Properties of ternary Sn-Ag-Bi solder alloys: Part I—Thermal properties and microstructural analysis

Bismuth additions of 1% to 10% were made to the 96.5Sn-3.5Ag (wt.%) alloy in an effort to develop a Sn-Ag-Bi ternary composition. A DSC evaluation of the melting properties of the 91.84Sn-3.33Ag-4.83Bi composition suggested the appearance of metastable, short-range order in the atomic structure as a result of low temperature, thermal aging. More extensive solid-state aging studies on 91.84Sn-3.33Ag-4.83Bi/Cu couples resulted in the growth of an intermetallic compound layer at the solder/substrate interface comprised of Cu₃Sn and the Cu₆Sn₅ sub-layers. The growth kinetics of the total layer thickness (x) as a function of solid-state aging time (t) and temperature (T) were represented by the following expression: $$x - x_o = A t^n \exp ( - Q/RT)$$ where x₀=0.57 × 10^?6 m; A=6.22 × 10^?3 m/sⁿ; n=0.46±0.15; and Q=49±8 kJ/mol. TEM analysis of the 91.84Sn-3.33Ag-4.83Bi composition indicated that solid-solution and precipitation strengthening mechanisms were a likely consequence of the Bi additions. Contact angle measurements, Cu/solder/Cu solder joint shear strength tests, and microhardness evaluations were also performed on the Sn-Ag-Bi alloys; those results are reported in Part II.     

Microstructure and mechanical properties of Sn–Zn–Bi–Cr lead-free solder

In this paper, the effect of trace addition of Cr on the mechanical properties and reliability on Sn–8Zn–3Bi solder alloys was investigated. It has been demonstrated that the microstructure of solder alloys was refined after doping traces of Cr. The elongation reaches up to 40.63% after doping 0.1% Cr; and the fracture mechanism converts from quasi-cleavage fracture into ductile fracture. With aging time of 0, 4, 9 and 16 days, mechanical property of Sn–8Zn–3Bi–0.3Cr alloy was improved slightly. It was found that the Sn–Zn–Cr phase was increased and Zn in alloy was consumed after aging, so that the amount of primary Zn phase was reduced and microstructure was improved.     

Effect of nano Al2O3 particles doping on electromigration and mechanical properties of Sn–58Bi solder joints

In the present study, the effect of Al₂O₃ nanoparticles on performances of Sn–58Bi solder were investigated in aspects of electro-migratio, shear strength and microhardness. The experimental results show that the Al₂O₃ nanoparticles significantly improved microstructure and mechanical performances of solder joints. With the addition of 0.5 wt% Al₂O₃, the intermetallic compounds (IMC) reduced from 2.5 μm to 1.27 μm after 288 aging hours at 85 °C. Furthermore, after electromigration test under a current density of 5 × 10³ A/cm² at 85 °C, Bi-rich layers formed at the anode side of both Al₂O₃ doped and plain solder. Moreover, the addition of Al₂O₃ nanoparticles reduced the mean thickness of Bi-rich layer. In addition, the growth rate of the IMC layer of Al₂O₃ doped solder decreased by 8% compared with the plain solder. Besides, the Al₂O₃ doped solder exhibited better performance than plain solder in microhardness after different aging times. While, the addition of Al₂O₃ significantly impeded the degradation of the shear strength of solder joint after aging for 48 and 288 h. Furthermore, it was worth noting that the fracture surface of doped solder showed a typical rough and ductile structure. However, plain solder exhibited a relatively smooth and fragile surface.     

Photoelectric properties of an injection photodetector based on alloys of II–VI compounds

A photosensitive structure with high room-temperature integrated sensitivity S _int ≈ 700 A/lm (14500 A/W) is fabricated based on alloys of II–VI compounds n-CdS _x Te_{1 ? x} and p-Zn _y Cd_{1 ? y} Te. Its photoelectric properties are studied at various illumination levels and bias voltages. It is found that diffusion and drift flows of nonequilibrium carriers are directed oppositely at low illumination levels and forward bias voltages. This effect leads to inversion of the photocurrent sign, which makes it possible to fabricate selective photodetectors with injection properties on its basis.     

