Mechanical behavior of nanoscale Cu/PdSi multilayers

Berkovich nanoindentation and uniaxial microcompression tests have been performed on sputter-deposited crystalline Cu/amorphous Pd_0.77Si_0.23 multilayered films with individual layer thicknesses ranging from 10 to 120 nm. Elastic moduli, strengths and deformation morphologies have been compared for all samples to identify trends with layer thicknesses and volume fractions. The multilayer films have strengths on the order of 2 GPa, from which Cu layer strengths on the order of 2 GPa can be inferred. The high strength is attributed to extraordinarily high strain hardening in the polycrystalline Cu layers through the inhibition of dislocation annihilation or transmission at the crystalline/amorphous interfaces. Cross-sectional microscopy shows uniform deformation within the layers, the absence of delamination at the interfaces, and folding and rotation of layers to form interlayer shear bands. Shear bands form where shear stresses are present parallel to the interfaces and involve tensile plastic strains as large as 85% without rupture of the layers. The homogeneous deformation and high strains to failure are attributed to load sharing between the amorphous and polycrystalline layers and the inhibition of strain localization within the layers.     

Oxygen-penetration-induced large hardness enhancement in nanoscale Nb/Ti multilayers

Nb/Ti multilayers with different modulation periods were prepared by magnetron sputtering deposition. Microstructure and mechanical properties were investigated by XRD, SEM and Nanoindentation. It turns out that hardness of samples increases with decreasing modulation wavelength (Λ) and then dropped at small Λ. Through scrutinizing other results in literature, we found that the coherent stress rather than modulus mismatch played more important role for hardness enhancement with decreasing Λ of the bcc-hcp multilayers. Annealing of samples in low (or high) vacuum at 400 C for 30 min led to large (or medium) enhancement of hardness, while the modulated structure was still maintained. It was found that annealing in low vacuum resulted in oxygen penetration into the multilayers as revealed by auger electron spectroscopy, but hardly changed metallic sheet resistivity. XRD results suggested that some niobium oxides were formed in Nb layers and oxygen distributed interstitially in Ti layers. The large hardness enhancement after annealing is mainly due to the strengthening effect from dispersive distribution of nano-scale niobium oxides and interstitial oxygen in the multilayers. In addition, interfaces between adjacent layers were more distinct after annealing which indicated good thermal stability of laminated structure.     

Cu/Nb纳米多层膜延性及其断裂行为

通过单轴拉伸试验研究恒定调制周期的聚酰亚胺基体Cu/Nb纳米金属多层膜的延性对调制比的依赖性,并采用聚焦离子束／扫描电子显微镜(FIB/SEM)截面定量表征技术深入分析多层膜的异质约束效应对断裂行为的影响.结果表明随着调制比的增加,多层膜的延性单调减小,出现由剪切型向张开型断裂模式的转变.当调制比小于某一临界值时,调制周期越小,多层膜延性越高;反之,则多层膜延性越差.这是由于软相Cu层对脆相Nb层中萌生的微裂纹扩展的约束作用.     

Study of growth processes and mechanical properties of nanoscale multilayered C/C films

The growth processes of carbon films deposited using two techniques (sputtering and RF plasma assisted chemical vapor deposition, PACVD), and the effects of stacking these films in a multilayered structure on their mechanical and adhesion properties were studied. The films deposited by the two techniques differed in composition, structure and hardness. They are termed “hard C” and “soft C” according to their synthesis process, sputtering and PACVD respectively. By means of stress measurements and angle-resolved X-ray photoelectron spectroscopy, the growth mechanism of the films was characterized when they were deposited on a silicon substrate and when one kind of carbon film was deposited onto the other kind, in order to simulate a multi-layer film formation. This study evidenced the effect of converting the individual layer surfaces into interfaces when building a multilayer film. On one hand, this conversion appeared to increase the compressive stress of the multilayer films for the lowest periodicity (14 nm and 7 nm of half-period). On the other hand, a strong correlation between the stress resulting from multilayering and the film elasto-plastic properties was found. A hardening effect, put in evidence by applying a nano-indentation “plasticity index”, was obtained for the most layered films (i.e. with the lowest modulation period) and this effect is discussed in relation with the existing models for multi-layer strengthening. The film adhesion to polyethylene terephtalate (PET) substrates was investigated. A beneficial effect of multi-layering on film adhesion was significant only when the half-width period went down to 14 nm and 7 nm. The adhesion improvement cannot be related to a reduced internal stress, since the most stressed films were also the most adherent. Instead the compressive stress found in the films with the lowest periodicity is thought to induce a stronger bonding of the soft C layer (polymer-like) to the PET substrate and to the hard C layers, through chain entanglement across the interface.     

