Origins of saccharin-induced stress reduction based on observed fracture behavior of electrodeposited Ni films

This research presents experimental results of an investigation aimed at understanding grain size driven mechanical processes in electrodeposited Ni thin films where saccharine additions are commonly used to improve mechanical properties. Ni films were fabricated using salfamate-based electro chemical baths, where it is empirically known that mmol/l concentrations of saccharine will reduce the observed tensile stress in addition to lowering the grain size up to a few nanometer scales. Some previous observations and several theoretical models suggest that saccharine incorporation results in sulfur segregation at grain boundaries. Since grain boundary formation is also associated with tensile stress evolution, a plausible hypothesis is that saccharine additions are directly altering grain boundary energetics. This suggests that saccharine additions should also have an observable effect on intergranular fracture in these films. To test this prediction, in situ stress measurements during film growth and fracture testing of these same films were compared. Lithographically patterned substrates were used to produce films with ordered arrays of uniform islands, which demonstrated island size effects on stress evolution, and enabled a well-defined notch geometry along one of the island boundaries to facilitate fracture experiments. In situ uniaxial tensile testing under in a scanning electron microscope was then used to obtain the fracture strength of such specimens. This technique provided real time recording of microscopic deformation during uniaxial tensile loading. The observed relationships among residual stress, grain size, and fracture strength were then analyzed with detailed models of both film growth and fracture.     

Quantitative characterization of the composition, thickness and orientation of thin films in the analytical electron microscope

Compositional variations in thin films can introduce lattice-parameter changes and thus create stresses, in addition to the more usual stresses introduced by substrate-film mismatch, differential thermal expansion, etc. Analytical electron microscopy comprising X-ray energy-dispersive spectrometry within a probe-forming field-emission gun scanning transmission electron microscope (STEM) is one of the most powerful methods of composition measurement on the nanometer scale, essential for thin-film analysis. Recently, with the development of improved X-ray collection efficiencies and quantitative computation methods it has proved possible to map out composition variations in thin films with a spatial resolution approaching 1-2 nm. Because the absorption of X-rays is dependent on the film thickness, concurrent composition and film thickness determination is another advantage of X-ray microanalysis, thus correlating thickness and composition variations, either of which may contribute to stresses in the film. Specific phenomena such as segregation to interfaces and boundaries in the film are ideally suited to analysis by X-ray mapping. This approach also permits multiple boundaries to be examined, giving some statistical certainty to the analysis particularly in nano-crystalline materials with grain sizes greater than the film thickness. Boundary segregation is strongly affected by crystallographic misorientation and it is now possible to map out the orientation between many different grains in the (S)TEM.     

Diffusion size effect

On the basis of the fact that the mean square displacement of atoms in the vicinity of the surface is higher than that in the bulk, a thin film diffusion size effect is introduced. In films of the order of 100 Å thick it leads to a marked increase in the diffusivity as the film thickness decreases. The size effect introduced is used to explain experimental data on the high diffusivity in thin films. The approach developed should also be taken into account when consideration is given to other thermally activated processes in thin films as well as in the vicinity of free surfaces, interfaces and grain boundaries (segregation, vacancy distribution and migration to the surface, phase formation etc.).     

A model for intergranular segregation/dilution induced by applied stress

A model for the effects of low applied stress on grain boundary segregation/dilution of solute has been suggested in the present paper. This model is based on the following assumptions: (1) The grain boundary is a weaker region on strength than the perfect crystalline in the interior of gain and will preferentially be deformed when a polycrystalline is exerted by an low applied stress. (2) Grain boundaries will work as sources of vacancies to emit vacancies when a compression stress is exerted on them and as sinks to absorb vacancies when a tension stress is exerted; (3) Oversaturated vacancies induced by the applied stress will be combined with the solute atoms to form vacancy-solute atom complexes, the diffusion rate of which is far greater than that of solute atoms in matrix; (4) The effects of applied stress on grain boundary segregation/dilution of solute will be controlled by the balance between the complex diffusion and the reverse solute atom diffusion. According to this model, there will be a critical time during stress aging, at which a maximum level of grain-boundary segregation/dilution will occur. This model can be corroborated by Shinoda and Nakamura's observation for phosphorus and Misra's observation for sulfur in steels. It can be expected that a new basis for understanding the low ductility intergranular fracture induced by applied stress will result from this new model.     

