Publications

An experimental study is described on the fracture toughness and micro-mechanisms associated with the initiation and propagation of cracks in metallic nickel containing marked gradients in grain size, ranging from ∼30 nm to ∼4 μm. Specifically, cracks are grown in a gradient structured (GS) nickel with grain-size gradient ranging from the coarse macro-scale to nano-scale (CG → NG) and vice versa (NG → CG), with the measured crack-resistance R-curves compared to the corresponding behavior in uniform nano-grained (NG) and coarse-grained (CG) materials. It is found that the gradient structures display a much-improved combination of high strength and toughness compared to uniform grain-sized materials. However, based on J-integral measurements in the gradient materials, the crack-initiation toughness is far higher for cracks grown in the direction of the coarse-to-nano grained gradient than vice versa, a result which we ascribe primarily to excessive crack-tip blunting in the coarse-grained microstructure. Both gradient structures, however, display marked rising R-curve behavior with exceptional crack-growth toughnesses exceeding 200 MPa.m½.

We report on measurements of Seebeck effect in single-crystal VO(2) microbeams across their metal-insulator phase transition. One-dimensionally aligned metal-insulator domain walls were reversibly created and eliminated along single VO(2) beams by varying temperature, which allows for accurate extraction of the net contribution to the Seebeck effect from these domain walls. We observed significantly lower Seebeck coefficient in the metal-insulator coexisting regime than predicted by a linear combination of contributions from the insulator and metal domains. This indicates that the net contribution of the domain walls has an opposite sign from that of the insulator and metal phases separately. Possible origins that may be responsible for this unexpected effect were discussed in the context of complications in this correlated electron material.

This work reports on zeolitic imidazolate framework (ZIF)-coupled microscale resonators for highly sensitive and selective gas detection. The combination of microscale resonators and nanoscale materials simultaneously permits the benefit of larger capture area for adsorption from the resonator and enhanced surface adsorption capacity from the nanoscale ZIF structure. Dielectrophoresis (DEP) was demonstrated as a novel method for directly assembling concentrated ZIF nanoparticles on targeted regions of silicon resonant sensors. As part of the dielectrophoretic assembly process, the first ever measurements of the Clausius-Mossotti factor for ZIFs were conducted to determine optimal conditions for DEP assembly. The first ever real-time adsorption measurements of ZIFs were also performed to investigate the possibility of inherent gas selectivity. The ZIF-coupled resonators demonstrated sensitivity improvement up to 150 times over a bare silicon resonator with identical dimensions, and real-time adsorption measurements of ZIFs revealed different adsorption time constants for IPA and CO2.

Disruptions in activity are core features of mood states in bipolar disorder (BD). In this study, we sought to identify activity patterns that discriminate between mood states in BD. Locomotor activity was collected by using actigraphy for 6 weeks in participants with interepisode BD Type I ( n = 37) or participants with no lifetime mood disorders ( n = 39). The 24-hr activity pattern of each participant-day was characterized and within-person differences in activity patterns were examined across mood states. Results showed that among participants with BD, depressive days are distinguished from other mood states by an overall lower activity level and a pattern of later activity onset, a midday elevation of activity, and low evening activity. No distinct within-person activity patterns were found for hypomanic/manic days. Given that activity can be monitored noninvasively for extended time periods, activity pattern identification may be leveraged to detect mood states in BD, thereby providing more immediate delivery of care.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Frictional energy dissipation at the interfaces of two-dimensional (2D) materials through the excitation and transfer processes of kinetic energy into the bulk can be easily influenced by an intercalated water film. An enhancement of friction on water-intercalated graphene has been observed. Is this frictional enhancement by confined water a general phenomenon? We address this issue by investigating the frictional behavior of confined water layers intercalated between single-layer molybdenum disulfide (MoS2), synthesized using chemical vapor deposition, and a silica substrate. The icelike water was intercalated by exposure to high-humidity air. We found that the intercalated water molecules morphologically deform the 2D MoS2 sheet, forming distinct subdomains after the exposure to high humidity. We found that the adsorption of the icelike water layer between the MoS2 and the silica leads to friction enhancement, compared with a pristine MoS2/silica sample, which is associated with additional phononic friction energy dissipation at the solid-liquid interface, as indicated by the phonon distribution analysis from the empirical force-field calculations. Moreover, the atomic stick-slip behavior shows that the lattice orientation of the hydrophilic MoS2 affects water molecule diffusion at the interface of the MoS2/silica substrate. Chemical mapping of the water-intercalated MoS2 on silica using scanning photoelectron microscopy and vacuum annealing processes shows water intercalation without changing the intrinsic composition of the MoS2 on silica.

We consider the problem of fair resource allocation in a system containing different resource types, where each user may have different demands for each resource. To address this problem, we propose Dominant Resource Fairness (DRF), a generalization of max-min fairness to multiple resource types. We show that DRF, unlike other possible policies, satisfies several highly desirable properties. First, DRF incentivizes users to share resources, by ensuring that no user is better off if resources are equally partitioned among them. Second, DRF is strategy-proof, as a user cannot increase her allocation by lying about her requirements. Third, DRF is envy-free, as no user would want to trade her allocation with that of another user. Finally, DRF allocations are Pareto efficient, as it is not possible to improve the allocation of a user without decreasing the allocation of another user. We have implemented DRF in the Mesos cluster resource manager, and show that it leads to better throughput and fairness than the slot-based fair sharing schemes in current cluster schedulers.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.