Buckling behaviors and adhesion energy of nanostructured Cu/X (X = Nb,Zr) multilayer films on a compliant substrate

Two sets of Cu/Nb (face-centered cubic (fcc)/body-centered cubic) and Cu/Zr (fcc/hexagonal close-packed) nanostructured multilayer films (NMFs) have been prepared on a flexible polyimide substrate, with a wide range modulation period (λ) from 250 down to 5 nm. The mechanical properties of the two NMFs have been measured upon uniaxial tensile testing and the buckling behaviors have been systematically investigated as a function of λ. A significant difference in the bucking behaviors was found between the two NMFs, with the buckles in the Cu/Nb NMF being mostly cracked, while the buckles were nearly crack-free in the Cu/Zr NMF. The different buckling behaviors, dependent on the constituent phases, are rationalized in the light of the disparity in mechanical properties. The criteria to characterize buckle cracking have been discussed with respect to the mechanical properties (e.g. yield strength, ductility and fracture toughness) of the NMFs. A modified energy balance model has been employed to estimate the adhesion energy of the NMFs on the polyimide substrate. Within the λ regime below a critical size (λ_crit) of ～50 nm a λ-independent adhesion energy of about 1.1 and 1.2 J m^?2 has been determined for the Cu/Nb and Cu/Zr NMFs, respectively, which agrees well with previous reports on the metal film/polymer substrate systems. Within the λ regime greater than λ_crit, however, the measured adhesion energy exhibit a strong size effect, i.e. increasing with increasing λ. The λ dependence of the evaluated adhesion energy is discussed in terms of the size-dependent deformation mechanism in NMFs. A micromechanics model has been utilized to quantify the critical modulation period of ～50 nm, where the deformation mechanism changes from dislocation pile-up to confined layer slip.     

Thermal effects on the process of electropolymerization of pyrrole on mild steel

Polypyrrole films were electrosynthesized on mild steel with oxalic acid electrolyte at varying temperatures (25–65 °C), current density (0.5–6.0 mA/cm²) and pH (2.0, 4.0, 7.0 and 8.5). The concentrations of pyrrole and oxalic acid were maintained at 0.1 M each in galvanostatic experiments. The results show that lower temperatures favor the formation of polypyrrole on mild steel in acidic media. At higher temperature and lower current densities, oscillations in electrode potentials were observed. The film quality was poorest at pH 7.0, while very adherent and compact films were obtained in alkaline medium. Higher temperature in alkaline media favors the formation of better polypyrrole films in contrast with acidic media.     

A comparative study of magnetron co-sputtered nanocrystalline titanium aluminium and chromium aluminium nitride coatings

A comparative study of magnetron co-sputtered (Ti,Al)N and (Cr,Al)N coatings was made. It was found that while both coatings followed similar development pattern with increasing nitrogen pressure, the (Cr,Al)N coatings achieved much higher deposition rate and hardness, suggesting the coatings had a great potential for many industrial applications.     

Synthesis and optical characterization of Pd–Au bimetallic nanoparticles dispersed within monolithic mesoporous silica

Au–Pd bimetallic nanoparticles dispersed within pores of monolithic silica was synthesized by soaking Au nanoparticles contained silica host into Pd precursor solution and subsequent step-annealing at temperature from 373 to 773 K. Theoretical optical spectra based on Mie-like model for Au_corePd_shell nanoparticles yield excellent agreement with the experimental results.     

Microstructural evolution in electroplated Cu thin films

The microstructural evolution of electroplated Cu films (0.89 to 3.0 μm thick) has been studied by texture analysis. Before annealing, the volume fraction of (1 1 1) grains decreases with increasing film thickness, while that of (1 0 0), (1 1 0), and randomly oriented grains increases. Annealing causes a decrease in the 1 1 1 fiber intensity in the thinnest films due to growth of randomly oriented grains and multiple twinning.     

Phase transformation kinetics and self-patterning in misfitting thin films

We outline a computational approach for the study of phase transformations in misfitting thin films and then investigate the kinetics of a transformation to assess the role of heterogeneous nucleation sites, formed in the vicinity of misfit dislocations, in the self-patterning of these systems. Both computer simulation and analytical methods are employed to analyze spatio-temporal correlations in the transforming phase as embodied, in particular, in the transformed volume fraction. To accomplish this, we first obtain an expression for the driving force for nucleation in terms of the strain energy stored in the film under site saturation conditions. This driving force forms the basis for simulations of nucleation and growth in films in which simulation parameters, such as film thickness and temperature, are systematically varied. We find that, in a fully coherent film, there is a retardation of the transformation at certain times that is associated with the constrained film geometry and that, in the presence of misfit dislocations, a range of kinetic behavior can be correlated with the relative magnitude of the driving forces and temperature. The implications of our results for pattern formation in these systems are then discussed.     