Peritectic melting of thin films,superheating and applications in growth of REBCO superconductors

Superheating of solids, an unconventional phenomenon in nature, can be achieved by suppressing the heterogeneous nucleation of melt at defect sites, such as free surfaces and internal grain boundaries. In recent years, experimental evidences have clearly proved that the YBCO (Y123) thin film with a free surface possesses a superheating capacity, which is mainly attributed to the film/substrate structures, distinctively consisting with low-energy surface and semi-coherent interface. Like most functional oxides, YBCO (denoted as α phase) is characterized by a peritectic melting: α → β + liq. Its superheating behavior certainly relates to this peritectic reaction. Furthermore, REBCO (RE123, RE: rare earth elements) thin films with high thermal stability have been successfully employed as seed materials in inducing the growth of REBCO materials, such as thick film, single crystal and single domain bulk. Therefore, this superheating property of thin films is of great importance in both scientific study and practical application. In this paper, the up-to-date researches covering on the superheating phenomenon of the α phase film, its mechanism and applications in growth of REBCO superconductors are reviewed, which is supposed to be valid for more thin films of functional oxides that have the same nature as the YBCO film/substrate.     

Engineering magnetic nanostructures with inverse hysteresis loops

Top-down lithography techniques allow the fabrication of nanostructured elements with novel spin configurations,which provide a new route to engineer and manipulate the magnetic response of sensors and electronic devices and understand the role of fundamental interactions in materials science.In this study, shallow nanostructure-pattemed thin films were designed to present inverse magnetization curves,i.e.,an anomalous magnetic mechanism characterized by a negative coercivity and negative remanence.This procedure involved a method for manipulating the spin configuration that yielded a negative coercivity after the patterning of a single material layer.Patterned NiFe thin films with trench depths between 15％-25％ of the total film thickness exhibited inverse hysteresis loops for a wide angular range of the applied field and the trench axis.A model based on two exchange-coupled subsystems accounts for the experimental results and thus predicts the conditions for the appearance of this magnetic behavior.The findings of the study not only advance our understanding of patterning effects and confined magnetic systems but also enable the local design and control of the magnetic response of thin materials with potential use in sensor engineering.     

Impurity segregation at grain boundaries in metals: Effects of applied stress

Impurity segregation at a ∑ = 5 tilt grain boundary in transition metals (α-Fe) has been studied using a tight-binding type electronic theory. The present theoretical calculation takes into account the effects of applied stress as well. Recent experimental observations on grain boundary segregation under applied stresses are qualitatively interpreted from the viewpoint of a chemical interaction between the solute atom and grain boundaries.     

Advances in piezoelectric thin films for acoustic biosensors,acoustofluidics and lab-on-chip applications

Recently, piezoelectric thin films including zinc oxide (ZnO) and aluminium nitride (AlN) have found a broad range of lab-on-chip applications such as biosensing, particle/cell concentrating, sorting/patterning, pumping, mixing, nebulisation and jetting. Integrated acoustic wave sensing/microfluidic devices have been fabricated by depositing these piezoelectric films onto a number of substrates such as silicon, ceramics, diamond, quartz, glass, and more recently also polymer, metallic foils and bendable glass/silicon for making flexible devices. Such thin film acoustic wave devices have great potential for implementing integrated, disposable, or bendable/flexible lab-on-a-chip devices into various sensing and actuating applications. This paper discusses the recent development in engineering high performance piezoelectric thin films, and highlights the critical issues such as film deposition, MEMS processing techniques, control of deposition/processing parametres, film texture, doping, dispersion effects, film stress, multilayer design, electrode materials/designs and substrate selections. Finally, advances in using thin film devices for lab-on-chip applications are summarised and future development trends are identified.     

Engineering Transport in Manganites by Tuning Local Nonstoichiometry in Grain Boundaries

Interface‐dominated materials such as nanocrystalline thin films have emerged as an enthralling class of materials able to engineer functional properties of transition metal oxides widely used in energy and information technologies. In particular, it has been proven that strain‐induced defects in grain boundaries of manganites deeply impact their functional properties by boosting their oxygen mass transport while abating their electronic and magnetic order. In this work, the origin of these dramatic changes is correlated for the first time with strong modifications of the anionic and cationic composition in the vicinity of strained grain boundary regions. We are also able to alter the grain boundary composition by tuning the overall cationic content in the films, which represents a new and powerful tool, beyond the classical space charge layer effect, for engineering electronic and mass transport properties of metal oxide thin films useful for a collection of relevant solid‐state devices.     

