Ab initio calculations have been performed on the structural and electronic properties of pure zirconia and yttria-stabilized cubic zirconia (YSZ) We use the local-density approximation to the exchange and correlation energy functional We expand the Kohn-Sham orbitals in plane waves and use norm-conserving fully separable pseudopotentials We find, in agreement with experiments that the most stable phase at zero temperature and pressure is the monoclinic baddelyte structure which transforms under pressure in the brookite orthorhombic phase We then study the properties of the YSZ cubic phase using a supercell of 96 atoms This is a defective structure where oxygen vacancies and yttrium substitutional impurities play a major role The pattern of relaxation around the defects is consistent with the most recent scattering data, as well as their relative interaction which leads to a next-nearest-neighbor attraction between vacancy and yttrium The analysis of the electronic properties show that single occupied color centers ${\mathrm{F}}^{+}$ are only marginally stable and decay into neutral, doubly occupied F centers and empty (doubly charged) vacancies Therefore, we found that the ${\mathrm{F}}^{+}$ center in YSZ is a negative Hubbard-$U$ site

Ab initio study of structural and electronic properties of yttria-stabilized cubic zirconia

The global electronic assembly community is striving to accommodate the replacement of Pb-containing solders, primarily Sn-Pb alloys, with Pb-free solders due to environmental regulations and market pressures. Of the Pb-free choices, a family of solder alloys based on the Sn-Ag-Cu (SAC) ternary eutectic (T eut. = 217°C) composition have emerged with the most potential for broad use across the industry, but the preferred (typically near-eutectic) composition is still in debate. This review will attempt to clarify the characteristic microstructures and mechanical properties of the current candidates and recommend alloy choices, a maximum operating temperature limit, and directions for future work. Also included in this review will be an exploration of several SAC + X candidates, i.e., 4th element modifications of SAC solder alloys, that are intended to control solder alloy undercooling and solidification product phases and to improve the resistance of SAC solder joints to high temperature thermal aging effects. Again, preliminary alloy recommendations will be offered, along with suggestions for future work.

Development of Sn–Ag–Cu and Sn–Ag–Cu–X alloys for Pb-free electronic solder applications

This study included a comparison of the baseline Sn-3.5Ag eutectic to one near-eutectic ternary alloy, Sn-3.6 Ag-1.0Cu and two quaternary alloys, Sn-3.6Ag-1.0Cu-0.15Co and Sn-3.6Ag-1.0 Cu-0.45 Co, to increase understanding of the beneficial effects of Co on Sn-Ag-Cu solder joints cooled at 1–3 C/sec, typical of reflow practice. The results indicated that joint microstructure refinement is due to Co-enhanced nucleation of the Cu6Sn5 phase in the solder matrix, as suggested by Auger elemental mapping and calorimetric measurements. The Co also reduced intermetallic interface faceting and improved the ability of the solder joint samples to maintain their shear strength after aging for 72 hr at 150 C. The baseline Sn-3.5Ag joints exhibited significantly reduced strength and coarser microstructures.

Alloying effects in near-eutectic Sn-Ag-Cu solder alloys for improved microstructural stability

The mechanical behavior of Sn-rich solder/Cu joints is highly sensitive to processing variables such as solder reflow time, cooling rate, and subsequent thermal aging. In this article, we focus on the lap shear behavior of Sn-3.5Ag/Cu joints as a function of solder yield strength and intermetallic thickness. Experimental results showed that the shear strength of the solder joints is primarily controlled by the mechanical properties of the solder, and not the intermetallic thickness. The thickness of intermetallic, however, controlled the fracture mode of the solder joints. At intermetallic thicknesses greater than 20 µm, brittle fracture between Cu6Sn5 and Cu3Sn was the most common failure mechanism. Finite-element simulations were carried out to evaluate the effect of solder properties and of intermetallic thickness and morphology on lap shear behavior. The finite-element simulations corroborated the experimental findings, i.e., that increased solder strength results in increased joint strength. The simulations also showed that thicker intermetallics, especially of nodular morphology, yielded higher local plastic shear strain and work hardening rate.

Influence of reflow and thermal aging on the shear strength and fracture behavior of Sn-3.5Ag solder/Cu joints

The electronic structure of crystalline Y2O3 is investigated by first-principles calculations within the local-density approximation (LDA) of the density-functional theory. Results are presented for the band structure, the total density of states (DOS), the atom-and orbital-resolved partial DOS. effective charges, bond order, and charge-density distributions. Partial covalent character in the Y-O bonding is shown, and the nonequivalency of the two Y sites is demonstrated. The calculated electronic structure is compared with a variety of available experimental data. The total energy of the crystal is calculated as a function of crystal volume. A bulk modulus B of 183 Gpa and a pressure coefficient B' of 4.01 are obtained, which are in good agreement with compression data. An LDA band gap of 4.54 eV at Gamma is obtained which increases with pressure at a rate of dE(g)/dP = 0.012 eV/Gpa at the equilibrium volume. Also investigated are the optical properties of Y2O3 up to a photon energy of 20 eV. The calculated complex dielectric function and electron-energy-loss function are in good agreement with experimental data. A static dielectric constant of epsilon(O)= 3.20 is obtained. It is also found that the bottom of the conduction band consists of a single band, and direct optical transition at Gamma between the top of the valence band and the bottom of the conduction band may be symmetry forbidden.

