PDSI Research Articles

Pd–Si complexes of the type ClPd(μ2-pyO)4SiR (R = Me, Ph, Bn, Allyl, κO-(pyO)PdCl(η3-allyl); pyO = pyridine-2-olate): The influence of substituent R on the Pd–Si bond

The reactions of organosiliconpyridine-2-olates (pyridyl-2-oxysilanes) RSi(pyO)3 (pyO = pyridine-2-olate, R = Me (1a), Ph (1b), Bn (1c) and Allyl (1d)) and [PdCl2(NCMe)2] in chloroform afforded the hexacoordinate silicon complexes RSi(μ2-pyO)4PdCl (R = Me (2a), Ph (2b), Bn (2c) and Allyl (2d), respectively), which feature a Pd–Si bond, in which the Pd atom is the formal lone pair donor toward Si. The new compounds 2b, 2c, 2d were characterized with multi-nuclear NMR spectroscopy and elemental analysis. The effect of the Si-bound substituent R on the trans-disposed Pd–Si bond was studied by single-crystal X-ray diffraction and computational analyses (e.g., Natural Localized Molecular Orbitals, NLMO; topological analyses of the electron density at the bond critical point with Quantum Theory of Atoms-In-Molecules, QTAIM). A structurally related byproduct, (η3-allyl)ClPd(pyO)Si(μ2-pyO)4PdCl 2d’, which formed along with target product 2d and features an Si–O bond trans to Pd–Si, was included in this systematic study. Another byproduct from the synthesis of 2d, the pentanuclear complex ClPd(μ2-pyO)2Si(μ2-pyO)2Pd(μ2-pyO)2Si(μ2-pyO)2PdCl (compound 3) was characterized crystallographically. This compound features pentacoordinate Si atoms within trigonal–bipyramidal Si(O4Pd) coordination spheres with equatorial Pd–Si bonds to the terminal Pd atoms. The Pd–Si bond situation in this compound was elucidated with the aid of computational analyses. QTAIM analyses of 3 in conjunction with a model compound PdSi4, which features two silyl groups and two silylene ligands, indicate topological properties of the electron density at the Pd–Si bond critical point which are similar to Pd–Si bonds of silyl and silylene compounds. The latter exhibit greater similarity, which indicates features of a Pd←Si bond. In contrast, NLMO analyses of 3 identify a polar covalent Pd–Si bond with predominant Pd contribution (formal Pd→Si donation).

Tunable Interfacial Electronic Pd-Si Interaction Boosts Catalysis via Accelerating O2 and H2O Activation.

Engineering the interfacial structure between noble metals and oxides, particularly on the surface of non-reducible oxides, is a challenging yet promising approach to enhancing the performance of heterogeneous catalysts. The interface site can alter the electronic and d-band structure of the metal sites, facilitating the transition of energy levels between the reacting molecules and promoting the reaction to proceed in a favorable direction. Herein, we created an active Pd-Si interface with tunable electronic metal-support interaction (EMSI) by growing a thin permeable silica layer on a non-reducible oxide ZSM-5 surface (termed Pd@SiO2/ZSM-5). Our experimental results, combined with density functional theory calculations, revealed that the Pd-Si active interface enhanced the charge transfer from deposited Si to Pd, generating an electron-enriched Pd surface, which significantly lowered the activation barriers for O2 and H2O. The resulting reactive oxygen species, including O2 -, O2 2-, and -OH, synergistically facilitated formaldehyde oxidation. Additionally, moderate electronic metal-support interaction can promote the catalytic cycle of Pd0 ⇆ Pd2+, which is favorable for the adsorption and activation of reactants. This study provides a promising strategy for the design of high-performance noble metal catalysts for practical applications.