Segregation effects in thin films

Atomic segregation and second-phase precipitation at surfaces and interfaces have a strong influence on many properties of bulk materials. Because these regions are different in chemistry and sometimes in lattice structure from the bulk, they can exert a pronounced influence on thin film properties, more so than in bulk materials owing to the large extent of the segregated and/or precipitated zones compared with the film thickness. Some of the similarities and the contrasting features of the segregation phenomena observed between thin films and bulk materials are noted. The thermodynamic aspects and diffusion kinetics which control segregation, the surface reactions and the formation of surface compounds in thin films are discussed. The influence of segregation and interfacial reactions on thin film properties and common experimental methods of studying segregation are briefly reviewed with selected examples.     

Phase transformations at interfaces: Observations from atomistic modeling

We review the recent progress in theoretical understanding and atomistic computer simulations of phase transformations in materials interfaces, focusing on grain boundaries (GBs) in metallic systems. Recently developed simulation approaches enable the search and structural characterization of GB phases in single-component metals and binary alloys, calculation of thermodynamic properties of individual GB phases, and modeling of the effect of the GB phase transformations on GB kinetics. Atomistic simulations demonstrate that the GB transformations can be induced by varying the temperature, loading the GB with point defects, or varying the amount of solute segregation. The atomic-level understanding obtained from such simulations can provide input for further development of thermodynamics theories and continuous models of interface phase transformations while simultaneously serving as a testing ground for validation of theories and models. They can also help interpret and guide experimental work in this field.     

Revealing Nanoscale Chemical Heterogeneities in Polycrystalline Mo‐BiVO4 Thin Films

The activity of polycrystalline thin film photoelectrodes is impacted by local variations of the material properties due to the exposure of different crystal facets and the presence of grain/domain boundaries. Here a multi‐modal approach is applied to correlate nanoscale heterogeneities in chemical composition and electronic structure with nanoscale morphology in polycrystalline Mo‐BiVO₄. By using scanning transmission X‐ray microscopy, the characteristic structure of polycrystalline film is used to disentangle the different X‐ray absorption spectra corresponding to grain centers and grain boundaries. Comparing both spectra reveals phase segregation of V₂O₅ at grain boundaries of Mo‐BiVO₄ thin films, which is further supported by X‐ray photoelectron spectroscopy and many‐body density functional theory calculations. Theoretical calculations also enable to predict the X‐ray absorption spectral fingerprint of polarons in Mo‐BiVO₄. After photo‐electrochemical operation, the degraded Mo‐BiVO₄ films show similar grain center and grain boundary spectra indicating V₂O₅ dissolution in the course of the reaction. Overall, these findings provide valuable insights into the degradation mechanism and the impact of material heterogeneities on the material performance and stability of polycrystalline photoelectrodes.     

Grain boundary segregation engineering in metallic alloys: A pathway to the design of interfaces

Grain boundaries influence mechanical, functional, and kinetic properties of metallic alloys. They can be manipulated via solute decoration enabling changes in energy, mobility, structure, and cohesion or even promoting local phase transformation. In the approach which we refer here to as ‘segregation engineering’ solute decoration is not regarded as an undesired phenomenon but is instead utilized to manipulate specific grain boundary structures, compositions and properties that enable useful material behavior. The underlying thermodynamics follow the adsorption isotherm. Hence, matrix-solute combinations suited for designing interfaces in metallic alloys can be identified by considering four main aspects, namely, the segregation coefficient of the decorating element; its effects on interface cohesion, energy, structure and mobility; its diffusion coefficient; and the free energies of competing bulk phases, precipitate phases or complexions. From a practical perspective, segregation engineering in alloys can be usually realized by a modest diffusion heat treatment, hence, making it available in large scale manufacturing.     

Achieving Ultralow Wear with Stable Nanocrystalline Metals

Recent work suggests that thermally stable nanocrystallinity in metals is achievable in several binary alloys by modifying grain boundary energies via solute segregation. The remarkable thermal stability of these alloys has been demonstrated in recent reports, with many alloys exhibiting negligible grain growth during prolonged exposure to near‐melting temperatures. Pt–Au, a proposed stable alloy consisting of two noble metals, is shown to exhibit extraordinary resistance to wear. Ultralow wear rates, less than a monolayer of material removed per sliding pass, are measured for Pt–Au thin films at a maximum Hertz contact stress of up to 1.1 GPa. This is the first instance of an all‐metallic material exhibiting a specific wear rate on the order of 10^?9 mm³ N^?1 m^?1, comparable to diamond‐like carbon (DLC) and sapphire. Remarkably, the wear rate of sapphire and silicon nitride probes used in wear experiments are either higher or comparable to that of the Pt–Au alloy, despite the substantially higher hardness of the ceramic probe materials. High‐resolution microscopy shows negligible surface microstructural evolution in the wear tracks after 100k sliding passes. Mitigation of fatigue‐driven delamination enables a transition to wear by atomic attrition, a regime previously limited to highly wear‐resistant materials such as DLC.     