Mechanical and electronic properties of Ca1−xMgxO alloys

The structural, mechanical, elastics anisotropy and electronic properties of Ca_1−xMg_xO in the cubic structure are investigated using density functional theory calculations. The lattice parameters, elastic constants and elastic modulus are in excellent agreement with the experimental and others theoretical data. The sound velocities and the Debye temperatures are calculated for all the Ca_1−xMg_xO alloys using the calculated elastic constants and elastic modulus. The elastic anisotropy are characterized by calculating several different anisotropic indexes and describing the three dimensional surface constructions. Finally, electronic structure studies show that Ca_1−xMg_xO alloys are direct band gap semiconductors.     

Contrast in Terahertz Images of Archival Documents—Part II: Influence of Topographic Features

We investigate the potential of terahertz time-domain imaging in reflection mode to reveal archival information in documents in a non-invasive way. In particular, this study explores the parameters and signal processing tools that can be used to produce well-contrasted terahertz images of topographic features commonly found in archival documents, such as indentations left by a writing tool, as well as sieve lines. While the amplitude of the waveforms at a specific time delay can provide the most contrasted and legible images of topographic features on flat paper or parchment sheets, this parameter may not be suitable for documents that have a highly irregular surface, such as water- or fire-damaged documents. For analysis of such documents, cross-correlation of the time-domain signals can instead yield images with good contrast. Analysis of the frequency-domain representation of terahertz waveforms can also provide well-contrasted images of topographic features, with improved spatial resolution when utilising high-frequency content. Finally, we point out some of the limitations of these means of analysis for extracting information relating to topographic features of interest from documents.     

Interrelationship of thermal parameters,microstructure and microhardness of directionally solidified Bi–Zn solder alloys

Due to the health and environmental concerns associated with lead usage, the research on alternative lead-free alloys for replacing lead-based solders is a global demand. Despite numerous studies on Sn-based lead-free solders in recent years, there is not yet a standard solder alloy able to cover the spectrum of properties furnished by the classic Sn-Pb alloy. In this sense, particular lead-free alloys compositions have been suited for specific needs and the options have been broadening when elements other than tin are used as the base component, such as indium, gold and bismuth. The later element is well known in the hall of lead-free alloys as an alloying option rather than as a base component. This study aims to establish interrelations of solidification thermal parameters (growth and cooling rates), microstructure features (primary and secondary dendrite arm spacings – λ₁ and λ₂; eutectic spacing – λ_E and interphase spacing – λ_int) and hardness of Bi-Zn alloys (1.5 wt% Zn-hypoeutectic, 2.7 wt% Zn-eutectic, and 5 wt% Zn-hypereutectic alloys) samples, which were directionally solidified in unsteady-state conditions under cooling rates similar to those of found in industrial soldering practice. Examination of the resulting microstructures by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) permitted the different phases morphologies to be characterized: Bi-rich trigonal dendrites and long Zn fibers, as primary phases of, hypoeutectic and hypereutectic alloys, respectively, immersed in a fiber-like eutectic mixture. The combined effects of macrosegregation, Zn alloying and representative scales of the phases forming the microstructure (λ_int, λ₂ and λ_E) on hardness of the Bi-Zn alloys are evaluated and Hall–Petch type equations relating λ_int, λ₂ and λ_E to hardness are proposed.     