Electronic, structural, and optical properties of crystalline yttria

A method of joining similar or dissimilar ceramic and ceramic composite materials, such as SiC continuous fiber ceramic composites, at relatively low joining temperatures uses a solventless, three component bonding agent effective to promote mechanical bond toughness and elevated temperature strength to operating temperatures of approximately 1200 degrees C. The bonding agent comprises a preceramic precursor, an aluminum bearing powder, such as aluminum alloy powder, and mixtures of aluminum metal or alloy powders with another powder, and and boron powder in selected proportions. The bonding agent is disposed as an interlayer between similar or dissimilar ceramic or cermaic composite materials to be joined and is heated in ambient air or inert atmosphere to a temperature not exceeding about 1200 degrees C. to form a strong and tough bond joint between the materials. The bond joint produced is characterized by a composite joint microstructure having relatively soft, compliant aluminum bearing particulate regions dispersed in a ceramic matrix.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1294&context=patents

Low temperature joining of ceramic composites

The shear deformation behavior of two lead-free solder compositions, Sn-3.5Ag (wt.%) and Sn-3.6Ag-1.0Cu (wt.%), both on copper substrates, was studied using an asymmetric four point bend technique. Four test joints were obtained from one master specimen of each composition, and each joint was subject to progressive loading, up to the maximum shear strength of the joint. One unstressed bar from each composition was retained as a reference. Each sample was metallo-graphically plished and lightly etched, and examined in a field emission scanning electron microscope (SEM) before shearing. Sheared joints were then re-examined in the SEM with no additional surface treatment. Compared with the traditional ring and plug method, the asymmetric four-point bend (AFPB) technique subjects the joints to a condition of pure shear, while providing an opportunity for unambiguous observation of microstructural features before and after shearing, without an intervening mechanical sectioning step. Shear banding in the Sn-rich matrix and crack nucleation in the vicinity of the intermetallic interface were observed at low displacements in the binary alloy. Evidence of non-homogeneous plastic flow in the matrix was seen at higher shear loading. No evidence of brittle fracture was observed in the Sn-3.6Ag-1.0 Cu alloy, with elastic deformation at low stress levels giving rise to plastic deformation at higher loading values. Results show that the AFPB technique is a viable approach to the study of shear loading on solder joints.

Shear deformation in Sn-3.5Ag and Sn-3.6Ag-1.0Cu solder joints subjected to asymmetric four-point bend tests

The asymmetrical four-point bend shear (AFPB) test method was used to measure the shear strength and creep properties through the stress relaxation experiments using three different Pb-free solder joint compositions in an assolidified condition. Since it was difficult to shear the uniform specimens and the local bending usually occurs at the inner loading points, the notches were introduced at the joint line to preferentially weaken this region. The stress analysis by finite element modeling showed that the straight notches transform the parabolic shear stress distribution in the uniform specimen into a relatively uniform shear distribution along the bond line in the notched specimens. Therefore, the shear strength results from the notched specimens are expected to be much more accurate. Experiments showed that both the Sn-3.6Ag-1Cu (wt.%) and Sn-3.6Ag-1Cu-0.45Co joints have superior strength and creep properties as compared to the Sn-3.5Ag joint. However, there was no statistical difference between the shear strength of the Sn-3.6Ag-1Cu and Sn-3.6Ag-1Cu-0.45 Co joints. Moreover, the difference between the creep resistance of these two types of joints was small.

Application of an asymmetrical four point bend shear test to solder joints

This study included a comparison of the baseline Sn–3.5Ag eutectic to one near-eutectic ternary alloy, Sn–3.6Ag–1.0Cu and two quaternary alloys, Sn–3.6Ag–1.0Cu–0.15Co and Sn–3.6Ag–1.0Cu–0.45Co, to increase understanding of the effects of Co on Sn–Ag–Cu solder joints cooled at 1–3 ◦ C/s., typical of reflow assembly practice. The results revealed joint microstructure refinement due to Co-enhanced nucleation of the Cu6Sn5 phase in the solder matrix, as suggested by Auger elemental mapping and calorimetric measurements. The Co also reduced intermetallic interface faceting and improved the ability of the solder joint samples to maintain their shear strength after aging for 72 h at 150 ◦ C. Some recent additional results with Co and Fe additions are consistent with this catalysis effect, where a reduced total solute level was tested. The baseline Sn–3.5Ag joints exhibited significantly reduced strength retention and coarser microstructures.

/pdf/effects-of-alloying-in-near-eutectic-tin-silver-copper-9gfu6hdzl7.pdf

Effects of Alloying in Near-Eutectic Tin-Silver-Copper Solder Joints

Compressive properties of polycrystalline yttrium oxide (Y{sub 2}O{sub 3}) were studied by slow-strain-rate experiments ({dot {var_epsilon}} = 5.7 {times} 10{sup {minus}6} s{sup {minus}1}) between room temperature and 1,600 C. It was shown that Y{sub 2}O{sub 3} fails in a brittle manner up to 1,000 C, and at 1,200 C and above plastic deformation becomes dominant. Plastic deformation of Y{sub 2}O{sub 3} takes place exclusively by dislocation motion. Maximum stress, yield stress, and elastic modulus decrease with increasing temperature, although the decrease at temperatures above 1,000 C is much more pronounced.

Compressive properties of yttrium oxide

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