The substrate influence on the magnetization of nickel electrodeposited thin films

The effects of diffusion of substrate copper atoms into the film as well as substrate stresses on both saturation magnetization and coercive force of annealed electrodeposited Ni films have been investigated.     

Effect of deposition parameters on microstructure of electrodeposited nickel thin films

Nickel thin films were prepared using electrodeposition process on a copper substrate. The effect of deposition parameters on film microstructure has been investigated with and without an organic additive (saccharin). Electrodeposition has been carried out using direct current electrodeposition (DCED) method and pulsed electrodeposition (PED) method. Significant reduction in crystallite size has been observed with the increase in saccharin concentration (~10 g/L) irrespective of the electrodeposition method. In PED, it has been observed that an increase in pulse width causes a drastic reduction in crystallite dimension (~15 nm) of the deposited Ni-film. Further PED process yielded needle-shaped Ni grains under controlled process conditions unlike in DCED, where spherical grain structure was observed in the micrographs. However, these needle-shaped grains change their microstructure on addition of saccharin to the bath. A phenomenological model is presented to explain the observed microstructural changes.     

Some observations with helium films

This appendix contains several comments relevant to the lectures of Roger Bowley.¹ We begin with some introductory remarks on third sound and proceed to utilize third sound to study the stability of flow in helium films. We conclude with a remark about the effective thermal conductivity of a helium film.This is an appendix to the series of lectures delivered by Roger Bowley at the Elba Summer School; much of this material was presented during one of Roger Bowley's lectures.     

Nanoindentation and micro-mechanical fracture toughness of electrodeposited nanocrystalline Ni-W alloy films

Nanocrystalline nickel-tungsten alloys have great potential in the fabrication of components for microelectromechanical systems. Here the fracture toughness of Ni-12.7 at.%W alloy micro-cantilever beams was investigated. Micro-cantilevers were fabricated by UV lithography and electrodeposition and notched by focused ion beam machining. Load was applied using a nanoindenter and fracture toughness was calculated from the fracture load. Fracture toughness of the Ni-12.7 at.%W was in the range of 1.49-5.14 MPa √m. This is higher than the fracture toughness of Si (another important microelectromechanical systems material), but considerably lower than that of electrodeposited nickel and other nickel based alloys.     

A modified layer-removal method for residual stress measurement in electrodeposited nickel films

Combining the traditional layer-removal method with a cantilever beam model, a modified layer-removal method is developed and used to measure residual stress in single and multi-layer electrodeposited nickel films with thickness of 2.5 μm. The out-of-plane displacement of the free tip of a cantilever beam is measured by the digital speckle correlation method. The results show that residual stress in a single semimat nickel film is compressive, while in a multi-layer system composed of dark, semimat and holophote nickel, residual stress in the surface layer is tensile. Residual stress decreases gradually with the increase of etching depths of single and multi-layer films. These findings are in qualitative agreement with nanoindentation tests, which confirms the reliability of the modified layer-removal method.     

A transmission electron microscope study of electrodeposited nickel films on (001) copper substrates

Nickel was electrodeposited on to (001) single-crystal copper films. Deposit thickness were in the range 10 Å to 500 Å. Bicrystals and stripped deposits were examined by transmission electron microscopy and high-resolution electron diffraction. Misfit, as a function of thickness, was determined from moiré fringe spacings, splitting of diffraction spots and the spacing of misfit dislocations. The misfit was found to be less than that of vapour-deposited films at corresponding thicknesses. The misfit as calculated from the spacing of misfit dislocations was not in agreement with the misfit as determined from moiré fringes or electron diffraction. These discrepancies might be due to foreign material incorporated in the deposit.     

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.     

Effects of simultaneous polishing on electrodeposited nanocrystalline nickel

Nanocrystalline nickel was electrodeposited by combining with the simultaneous polishing of free particles. The polishing could strengthen the nickel deposition by affecting the structure of the deposits and refining grains. Nickel deposition with grain size of 30-80 nm and tensile strength of about 1400 MPa was achieved.     

Some electron microscope observations of alumina whisker/nickel composites

Alumina whiskers separated from Thermokinetic Fibres sapphire wool mat type 1A have been incorporated into nickel by roll-bonding and by hot-pressing. The roll-bonded composites containing up to 5 vol % whiskers were successfully electrothinned to allow transmission electron microscope examination of alumina whiskers contained in the nickel matrix. In the hot-pressed composites which contained 20 vol % whiskers, examination of the whiskers was only possible after extraction from the composite. The roll-bonding process resulted in whisker break-up but the whisker segments retained their mono-crystalline form. The whiskers in these composites also retained extensive dislocation tangles in the adjacent nickel matrix after annealing at temperatures up to 1400° C. The whiskers extracted from the as-hot-pressed composites had a monocrystalline form and gave no indication of spheroidisation. In hot-pressed samples annealed at 1400° C for up to 100 h the whiskers exhibited evidence of spheroidisation. In some instances the whiskers developed strong crystallographic morphologies with faces parallel to the <11¯20> directions of the approximated close-packed hexagonal lattice of-alumina.     

Some observations on cleavage fracture in high-strength aluminium alloy 7010

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Some observations on thin β-tin films by electron diffraction

Forbidden “110-type” reflections are present on some patterns of tin single-crystal films. Their intensities may vary in a large range and these reflections do not appear with polycrystalline samples. The other forbidden “002-type” reflections which were observed by other authors and ourselves are probably due to secondary elastic scattering, but the kind of “forbidden” spots studied here can be explained by dynamic interactions.     

