Nanotribological Characteristics of Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript>, Cu<Subscript>3</Subscript>Sn,and Ni<Subscript>3</Subscript>Sn<Subscript>4</Subscript> Intermetallic Compounds

Nanotribological characteristics, including the coefficient of friction, wear coefficient, and wear resistance, of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ intermetallic compounds developed by the annealing of Sn–Cu or Sn–Ni diffusion couples were investigated in this work. The scratch test conditions combined a constant normal load of 10 mN, 20 mN, or 30 mN and a scratch rate of 0.1 μm/s, 1 μm/s, or 10 μm/s. Experimental results indicated that, as the normal load increases, the pile-up grows taller and the scratch deepens, leading to a greater coefficient of friction and wear coefficient, and reduced wear resistance. Moreover, the scratch rate does not have a significant effect on the nanotribological characteristics except for those of Cu₆Sn₅ and Cu₃Sn under a normal load of 10 mN. Though the hardness of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ is similar, Ni₃Sn₄ appears to be more prone to wear damage.     

The nanoindentation characteristics of Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript>, Cu<Subscript>3</Subscript>Sn,and Ni<Subscript>3</Subscript>Sn<Subscript>4</Subscript> intermetallic compounds in the solder bump

In general, formation and growth of intermetallic compounds (IMCs) play a major role in the reliability of the solder joint in electronics packaging and assembly. The formation of Cu-Sn or Ni-Sn IMCs have been observed at the interface of Sn-rich solders reacted with Cu or Ni substrates. In this study, a nanoindentation technique was employed to investigate nanohardness and reduced elastic moduli of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ IMCs in the solder joints. The Sn-3.5Ag and Sn-37Pb solder pastes were placed on a Cu/Ti/Si substrate and Ni foil then annealed at 240°C to fabricate solder joints. In Sn-3.5Ag joints, the magnitude of the hardness of the IMCs was in the order Ni₃Sn₄>Cu₆Sn₅>Cu₃Sn, and the elastic moduli of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ were 125 GPa, 136 GPa, and 142 GPa, respectively. In addition, the elastic modulus of the Cu₆Sn₅ IMC in the Sn-37Pb joint was similar to that for the bulk Cu₆Sn₅ specimen but less than that in the Sn-3.5Ag joint. This might be attributed to the strengthening effect of the dissolved Ag atoms in the Cu₆Sn₅ IMC to enhance the elastic modulus in the Sn-3.5Ag/Cu joint.     

Microstructures and Thermoelectric Properties of an Annealed Ti<Subscript>0.5</Subscript>(Hf<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>)<Subscript>0.5</Subscript>NiSn<Subscript>0.998</Subscript>Sb<Subscript>0.002</Subscript> Ribbon

The thermoelectric half-Heusler compound Ti_0.5(Hf_0.5Zr_0.5)_0.5NiSn_0.998Sb_0.002 was fabricated by spin-casting and subsequent annealing. ZT at room temperature increased with annealing time through an increase in absolute Seebeck coefficients despite a decrease in electrical conductivity. ZT reached 0.10 after annealing at 1050 K for 48 h. In powder x-ray diffraction analysis, each half-Heusler peak was accompanied by a bump at the high-angle side, corresponding to a minor Ti-rich half-Heusler phase. The quantity and Ti composition of the minor phase increased with annealing time, although those of the major half-Heusler phase were almost constant. In transmission electron microscopic analysis, granular domains, several nanometers in size, with atomic ordering or disordering were observed. Thermoelectric properties were␣improved by annealing through the growth of heterogeneous microstructures of the Ti-rich and Ti-poor half-Heusler grains and of the granular domains.     

