microscopy. While the physically-derived samples have somewhat finer grain sizes, the Sn-doped chemically-derived samples are more readily densified, appear to be highly stoichiometric, and are free of the anat ase phase.

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