电沉积制备Ni-S涂层电极的研究

以泡沫镍为基体电沉积制备了Ni-S涂层电极.通过对电流密度、电沉积温度、电沉积时间和镀液pH值等条件对涂层析氢性能影响的研究,确定了适宜的电沉积工艺条件;电极涂层的XRD、SEM测试结果表明涂层为非晶态结构、表面颗粒大小均匀且具较大表面积;电解过程中出现了Ni3S2由非晶态向晶态的转变,由于Ni3S2具有较强的吸附氢能力,是析氢反应的催化活性中心,电解过程中Ni3S2的出现降低了电极的析氢电位.模拟电解水实验结果表明泡沫镍基Ni-S涂层电极较Raney-Ni电极具有更好的析氢活性.     

Effect of temperature on the evolution of diffusivity, microstructure and hardness of nanocrystalline nickel films electrodeposited at low temperatures

Most nickel (Ni) films galvanostatically electrodeposited at 40-50 °C exhibited low hardness about 4 GPa and rough surface. In this article, we have investigated Ni electrodeposition at low temperatures of 5-20 °C in order to enhance the hardness and smoothness of films and performed by potentiostatic mode instead of galvanostatic mode to avoid the low-temperature precipitation of electrolyte agents. Effect of temperature on the evolution of diffusion coefficient, deposition rate, morphology and hardness was studied. Electrodeposition at low temperature without hard-element addition can reduce diffusion rate and produce the fine-grain, smooth morphology and dense film together with compressive residual stress to enhance hardness up to 6.18 GPa at 5 °C. The growth and hardening mechanism of low-temperature electrodeposited Ni were further discussed in details.     

Some aspects of the fracture mechanics of thin flexible films

A theory is presented for the determination of a number of physical constants which affect the fracture behaviour of thin plastic and viscoelastic materials. The theory is based on a relatively simple consideration of the energy balance in a tensile-test-strip with an edge-crack.

The total energy needed up to fracture was measured, for each material, for 90 samples approximately, varying both the total length of the test-trips, and the length of the initial cracks. In the first instance, the width of the test-strips was kept constant at a value of 15 mm.

Then, with the help of a special version of a multiple linear regression, values of physical parameters were obtained. Three materials were investigated, viz. a polyethylene, a polyester/polyethylene laminate and a polyamide/polyethylene laminate.

Initial crack lengths between 0.1 and 1 mm were investigated, in two different ways, viz. with a static tensile test and with an impact tensile test.

For each material, considerable differences between static and dynamic behaviour has been measured; a qualitative explanation, however, of these differences can not yet be given.

Secondly, the number of data per material was reduced, in order to get an impression of the reliability of the applied method of approximation. It appeared that even a set of four test-strips for one material produced significant results.

Next, an effort has been made to extrapolate the values obtained for test-strips with a width of 10 and 5 mm, respectively. In general, this extrapolation appeared to be inadmissable, implying that the underlying theory should only be considered as a rough approximation of a very complex reality.

From the experiments and calculations it is concluded that in general the method adopted is suitable to quantify the fracture behaviour of thin films in industrial practice.     

Enhancement of the coercivity of electrodeposited nickel nanowire arrays

In the present study, the single-crystal Ni nanowire arrays with a preferred growth along the [110] direction have been prepared by the deposition of Ni into the alumina template with nanopores at a current density of 2.0 mA/cm². The single-crystal Ni nanowire arrays show a magnetic anisotropy with the easy axis parallel to the nanowires and an enhanced coercivity as compared with the polycrystalline Ni nanowire arrays. A large coercivity of 1110 Oe together with a high remanence M_r = 0.92M_s is observed for 15-nm diameter single-crystal Ni nanowire arrays. The preferred growth mechanism of the single-crystal nanowires is briefly discussed.     

间接超声下电解液温升及微棱镜镍层反光性能

采用间接超声搅拌方式电化学沉积生长了微棱镜反光材料工作模。研究了间接超声电沉积带来的温升原因,以及温升和空化综合作用对工作模反光性能的影响。采用X射线衍射对电沉积镍层的结构进行分析,逆反射标志测量仪测量了镍铸层的表面反光系数,并采用3D激光电子扫描显微镜观测了镍铸层的表面形貌和微棱侧面粗糙度。结果表明,在间接超声作用下,随着时间的增加,电解液温度单调增加,工作模侧面粗糙度和反光系数呈现折线变化。低频时,超声空化起主要作用;高频时,超声温升起主要作用。在70kW/m2和40kHz时,工作模的粗糙度最低,反光系数最高。     

On the structure and lattice faults of electrodeposited nickel

The influence of bath pH on the structure and lattice faults of nickel deposited from a sulphate bath was investigated by X-ray diffraction. A special method was developed for separating the effects of macrostrains and deformation stacking faults on the peak positions.Measurable amounts of twin faults and macrostrains along the cube edges were found in samples kept at room temperature. No deformation faults or strains along the cube diagonals were found. The faults and the strains disappeared by heating to about 300 to 400° C. The coherent domain size was also found to increase in the same region.Most of the phenomena observed were probably caused by co-deposition of colloidal nickel hydroxide into the cathode. The twin faults were possibly caused, at least indirectly, by co-deposited hydrogen.     

Some observations on Sih's strain energy density approach for fracture prediction

International Journal of Fracture -     

Some observations on the path stability in fracture propagation for biaxially stressed plates

The path stability problem for fracture starting from a straight crack in a biaxially stressed plate is investigated. The analysis is performed by employing a simple equivalent crack in order to obtain an approximate evaluation of the Stress Intensity Factors of the S-shaped crack which may be generated by the fracture propagation. It is shown that crack propagation deviates from the original direction when the applied stress parallel to the crack plane becomes higher than that perpendicular to it. The trend of the fracture trajectories and their sensitivity to small deviations caused by irregularities have also been determined for various values of the stress biaxiality ratio. Some comparisons with previous analyses are made.