Multilayer crystal-amorphous Pd-based nanosheets on Si/SiO2 with interface-controlled ion transport for efficient hydrogen storage

This contribution shows an unusually high hydrogen storage of multilayer amorphous (A)-crystalline (C) Pd–Si based nanosheets when stacked in the right order. Samples with A/C/A/C/A stacking sequence exhibit 40 and 12 times larger hydrogen sorption than monolithic crystalline and amorphous samples, respectively. The maximum capacitance calculated from the fitting of electrochemical impedance measurements of the same sample is twice larger than that of the conventional polycrystalline Pd films of similar thickness. Five times higher diffusion coefficient calculated from modified Cottrell equation is obtained compared to specimens with C/A/C/A/C stacking. For the A/C/A/C/A multilayers, nanobubbles with diameters of 1–2 nm are homogeneously distributed at Si/SiO2 interface, and PdHx crystal formation in these regions confirms hydrogen-metal interactions. Furthermore, corrosion-resistant amorphous top layer permits larger amounts of hydrogen ion transfer to inner layers. Thus, hydrogen storage and production can be enhanced by smart design of multilayers targeted for proton exchange membrane electrolysis or fuel cells.

Diffusion bonding of Ti‐coated Cf‐SiCf/SiBCN composites to Nb using Ag–Pd interlayer

AbstractThe Ti‐coated Cf‐SiCf/SiBCN ceramic was diffusion‐bonded to Nb using an Ag–Pd interlayer. At a ceramic/Ag–Pd interface, it is found that Ti first reacted with C and Pd to form a TiCx/Ti(Pd) double‐layer structure, and then Ti(Pd) completely converted into TiCx with the increase of holding time or temperature. Meanwhile, the infiltration of Pd along TiCx grain boundary reacted with Si to form brittle Pd2Si compound. By contrary, only a simple NbPd3 layer was formed at the Nb/Ag–Pd interface during the whole process. A maximum shear strength of 16 ± 3 MPa is obtained for the joint prepared at 900°C for 30 min. The plastic deformation of the Ag–Pd interlayer and pullout of fibers inside ceramic contributed to the superior performance. Nevertheless, as the holding time and temperature reached 90 min and 950°C, the high chemical affinity of the Pd–Si system and enhanced atomic diffusion led to the massive formation of Pd2Si, which increased the joint brittleness and degraded the ceramic performance.

Barrier and Packaging Level CMP Optimization

The yield of chip packaging operations is a function of the surface finish that is solderable and wire-bondable. It has been shown that the use of palladium (Pd) plated lead-frame for packaging has improved the processing cost and reliability by simplifying the process integration [1]. Pd can also be used as a sacrificial layer to protect the copper (Cu) substrate material from oxidation and interdiffusion before the SnPb solder application. In packaging applications, Pd is advantageous in forming a Ni(Pd)Si allowing a lower sheet resistance at higher temperatures as compared to the NiSi in addition to much better surface morphology [2].The new integration schemes of Pd into the circuitry at the packaging level also require a CMP application where the Pd is deposited on the dielectric layer with bond-pad recess and polished to form a Pd based pad surrounded by Ni [3]. In combination with the applications of the 2.5 and 3-D microelectronics packaging technologies, the CMP electivity becomes critical where a competitive removal rate is desired between the Cu, Cu-barrier (TaN) and Ni filmsIt has been demonstrated on W-CMP in an earlier study that the 1:1:1 removal rate selectivity of the W/Ti/TiN layers can be achieved through adjusting CMP formulations in terms of the slurry chemistry and the particle concentrations. The systematic study has demonstrated a methodology for developing optimal CMP configurations for the newly introduced films to CMP applications based on electrochemical evaluations [5].In this paper, we study the removal rate selectivity for Pd integrated packaging level CMP applications through electrochemical and surface energy evaluations in commercial bulk and barrier CMP slurries. It can be seen that the thin film dissolution rates relate to the surface passivation as well as the surface energy measured by the contact angle evaluations on the Cu, Pd, TaN and Ni wafer coupons. Pd being a novel metal shows strong passivation that results in minimal dissolution in the slurry environment which requires alternative formulations for CMP slurries to promote the removal rate performance.

