T. Waitz

Quantifying thiol ligand density of self-assembled monolayers on gold nanoparticles by inductively coupled plasma-mass spectrometry.

Gold nanoparticles (GNPs) are often used as colloidal carriers in numerous applications owing to their low-cost and size-controlled preparation as well as their straightforward surface functionalization with thiol containing molecules forming self-assembling monolayers (SAM). The quantification of the ligand density of such modified GNPs is technically challenging, yet of utmost importance for quality control in many applications. In this contribution, a new method for the determination of the surface coverage of GNPs with thiol containing ligands is proposed. It makes use of the measurement of the gold-to-sulfur (Au/S) ratio by inductively coupled plasma mass spectrometry (ICP–MS) and its dependence on the nanoparticle diameter. The simultaneous ICP–MS measurement of gold and sulfur was carefully validated and found to be a robust method with a relative standard uncertainty of lower than 10%. A major advantage of this method is the independence from sample preparation; for example, sample loss during the washing steps is not affecting the results. To demonstrate the utility of the straightforward method, GNPs of different diameters were synthesized and derivatized on the surface with bifunctional (lipophilic) ω-mercapto-alkanoic acids and (hydrophilic) mercapto-poly(ethylene glycol) (PEG)n-carboxylic acids, respectively, by self-assembling monolayer (SAM) formation. Thereby, a size-independent but ligand-chain length-dependent ligand density was found. The surface coverage increases from 4.3 to 6.3 molecules nm–2 with a decrease of ligand chain length from 3.52 to 0.68 nm. Furthermore, no significant difference between the surface coverage of hydrophilic and lipophilic ligands with approximately the same ligand length was found, indicating that sterical hindrance is of more importance than, for example, intermolecular strand interactions of Van der Waals forces as claimed in other studies.

Size effects on martensitic phase transformations in nanocrystalline NiTi shape memory alloys

Abstract Results of a systematic study are presented to review various effects of crystal size on the martensitic phase transformations in nanocrystalline NiTi shape memory alloys. The transformation temperatures and the transformed volume fraction strongly decrease with decreasing grain size less than about 100 nm. Transformation to martensite is not observed in grains smaller than a critical grain size of about 50 nm. The nanograins significantly impact the morphology of B19′ martensite composed of (001) compound twins that occur at an atomic scale and violate the well established theory of martensite formation. Self-accommodation occurs by a herringbone morphology of two twinned variants. Contrary to the martensite, grain size hardly impacts the transformation to the R-phase. The experimental results are explained by a size dependent transformation barrier that accounts for the suppression of the martensitic transformation, its thermal stability and unique morphology in the nanograins.

The f.c.c. to h.c.p. martensitic phase transformation in CoNi studied by TEM and AFM methods

Abstract The thermally induced martensitic phase transformation from the high temperature (f.c.c.) to the low temperature (h.c.p.) phase was studied in a Co 32% Ni single crystal by transmission electron microscopy (TEM), atomic force microscopy (AFM) and light microscopy with multiple beam interferometry (MBI). Quantitative analysis of the TEM results shows that the transformation takes place by consecutive glide of partial dislocations of the same Shockley partial Burgers vector on every other close packed plane. The tapering h.c.p. lamellae contain high shear strains and cause long range internal stresses that facilitate transformation induced plasticity. The AFM and MBI results show that the transformation shear strains are compensated on a mesoscopic scale. This indicates that the transformation induced stresses trigger the formation of new self-accommodating h.c.p. lamellae by an autocatalytic process. It should be pointed out that the reverse transformation (h.c.p. → f.c.c.) previously investigated in the same material showed the occurrence of a different transformation mechanism based on an atomistic compensation.

Thermal stability and phase transformations of martensitic Ti-Nb alloys.

Abstract Aiming at understanding the governing microstructural phenomena during heat treatments of Ni-free Ti-based shape memory materials for biomedical applications, a series of Ti–Nb alloys with Nb concentrations up to 29 wt% was produced by cold-crucible casting, followed by homogenization treatment and water quenching. Despite the large amount of literature available concerning the thermal stability and ageing behavior of Ti–Nb alloys, only few studies were performed dealing with the isochronal transformation behavior of initially martensitic Ti–Nb alloys. In this work, the formation of martensites (α′ and α″) and their stability under different thermal processing conditions were investigated by a combination of x-ray diffraction, differential scanning calorimetry, dilatometry and electron microscopy. The effect of Nb additions on the structural competition in correlation with stable and metastable phase diagrams was also studied. Alloys with 24 wt% Nb or less undergo a transformation sequence on heating from room temperature to 1155 K. In alloys containing >24 wt% Nb α″ martensitically reverts back to β0, which is highly unstable against chemical demixing by formation of isothermal ωiso. During slow cooling from the single phase β domain α precipitates and only very limited amounts of α″ martensite form.