MmM5/Mg multi-layer hydrogen storage thin films prepared by dc magnetron sputtering

MmNi_3.5(CoAlMn)_1.5/Mg (here Mm denoted for mischmetal) multi-layer thin films were deposited on (0 0 1) Si substrate by direct current (dc) magnetron sputtering with dual-target. X-ray diffraction (XRD) and scanning electron microscopy analysis revealed that the microstructure of the MmNi_3.5(CoAlMn)_1.5 layer is amorphous and/or nanocrystalline and that the microstructure of the Mg layer is fine grained crystalline with preferential orientation. Phase analysis of hydrogenated and dehydrogenated MmNi_3.5(CoAlMn)_1.5/Mg multi-layer thin films proved that an apparent absorption of hydrogen in the Mg layer occurs at temperatures higher than 200 °C and that the hydrogen absorbed can be fully released at 250 °C.     

Transparent conductive Sb-doped SnO2/Ag multilayer films fabricated by magnetron sputtering for flexible electronics

The effects of an embedded silver layer and substrate temperature on the electrical and optical properties of Sb-doped SnO₂ (ATO)/silver (Ag) layered composite structures on polyethylene naphthalate substrates have been investigated. The highest conductivity of ATO/Ag multilayer films was obtained with a carrier concentration of 1.5 × 10²² cm^?3 and a resistivity of 2.4 × 10^?5 Ω cm at the optimum Ag layer thickness and substrate temperature. The photopic averaged transmittance and Haacke figure of merit are 81.7%, and 21.7 × 10^?3 Ω^?1, respectively. In addition, a conduction mechanism is proposed to elucidate the mobility variation with increased Ag thickness. We also describe the influence of substrate temperature on the structural, electrical and optical properties of the ATO/Ag multilayer films, and propose a mechanism for the changes in electrical and optical properties at different substrate temperatures.     

Solidification study of Al–Co–Cu alloys using the Bridgman method

The solidification of a series of Bridgman-grown Al–Co–Cu alloys with compositions in the vicinity of the quasicrystal was studied by powder X-ray diffraction (XRD), differential thermal analysis (DTA), electron microprobe analysis, and optical microscopy. The phase equilibria and microstructure of solidified alloys are presented; the temperatures of the involved solidification reactions were determined. These experimental data were used to construct a solidification phase diagram as to understand the crystallization path. The decagonal (D) AlCoCu quasicrystals form incongruently, but they can be primarily solidified from off-stoichiometric melts.     

Texture of TiNi shape memory alloy sheets produced by roll-bonding and solid phase reaction from elementary metals

We have developed a new process to produce Ti–Ni shape memory alloys from titanium and nickel sheets. This process is composed of repetitive roll-bonding for producing a Ti/Ni laminated sheet from them, and of subsequent heat treatment for forming a TiNi intermetallic phase by reactive diffusion. The Ti/Ni laminated sheet changed to a nearly single phase TiNi material through the formation of Ti₂Ni and Ni₃Ti intermediate products. TiNi sheets obtained by this process consisted of a near <111>//ND texture such as {223}<110> in the B2 austenite phase. Recoverable strain associated with martensitic transformation of TiNi was fairly isotropic in the sheet plane due to the relatively isotropic character of {111}<uvw> texture. The formation of such texture through phase transformations by reactive diffusion has been discussed in view of texture inheritance.     

Magnetic phase transitions in Nd1−xYxMn2Ge2 (0 ≤ x ≤ 0.6)

The structure and magnetic properties of Nd_1−xY_xMn₂Ge₂ (0.0≤x≤0.6) were studied by X-ray powder diffraction and magnetization measurements. All compounds crystallize in the ThCr₂Si₂-type structure with space group I4/mmm. Substitution of Y for Mn led to a linear decrease in the lattice constants and the unit cell volume. Increasing substitution of Y for Nd in NdMn₂Ge₂ shows a depression of ferromagnetic ordering and the gradual development of antiferromagnetic ordering.     

Influence of Mn and Cu addition on the hyperfine parameters of amorphous and nanocrystalline FeCoNbB alloys

The effects of Cu and Mn addition on the hyperfine field of FeCoNbB HITPERM alloys are discussed from Mössbauer spectrometry. Amorphous and nanocrystalline samples at different stages of the nanocrystallization were studied. The effect of Cu addition correlates with the observed refinement of the microstructure. Mn mainly partitions to the matrix, decreasing the average hyperfine field of the amorphous matrix, although some Mn remains in the nanocrystalline grains, presumably, in a concentration below the maximum solubility of Mn in -Fe.     

Hydrothermal synthesis and characterization of a new layered compound Li2VGeO5

The new compound Li₂VGeO₅ with a layered structure has been synthesized at 580 °C via the hydrothermal method. The compound crystallizes in the space group P4/n of the tetragonal system with two formula units in a cell of dimensions a=6.5187(9) Å, c=4.5092(9) Å (T=298 K), V=191.61(5) Å³. The structure is composed of layers made of repeating [(VO₅)(GeO₄)]¹⁻ units. Li⁺ ions reside between the layers. The magnetic susceptibility data show an antiferromagnetic coupling below 5 K with C=0.47 emu K mol⁻¹, and θ=−13 K with μ_eff=1.89μ_B for each Li₂VGeO₅ unit.