Magnetoresistance and the planar Hall effect in multilayer Co<Subscript>0.45</Subscript>Fe<Subscript>0.45</Subscript> Zr<Subscript>0.1</Subscript>/(a-Si) structures

The magnetic and magnetotransport properties of multilayer Co_0.45 Fe_0.45Zr_0.1/(a-Si) nanostructures with a 0.7-to 3.5-nm-thick amorphous silicon layer and a 2.5-to 3.5-nm-thick metal layer that are produced via ion-beam sputtering are investigated. It is demonstrated that the resistance of these structures depends on temperature as R _xx ∝-log T, which is typical of metal/dielectric nanocomposites on the metal side of the percolation transition. A negative magnetoresistance (～0.15%) is observed at a thickness of the amorphous silicon layers of no greater than 1 nm. This effect is related to spin-dependent electron transitions between the neighboring layers in the presence of the antiferromagnetic exchange interaction between them. Under the same conditions, a transverse (between the Hall probes) magnetoresistive effect amounts to 6–9%. This phenomenon is related to the anisotropic magnetoresistance and the planar Hall effect, which has not been observed in metal/dielectric nanocomposites in the vicinity of the percolation transition. It is demonstrated that a magnetic memory cell based on such a film structure with the induced magnetic anisotropy can be created.     

Low-Temperature Transport Properties of Sn<Subscript>24</Subscript>P<Subscript>19.3</Subscript>Br<Subscript>8</Subscript> and Sn<Subscript>17</Subscript>Zn<Subscript>7</Subscript>P<Subscript>22</Subscript>Br<Subscript>8</Subscript>

We report on temperature-dependent thermal conductivity, resistivity, and Seebeck coefficient of two polycrystalline Br-containing Sn-clathrate compounds with the type I crystal structure. Interstitial Br atoms reside inside the polyhedral cavities formed by the framework, resulting in hole conduction. The framework bonding directly influences the transport properties of these two compositions. The transport properties of these two clathrates are compared with those of other Sn-clathrates. We also discuss our results in terms of the potential for thermoelectric applications.     

Optical and Dielectric Properties and Microstructures of rf-Magnetron Sputtered ZnO-Doped Zr<Subscript>0.8</Subscript>Sn<Subscript>0.2</Subscript>TiO<Subscript>4</Subscript> Films on Indium Tin Oxide/Glass Substrates

Optical and dielectric properties and microstructures of ZnO-doped (Zr_0.8Sn_0.2)TiO₄ thin films prepared by radiofrequency (rf)-magnetron sputtering on indium tin oxide/glass substrates at different rf powers and substrate temperatures have been investigated. Selected-area diffraction patterns showed that the deposited films exhibited a polycrystalline microstructure. All films exhibited the ZnO-doped (Zr_0.8Sn_0.2)TiO₄ structure with the (111) orientation perpendicular to the substrate surface. The grain size as well as the deposition rate of the film increased with an increase in both rf power and substrate temperature. At an annealing temperature of 700°C, the ZnO-doped (Zr_0.8Sn_0.2)TiO₄ film possessed a dielectric constant of 47 at 10 MHz, a dissipation factor of 0.02 at 10 MHz, a leakage current density of 7.35 × 10⁻⁹  A/cm² at an electrical field of 1 kV/cm, average transmission in the visible range of over 70%, and an optical bandgap of 3.6 eV. This film will allow fabrication of fully transparent semiconductor devices such as a resistive random-access memory (RRAM) and thin-film transistors (TFTs) completely based on ZnO-doped (Zr_0.8Sn_0.2)TiO₄ thin films.     

Thermoelectric Properties of Sb-Doped Mg<Subscript>2</Subscript>Si<Subscript>0.3</Subscript>Sn<Subscript>0.7</Subscript>

Mg₂(Si_0.3Sn_0.7)_1−y Sb _y (0 ≤ y ≤ 0.04) solid solutions were prepared by a two-step solid-state reaction method combined with the spark plasma sintering technique. Investigations indicate that the Sb doping amount has a significant impact on the thermoelectric properties of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y compounds. As the Sb fraction y increases, the electron concentration and electrical conductivity of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y first increase and then decrease, and both reach their highest value at y = 0.025. The sample with y = 0.025, possessing the highest electrical conductivity and one of the higher Seebeck coefficient values among all the samples, has the highest power factor, being 3.45 mW m⁻¹ K⁻² to 3.69 mW m⁻¹ K⁻² in the temperature range of 300 K to 660 K. Meanwhile, Sb doping can significantly reduce the lattice thermal conductivity (κ _ph) of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y due to increased point defect scattering, and κ _ph for Sb-doped samples is 10% to 20% lower than that of the nondoped sample for 300 K < T < 400 K. Mg₂(Si_0.3Sn_0.7)_0.975Sb_0.025 possesses the highest power factor and one of the lower κ _ph values among all the samples, and reaches the highest ZT value: 1.0 at 640 K.     