On size effect of cleavage cracking in polycrystalline thin films

A crack trapping model is developed for the fracture resistance of high-angle grain boundaries in free-standing brittle thin films, based on which a new size effect is predicted. In addition to the crystallographic misorientations, the grain boundary toughness is also dependent on the film thickness, primarily due to the geometrically necessary crack front branching.     

Stress behavior of FCC metallic thin films during thermal evaporation

The development of stress in metallic thin films, monitored by in-situ curvature measurements during deposition, is analyzed. Three distinct stress regions including initial compressive, broad tensile, and incremental compressive stress were reported in terms of the film thickness (deposition time) by F. Spaepen. An experimental set-up was assembled for the in-situ curvature measurements utilizing vacuum thermal evaporation and multi-beam laser reflection points arrayed in x- and y-axis. The change in the spacing of laser reflected points was converted to the curvature of specimen, in turn, to instantaneous stress levels in the growing films using Stoney's formula. To investigate the effect on the distinct stress regions, the flux of the depositing metallic atoms was used as an experimental variable in this study. For the lowest flux cases for Cu and Ag, an additional second compressive stress stages after tensile maximum stress was observed in this study. Initial compressive part and tensile maximum stress regions appeared in shorter period of time for the thin films deposited at higher flux of atoms. Thus the flux of depositing atoms may affect the mechanisms of each stage. The initial compressive stress is conjectured to stem from the state of thin film surfaces; dynamic and relaxed surface. A broad tensile region is reported from the fact that the reduction of excess volume associated with grain boundaries and/or the coalescence of grains for high mobility materials. The incremental compressive stress region may be related to surface state and atomic mobilities.     

Effect of copper-oxide segregation on the dielectric properties of CaCu3Ti4O12 thin films fabricated by pulsed-laser deposition

We investigated the effects of post-deposition cooling conditions on the surface morphologies and dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) thin films grown by pulsed-laser deposition on Pt/TiO₂/SiO₂/Si substrates. CCTO thin films cooled under the typical cooling parameters, i.e., slow cooling (3 °C/min) at high oxygen pressure (66 kPa) showed a severe segregation of nanoparticles near the grain boundaries, which was identified to be copper oxide from electron probe micro analyzer mapping. On the other hand, we could not observe any segregation on the film surface when the samples were cooled fast (∼ 20 °C/min) at relatively low oxygen pressure (100 Pa). The dielectric constant, ε_r, of CCTO thin films deposited at 750 °C with severe surface segregation (ε_r ∼ 750 at 10 kHz) was found to be much lower than that (ε_r ∼ 2000 at 10 kHz) of CCTO thin films with smooth surface. As the copper-oxide segregation becomes more serious, which preferentially occurs at relatively high ambient oxygen pressure and temperature, the degradation in the dielectric properties of CCTO films becomes larger. The variation of dielectric constant of CCTO films with no copper-oxide segregation could be related to the presence of an impurity phase at grain boundaries.     

Finite element simulation of complex interfacial segregation phenomena in dilute alloys

Segregation of trace elements on a surface, at grain boundaries or more generally in any interface can have important consequences: adhesion of thin films, catalytic activity, embrittlement of steels by P or of nickel alloys by S, reinforcement of nickel alloys by B, etc. Segregation kinetics can be simulated by a finite element (FE) approach, by implementing the Darken–Du Plessis equation at the interface and Fick’s diffusion laws in the bulk. It is then possible to simulate segregation kinetics in non-isothermal conditions, and to couple segregation and macroscopic heat transfer calculations. A previously developed model is here adapted to the case of complex interfacial segregation phenomena: (i) segregation of a single species with a solute–solute or solute–solvent interaction, (ii) co-segregation of two species with a site competition in the interface, and (iii) segregation of a single species at an interface between two phases. Results are compared with available experimental data.