Transport Properties of Bulk Thermoelectrics—An International Round-Robin Study, Part I: Seebeck Coefficient and Electrical Resistivity

Recent research and development of high-temperature thermoelectric materials has demonstrated great potential for converting automobile exhaust heat directly into electricity. Thermoelectrics based on classic bismuth telluride have also started to impact the automotive industry by enhancing air-conditioning efficiency and integrated cabin climate control. In addition to engineering challenges of making reliable and efficient devices to withstand thermal and mechanical cycling, the remaining issues in thermoelectric power generation and refrigeration are mostly materials related. The dimensionless figure of merit, ZT, still needs to be improved from the current value of 1.0 to 1.5 to above 2.0 to be competitive with other alternative technologies. In the meantime, the thermoelectric community could greatly benefit from the development of international test standards, improved test methods, and better characterization tools. Internationally, thermoelectrics have been recognized by many countries as a key component for improving energy efficiency. The International Energy Agency (IEA) group under the Implementing Agreement for Advanced Materials for Transportation (AMT) identified thermoelectric materials as an important area in 2009. This paper is part I of the international round-robin testing of transport properties of bulk thermoelectrics. The main foci in part I are the measurement of two electronic transport properties: Seebeck coefficient and electrical resistivity.     

Sn addition on the tensile properties of high temperature Zn–4Al–3Mg solder alloys

The Zn–4Al–3Mg based solder alloy is a promising candidate to replace the conventional Pb–5Sn alloy in high-temperature electronic packaging. In this study, the tensile properties of Zn–4Al–3Mg–xSn alloys (x = 0, 6.8 and 13.2 wt.%) at high temperatures (e.g., 100 °C, and 200 °C) were investigated. It was found that the uniaxial tensile strength (UTS) of Zn–4Al–3Mg–xSn solder alloys all decrease monotonously with the increment of temperature. The elongation ratio at 100 °C is superior to that at room temperature whereas follows a significant drop at 200 °C. The microstructure observations show that a typical brittle fracture of Zn–4Al–3Mg alloy occurs at room temperature and 200 °C under normal tension, whereas a ductile fracture is found at 100 °C. The 6.8 wt.% Sn addition in Zn–4Al–3Mg alloy causes a dramatic decrease of yield strength, and a slight deterioration of the ductility.     

Optical properties of molecular beam epitaxy-grown ZnSexTe1−x II–VI semiconductor alloys

A series of ZnSe_xTe_1−x alloys with varying compositions that cover the entire range between the two binaries ZnSe and ZnTe were grown to determine the index of refraction, n, as a function of both wavelength and the alloy concentration. The ZnSe_xTe_1−x alloys were all grown directly on GaAs (100) substrates using molecular beam epitaxy. X-ray diffraction experiments were performed to determine the quality and the compositions of each of the ZnSe_xTe_1−x specimens. The n was then measured with an accuracy of at least 0.1% at four discrete wavelengths using a prism coupler method. Since these data points are inadequate to ascertain, an accurate dispersion of the index of refraction, we next performed reflectivity measurements on each of the samples to complement the prism coupler data. The reflectivity and the prism coupler data allow us to arrive at the dispersion relations of the indices of refraction of ZnSe_xTe_1−x ternary alloys very accurately. We also find that, unlike most semiconductors, the values of n of ZnSe_xTe_1−x alloys do not follow an inverse relationship with the energy gap.     

Structural,optical and electrical properties of MgxZn1−xO ternary alloy films

The structural, optical and electrical properties of Mg_xZn_1−xO (x=0.05–0.3) ternary alloy thin films deposited by the sol–gel method on the glass substrate were investigated. The presence of Mg in deposited samples was confirmed through EDAX. XRD spectra revealed that the deposited Mg doped ZnO films were polycrystalline in nature. The optical band gap of the films was tailored between 3.2 and 3.45 eV by varying Mg mole concentration between 0.05 and 0.3. I–V characteristics showed decrease in current with increase in the Mg mole concentration. These results explore the applicability of MgZnO to form effective and efficient heterostructures with ZnO as an active layer for efficient carrier confinement in light emitting devices.     