Formation and Growth of Intermetallic Compound Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript> at Early Stages in Lead-Free Soldering

In this work, the early stages of the formation and growth of the intermetallic compound Cu₆Sn₅ during soldering reactions between a Cu substrate and liquid Sn are examined through phase-field simulations. The liquid Sn-based solder (L phase) and the copper substrate (α phase) are considered to be under metastable equilibrium conditions that eventually lead to nucleation of the Cu₆Sn₅ intermetallic compound (IMC) (η phase) at the solid/liquid interface. Nucleation is incorporated into the model through a classical treatment considering that individual nucleation events follow a Poisson distribution function. The driving forces for the nucleation and phase transformations are obtained by coupling the phase-field simulations to CALPHAD models. In the phase-field simulations, physical properties such as liquid surface as well as IMC interfacial energies are treated parametrically to probe the behavior of the system under various growth conditions. The simulations are compared with previous works and are shown to have good (qualitative) agreement with recent detailed studies on the early stages of the interaction between Cu and liquid Sn.     

Thermoelectric Properties of Co<Subscript>4</Subscript>Sn<Subscript>6</Subscript>Se<Subscript>6</Subscript> Ternary Skutterudites

A ternary ordered variant of the skutterudite structure, the Co₄Sn₆Se₆ compound, was prepared. Polycrystalline samples were prepared by a modified ceramic method. The electrical conductivity, the Seebeck coefficient and the thermal conductivity were measured over a temperature range of 300–800 K. The undoped Co₄Sn₆Se₆ compound was of p-type electrical conductivity and had a band gap E _g of approximately 0.6 eV. The influence of transition metal (Ni and Ru) doping on the thermoelectric properties was studied. While the thermal conductivity was significantly lowered both for the undoped Co₄Sn₆Se₆ compound and for the doped compounds, as compared with the Co₄Sb₁₂ binary skutterudite, the calculated ZT values were improved only slightly.     

Thermoelectric Properties of Zr<Subscript>3</Subscript>Mn<Subscript>4</Subscript>Si<Subscript>6</Subscript> and TiMnSi<Subscript>2</Subscript>

The Seebeck coefficient, electrical resistivity, and thermal conductivity of Zr₃Mn₄Si₆ and TiMnSi₂ were studied. The crystal lattices of these compounds contain relatively large open spaces, and, therefore, they have fairly low thermal conductivities (8.26 Wm⁻¹ K⁻¹ and 6.63 Wm⁻¹ K⁻¹, respectively) at room temperature. Their dimensionless figures of merit ZT were found to be 1.92 × 10⁻³ (at 1200 K) and 2.76 × 10⁻³ (at 900 K), respectively. The good electrical conductivities and low Seebeck coefficients might possibly be due to the fact that the distance between silicon atoms in these compounds is shorter than that in pure semiconductive silicon.     

Charge transport in thin layers of ferroelectric Hf<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>O<Subscript>2</Subscript>

The mechanism responsible for the charge transport in thin ferroelectric Hf_0.5Zr_0.5O₂ films has been studied. It is shown that in these films the transport mechanism is phonon-assisted tunneling between the traps. The optimal thickness of dielectric film for TiN/Hf_0.5Zr_0.5O₂/Pt structures is determined. As a result of comparing the experimental current–voltage (I–V) characteristics of TiN/Hf_0.5Zr_0.5O₂/Pt structures with the calculated ones, the thermal and optical energies of the traps are determined and the concentration of the traps is estimated. A comparison between the transport properties of ferroelectric and amorphous Hf_0.5Zr_0.5O₂ films is carried out. It is shown that the charge transport mechanism in this dielectric does not depend on its crystalline phase. A method for decreasing leakage currents in Hf_0.5Zr_0.5O₂ is proposed. A study of the resource of repolarization cycles for TiN/Hf_0.5Zr_0.5O₂/TiN metal-dielectric-metal (MDM) structures fully grown by atomic layer deposition (ALD) has been carried out.     

