Protein Science (Journal)

Lactococcus lactis is a promising host for (membrane) protein overproduction. Here, we describe a protocol for incorporation of selenomethionine (SeMet) into proteins expressed in L. lactis. Incorporation efficiencies of SeMet in the membrane protein complex OpuA (an ABC transporter) and the soluble protein OppA, both from L. lactis, were monitored by mass spectrometry. Both proteins incorporated SeMet with high efficiencies (>90%), which greatly extends the usefulness of the expression host L. lactis for X-ray crystallography purposes. The crystal structure of ligand-free OppA was determined at 2.4 Å resolution by a semiautomatic approach using selenium single-wavelength anomalous diffraction phasing.

PHYSICAL REVIEW B 78, 024109 (2008)

We have studied the effect of Ca-doping in single-crystal Tb1−xCaxMnO3(x<=0.1) on the crystal and magnetic structure, magnetocapacitance, and electric polarization. For low doping (x=0.05), the presence of Mn4+ ions gives rise to a state with behavior resembling that of a relaxor ferroelectric. The coherence length of the Mn magnetic spin spiral is reduced, while the Mn-modulation wave vector is unchanged. For doping larger than 5%, the ferroelectric state is suppressed, which we ascribe to breakdown of the spiral magnetic structure.

Journal of Physics: Condensed Matter 19, 476214 (2007)

An experimental study on Mn-dopant-induced effects in Zn1−xMnxO was performed for samples with x = 0.02, 0.04, 0.06, and 0.08, which were prepared by solid state reaction at 1200 ◦C. The result of x-ray diffraction refinement analysis did not turn up evidence of MnxOy cluster inclusion over the entire doping range, while the structural parameter variations indicate a solubility limit of about 6% for Mn2+ and incorporation of Mn ions of higher valency at higher dopant levels. The magnetization data exhibit mostly intrinsic antiferromagnetic instead of ferromagnetic properties, as predicted by more recent theoretical works. The substitutional incorporation of the Mn in ZnO was further confirmed by the associated broad-band photoluminescence spectra. The suggested incorporation of Mn ions of higher valency at higher dopant concentration was also supported by the related x-ray photoemission data, and in agreement with a previous theoretical prediction.

Acta Materialia, Volume 55, Issue 19, November 2007, Pages 6408-6415

Abstract:

This paper reports a study of the indentation of a model polycrystal using two-dimensional discrete dislocation plasticity. The polycrystal consists of square grains having the same orientation. Grain boundaries are modelled as being impenetrable to dislocations. Every grain has three slip systems, with a random distribution of initial sources and obstacles, and edge dislocations that glide in a drag-controlled manner. The indenter is wedge shaped, so that the indentation depth is the only geometrical length scale. The microstructural length scale on which we focus attention is the grain size, which is varied from 0.625 to 5 μm. While the predicted uniaxial yield strength of the polycrystals follows the Hall–Petch relation, this grain size dependence couples to the dependence on indentation depth. Polycrystals with a sufficiently large grain size exhibit the same “smaller is harder� dependence on indentation depth as single crystals, but an inverse indentation depth dependence occurs for fine-grained materials. For sufficiently deep indentation, the predicted nominal hardness is found to scale with grain size d according to $H_N=H_N^\infty(1+d^*/d)^{1/2}$, where $H_N^\infty$ is the single-crystal nominal hardness and $d^*$ is a material length scale.

Keywords: Discrete dislocations; Nano-indentation; Polycrystals; Size effects; Grain-size effects

Copyright © 2007 Acta Materialia Inc. Published by Elsevier Ltd.

PhD thesis series: Zernike Institute for Advanced Materials, University of Groningen

Since a long time, indentation testing is a commonly used technique to measure the properties of materials such as hardness and Young's modulus. Recently, nano-indentation is increasingly employed to investigate the properties of very small volumes of material as encountered in today's miniature technology. While an indentation experiment is performed relatively easily, the interpretation of the outcome is far from being trivial. The minimum requirement is the availability of a theory for the deformation processes taking place during indentation. As far as the plastic deformation of metals is concerned, this is precisely the scientific challenge because plasticity in volumes of cubic micrometers is size dependent. While the origin and description of this size dependence are subject of intense debate, one thing is certain: classical plasticity theories do not apply at length scales of tens of micrometers and below since these theories do not contain a material length scale and are therefore size independent.

In this research, a model is adopted that does have an inherent material length scale and aims at bridging the length scale gap between atomistics and continuum theories: discrete dislocation plasticity. In this theory, plasticity is viewed as originating from the collective motion of dislocations, which are described as line defects in a linear elastic continuum.

Experiments have convincingly demonstrated that indentation at the nano or micro scale also reveal a size effect.