Study of the Properties of II–VI and III–V Semiconductor Quantum Dots

The specific features of the electron spectra of II–VI and III–V semiconductor quantum dots are studied experimentally and theoretically. Analysis of the samples makes it possible to estimate the positions of the first three levels in the electron spectrum of the quantum object. Good qualitative and quantitative agreement between the experimental results and theoretical estimates is attained. It is shown that the mechanism of the experimentally observed field-emission current through a quantum dot is satisfactorily described by Morgulis–Stratton theory in experimental conditions.     

First-principles prediction of the structural and electronic properties of zinc blende GaNxAs1−x alloys

To investigate the structural and electronic properties of zinc blende GaN_xAs_1?x alloys, we performed full-potential linearized augmented plane wave (FP-LAPW) calculations based on density functional theory. We assessed GaN_xAs_1?x alloys for 0≤x≤1 using 16-atom special quasi-random structures. The generalized gradient approximation (GGA) of Wu and Cohen was used as the exchange correlation potential to calculate the structural and electronic properties of GaN_xAs_1?x. In addition, the alternative GGA proposed by Engel and Vosko and the modified Becke–Johnson potential were used for better reproduction of the band structure and electronic properties. The equilibrium lattice parameters and bulk modulus were calculated and analyzed for binary and ternary alloys. The lattice constants for GaN_xAs_1?x positively deviate from Vegard's law with an upward bowing parameter of ?0.4708 Å. All our materials are direct-bandgap semiconductors for which the valence band maximum is located at Γ_v and the conduction band minimum at Γ_c. We observed that the direct bandgap of GaN_xAs_1?x increases nonlinearly with x. To shed light on the bandgap trend for increasing nitrogen concentrations in GaN_xAs_1?x, we used the atoms-in-molecule formalism. Special attention was paid to the increase in charge transfer for the nitrogen atom and to ionicity as a function of increasing x concentration.     

Study of mechanical properties of polymer coatings at and near the surface with scanning probe microscope

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Density functional perturbation theory calculations of vibrational and thermodynamic properties of Zn1−xBexO alloys

The elastic, phonon and thermodynamic properties of Zn_1−xBe_xO alloy are investigated by performing density functional theory (DFT) and density functional perturbation theory (DFPT) calculations. The calculated lattice parameters decreases with the increase of Be content that is in good agreement with the available theoretical and experimental data. The effect of Be composition on elastic constants was investigated for Zn_1−xBe_xO alloys. Phonon dispersion curves show that Zn_1−xBe_xO are dynamically stable. Thermodynamic properties, including Helmholtz free energy, enthalpy, entropy and heat capacity, were evaluated under quasi-harmonic approximation using the calculated phonon density of states. Finally, the results show that Zn_1−xBe_xO alloys with lower Be content are more thermodynamically stable. The agreement between the present results and the known data that are available only for ZnO and BeO is generally satisfactory.     

Optical Properties of Silicon—doped InGaN and GaN Layers

The optical properties of Silicon—doped InGaN and GaN grown on sapphire by MOCVD have been investigated by photoluminescence (PL) method. At room temperature, the band—gap peak of InGaN is 437.0 nm and its full width of half—maximum (FWHM) is about 14.3 nm. The band—gap peak and FWHM for GaN are 364.4 nm and 9.5 nm, respectively. By changing the temperature from 20 K to 293 K, it is found that the PL intensity of samples decreases but the FWHM broadens with the increasing of the temperature.GaN sample shows red—shift, InGaN sample shows red—blue—red—shift. The temperature dependence of peak energy shift is studied and explained.     

Fermi level pinning and electrical properties of irradiated CdxHg1−x Te alloys

The composition dependence of the local neutrality level E _lnl in Cd_xHg_1?x Te alloys has been studied theoretically. It is shown that for compositions with x<0.47 the E _lnl level lies within the conduction band, and at x>0.47 it lies in the upper half of the band gap. The identification of the calculated E _lnl values with the limiting position of the Fermi level (F _lim) in Cd_xHg_1?x Te irradiated with high-energy particles suggests that irradiation is responsible for the n ⁺-or n-type conduction in the material.