Development of a Thermoelectric Module Using the Heusler Alloy Fe<Subscript>2</Subscript>VAl

A thermoelectric module was constructed using a Heusler Fe₂VAl sintered alloy prepared by pulse-current sintering (PCS) using mechanically alloyed powder. A large-amplitude vibration mill was developed for powder preparation. The processing rate was increased 50-fold over that of laboratory planetary ball milling. In all, 25 pieces of millimeter-sized sintered alloy, which is useful for thermoelectric devices without cutting, were fabricated simultaneously using a single PCS process. The thermoelectric module consisting of the high-strength sintered alloy and electrode joint showed high durability. This module stably generated electric power on the exhaust pipe of a running motorcycle.     

Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript> Morphology Transition and Its Effect on Mechanical Properties of Eutectic Sn-Ag Solder Joints

The morphologies of Cu₆Sn₅ grains formed at the interface between Sn-3.5Ag (wt.% unless otherwise specified) and Cu substrates were studied in this work. Reflow experiments were performed for 60 s at peak temperatures of 513 K, 533 K, 543 K, and 553 K. Two morphologies of interfacial Cu₆Sn₅ grains were observed in wetting reactions: prism type, above 543 K, and scallop type, below 533 K. During aging, the two morphologies gradually transitioned to layer type. These three morphologies could be transformed into each other as long as the corresponding condition changed. The morphology transition of Cu₆Sn₅ in the wetting reaction was explained by the change in Jackson’s parameter with temperature. In addition, the effect of the Cu content in molten solder on interfacial Cu₆Sn₅ grains was examined. Significant differences in shear strength were observed for solder joints with different interfacial Cu₆Sn₅ morphologies in the case of a lower shear height. Joint strength is discussed in terms of the microstructure of the solder matrix and the morphology of interfacial Cu₆Sn₅ grains.     

Fabrication of an Fe<Subscript>80.5</Subscript>Si<Subscript>7.5</Subscript>B<Subscript>6</Subscript>Nb<Subscript>5</Subscript>Cu Amorphous-Nanocrystalline Powder Core with Outstanding Soft Magnetic Properties

In this study, the melt spinning method was used to develop Fe_80.5Si_7.5B₆Nb₅Cu amorphous ribbons in the first step. Then, the Fe_80.5Si_7.5B₆Nb₅Cu amorphous-nanocrystalline core with a compact microstructure was obtained by multiple processes. The main properties of the magnetic powder core, such as micromorphology, thermal behavior, permeability, power loss and quality factor, have been analyzed. The obtained results show that an Fe_80.5Si_7.5B₆Nb₅Cu amorphous-nanocrystalline duplex core has high permeability (54.8–57), is relatively stable at different frequencies and magnetic fields, and the maximum power loss is only 313 W/kg; furthermore, it has a good quality factor.     

Thickening kinetics of interfacial Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript> and Cu<Subscript>3</Subscript>Sn layers during reaction of liquid tin with solid copper

Thickening behavior of interfacial η (Cu₆Sn₅) phase and ɛ (Cu₃Sn) phase intermetallic layers was investigated in liquid tin/solid copper reaction couples over reaction times from 30 sec to over 4,000 min and temperatures from 250°C to 325°C. A scanning electron microscope (SEM) was used to quantify the interfacial microstructure at each processing condition. The η developed with a scalloped morphology, while the ɛ always grew as a somewhat undulated planar layer in phase with the η. The thickness of each phase was quantitatively evaluated from SEM micrographs using imaging software. Thickening kinetics of the ɛ and η compounds were modeled using time- and temperature-dependent empirical power-law equations. From the model, values for the kinetic exponent, rate constant, and activation energy were established for each intermetallic layer. Measured values for the kinetic exponents and activation energies suggest that thickening of the η is controlled by a grain-boundary diffusion mechanism, and growth of the ɛ occurs by solid-state diffusion, probably grain-boundary diffusion.     

Finite Element Modeling of Stress Evolution in Sn Films due to Growth of the Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript> Intermetallic Compound

We use finite element simulations to quantitatively evaluate different mechanisms for the generation of stress in Sn films due to growth of the Cu₆Sn₅ intermetallic phase at the Cu-Sn interface. We find that elastic and plastic behavior alone are not sufficient to reproduce the experimentally measured stress evolution. However, when grain boundary diffusion is included, the model results agree well with experimental observations. Examination of conditions necessary to produce the observed stresses provides insight into potential strategies for minimizing stress generation and thus mitigating Sn whisker growth.     