Comparative method evaluation for size and size-distribution analysis of gold nanoparticles

Gold nanoparticles (GNPs) are popular colloidal substrates in various sensor, imaging, and nanomedicine applications. In separation science, they have raised some interest as a support for sample preparation. Reasons for their popularity are their low cost, ability for size-controlled synthesis with well-defined narrow nanoparticle size distributions, as well as straightforward surface functionalization by self-assembling (thiol-containing) molecules on the surface, which allows flexible introduction of functionalities for the selective capture of analytes. Most commonly, the method of first choice for size determination is dynamic light scattering (DLS). However, DLS has some serious shortcomings, and results from DLS may be misleading. For this reason, in this contribution several distinct complementary nanoparticle sizing methodologies were utilized and compared to characterize citrate-capped GNPs of different diameters in the range of 13-26 nm. Weaknesses and strengths of DLS, transmission electron microscopy, asymmetrical-flow field-flow fractionation and nanoelectrospray gas-phase electrophoretic mobility molecular analysis are discussed and the results comparatively assessed. Furthermore, the distinct GNPs were characterized by measuring their zeta-potential and surface plasmon resonance spectra. Overall, the combination of methods for GNP characterization gives a more realistic and comprehensive picture of their real physicochemical properties, (hydrodynamic) diameter, and size distribution.

Martensitic phase transformations of nanocrystalline NiTi shape memory alloys processed by repeated cold rolling

Abstract The impact of grain size on the martensitic phase transformations of bulk nanocrystalline NiTi shape memory alloys processed by repeated cold rolling was systematically studied by differential scanning calorimetry and transmission electron microscopy. With decreasing grain size, the formation of the martensite is strongly suppressed and its thermal stability decreases. The effect of grain size on the intermediate R-phase is much smaller than that observed in the case of the martensite. Reversible and irreversible contributions to the Gibbs free energy of the martensite were obtained that are larger than those arising from the formation of martensite in coarse grains. Considering the dependence of the energy barrier on the transformation eigenstrain and the grain size, the experimental results were modelled within the general thermodynamic framework of the martensitic phase transformation.

Mechanical properties, structural and texture evolution of biocompatible Ti-45Nb alloy processed by severe plastic deformation

Biocompatible β Ti-45Nb (wt%) alloys were subjected to different methods of severe plastic deformation (SPD) in order to increase the mechanical strength without increasing the low Young׳s modulus thus avoiding the stress shielding effect. The mechanical properties, microstructural changes and texture evolution were investigated, by means of tensile, microhardness and nanoindentation tests, as well as TEM and XRD. Significant increases of hardness and ultimate tensile strength up to a factor 1.6 and 2, respectively, could be achieved depending on the SPD method applied (hydrostatic extrusion - HE, high pressure torsion - HPT, and rolling and folding - R&F), while maintaining the considerable ductility. Due to the high content of β-stabilizing Nb, the initial lattice structure turned out to be stable upon all of the SPD methods applied. This explains why with all SPD methods the apparent Young׳s modulus measured by nanoindentation did not exceed that of the non-processed material. For its variations below that level, they could be quantitatively related to changes in the SPD-induced texture, by means of calculations of the Young׳s modulus on basis of the texture data which were carefully measured for all different SPD techniques and strains. This is especially true for the significant decrease of Young׳s modulus for increasing R&F processing which is thus identified as a texture effect. Considering the mechanical biocompatibility (percentage of hardness over Young׳s modulus), a value of 3-4% is achieved with all the SPD routes applied which recommends them for enhancing β Ti-alloys for biomedical applications.

Transformation dislocations in Co–Fe

Abstract High-resolution transmission electron microscopy (HRTEM) studies indicate a compensation of the strain fields of the dislocations involved in the hexagonal close packed to double hexagonal close packed transformation of Co–Fe. In the HRTEM images only the edge component of the dislocations can be determined by Burgers circuits whereas the screw components are expected to cause an Eshelby twist in the thin foils. The influence of the Eshelby twist on HRTEM images was determined by contrast simulations. Dislocation arrangements with compensating screw components can be distinguished form those with non-compensating screw components.

Crystalline-to-amorphous transformation in intermetallic compounds by severe plastic deformation

Process of nanostructure formation and crystalline-to-amorphous transformation (CTAT) by high pressure torsion have been studied for various intermetallic compounds, such as, TiNi, ZrCu and Ni3Al. These compounds have been chosen to elucidate the effect of crystal structure on CTAT. Crystal structures of the samples were B2 or B19’ martensite for TiNi, Cm martensite for ZrCu and L12 for Ni3Al.

Nanoscale Phase Transformations in Functional Materials

The in-depth knowledge of the complex mechanisms of phase transformations of nanoscale functional materials is a prerequisite for controlling their properties. With a special emphasis on ferroic systems, the present chapter gives a review on phase transformations of various nanostructured functional materials. The review includes their size dependent properties, as well as corresponding physical concepts of nonextensive nanothermodynamics, phase fluctuations, critical temperatures, scaling laws, transition pathways, and domain formation.