Density-Functional Study of the Electronic Structure and Optical Properties of Transparent Conducting Oxides In<Subscript>4</Subscript>Sn<Subscript>3</Subscript>O<Subscript>12</Subscript> and In<Subscript>4</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript>

The electronic structure and optical properties of In₄Sn₃O₁₂ and In₄Ge₃O₁₂ are studied by the projector-augmented-wave method based on the density-functional theory within the generalized gradient approximation. The cation ordering of the two compounds is explored by means of first-principles calculations. It is found that the valence-band maximum of the materials is determined by the d states of metal elements and O-2p states; the conduction-band minimum is occupied by an admixture of the O-2p states, In-5s states, and Sn-5s or Ge-4s states, respectively. The two compounds are direct-bandgap semiconductors. The low intensity of the absorption coefficient, reflectivity, and loss function shows that they are good transparent conducting oxides.     

Effect of Sb Doping on the Thermoelectric Properties of Mg<Subscript>2</Subscript>Si<Subscript>0.7</Subscript>Sn<Subscript>0.3</Subscript> Solid Solutions

This study focuses on Sb-doped Mg₂(Si,Sn) thermoelectric material. Samples were successfully fabricated using a hybrid synthesis method consisting of three different processes: induction melting, solid-state reaction, and a hot-press sintering technique. We found that the carrier concentration increased with Sb content, while the Seebeck coefficient exhibited a decreasing trend. Sb doping was shown to improve the power factor and thermoelectric figure of merit compared with the undoped material, yielding a peak figure of merit (ZT) of ~0.55 at 620 K, while leaving the band gap of Mg₂Si_0.7Sn_0.3 almost unchanged.     

Electrical and Thermal Conductivity and Conduction Mechanism of Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> Alloy

Ge₂Sb₂Te₅ alloy has drawn much attention due to its application in phase-change random-access memory and potential as a thermoelectric material. Electrical and thermal conductivity are important material properties in both applications. The aim of this work is to investigate the temperature dependence of the electrical and thermal conductivity of Ge₂Sb₂Te₅ alloy and discuss the thermal conduction mechanism. The electrical resistivity and thermal conductivity of Ge₂Sb₂Te₅ alloy were measured from room temperature to 823 K by four-terminal and hot-strip method, respectively. With increasing temperature, the electrical resistivity increased while the thermal conductivity first decreased up to about 600 K then increased. The electronic component of the thermal conductivity was calculated from the Wiedemann–Franz law using the resistivity results. At room temperature, Ge₂Sb₂Te₅ alloy has large electronic thermal conductivity and low lattice thermal conductivity. Bipolar diffusion contributes more to the thermal conductivity with increasing temperature. The special crystallographic structure of Ge₂Sb₂Te₅ alloy accounts for the thermal conduction mechanism.     

Low-Temperature Thermoelectric Properties of Co<Subscript>0.9</Subscript>Fe<Subscript>0.1</Subscript>Sb<Subscript>3</Subscript>-Based Skutterudite Nanocomposites with FeSb<Subscript>2</Subscript> Nanoinclusions

Bulk thermoelectric nanocomposite materials have great potential to exhibit higher ZT due to effects arising from their nanostructure. Herein, we report low-temperature thermoelectric properties of Co_0.9Fe_0.1Sb₃-based skutterudite nanocomposites containing FeSb₂ nanoinclusions. These nanocomposites can be easily synthesized by melting and rapid water quenching. The nanoscale FeSb₂ precipitates are well dispersed in the skutterudite matrix and reduce the lattice thermal conductivity due to additional phonon scattering from nanoscopic interfaces. Moreover, the nanocomposite samples also exhibit enhanced Seebeck coefficients relative to regular iron-substituted skutterudite samples. As a result, our best nanocomposite sample boasts a ZT = 0.041 at 300 K, which is nearly three times as large as that for Co_0.9Fe_0.1Sb₃ previously reported.