Torsional resonator of Pd–Si–Cu metallic glass with a low rotational spring constant

In this study, a Pd–Si–Cu metallic glass was applied to a torsional resonator with long torsion beams. It is challenging to obtain the excellent mechanical performances of metallic glass when applied to torsional resonators with a low spring constant. The resonant frequency is evaluated using a laser Doppler vibrometer, and a fundamental resonant frequency of 1438 Hz and a torsional resonant frequency of 1908 Hz are obtained. When the resonant frequency is compared to the simulation results using the finite element method, a tensile stress of 25 MPa is expected for the beam part combining the residual stress and deformation. The rotational spring constant is 8.8 × 104 pNm/rad, and the displacement of the stage per unit torque is estimated to be 5.7 pm/fNm. We propose a torsional resonator with metallic glass without residual stress and initial deformation for the detection of small torques such as quantum spins.

Spin–orbit torque generated by a ferromagnet/a metallic glass bilayer

Large charge-spin conversion in a metallic glass is expected due to disordered atomic structure. Here, we report spin–orbit torque in ferromagnet/metallic glass bilayer, in which spin current generated in the metallic glass injects into the ferromagnetic layer and exerts spin-transfer torque. The effective spin-Hall angle in Pd–Si binary alloy/Ni–Fe bilayer is evaluated by modulation of damping and spectrum shape analysis in spin-torque ferromagnetic resonance experiment. Our results indicate that Pd–Si metallic glass has large charge-spin conversion efficiency compared with Pd.

Influence of topological structure and chemical segregation on the thermal and mechanical properties of Pd–Si nanoglasses

Metallic nanoglasses are non-crystalline solids with interfacial regions, typically characterized by a modified short-range order and compositional gradients. These interfaces can act as nucleation sites for the formation of shear transformation zones during mechanical deformation, which gives rise to a deformation behavior distinct from the bulk glass counterpart. While various studies have investigated nanoglasses experimentally (mostly Fe–Sc) and in computer simulations (typically Cu–Zr), there is hitherto no study comparing compositionally identical nanoglasses and conventional metallic glasses by experiments and simulations. In this contribution, we investigate Pd–Si as a model system and compare nanoglasses produced by inert gas condensation with melt-spun ribbons. Molecular dynamics simulations and atom probe tomography provide evidence that glass–glass interfaces are primarily topological and chemical defects in this particular system. Differential scanning calorimetry shows a decrease in the glass transition and crystallization temperature of the nanoglasses compared to melt-spun ribbons. Nanoindentation and micropillar tests on Pd–Si metallic nanoglasses, however, provide evidence for shear band formation in both sample types, the melt-spun ribbons and nanoglass. Shear bands in the nanoglass samples appear more diffuse as compared to melt-spun ribbons. This is also evident from the reduced strain localization in the nanoglass. It is concluded that the topological inhomogenieties induced by forming glass–glass interfaces significantly affect the mechanical properties of nanoglasses.

Electrocatalytic Behavior of Hydrogenated Pd-Metallic Glass Nanofilms: Butler-Volmer, Tafel, and Impedance Analyses

Electrocatalytic activity and sorption behavior of hydrogen in nanosized Pd–Si–(Cu) metallic glass thin film and Pd thin film electrodes sputtered on a Si/SiO2 substrate were investigated by linear sweep voltammetry, cyclic voltammetry, and electrochemical impedance spectroscopy. The electrode MG4 (Pd69Si18Cu13) exhibits the best performance with the highest electrocatalytic activity in the hydrogen evolution region with less than half of the Tafel slope of Pd thin film of the same thickness and lowest overpotential at 10 mA cm−2. A new approach has been adopted by a nonlinear fitting of the entire region of the polarization curve (far- and near-equilibrium cathodic and anodic regions) to the Butler-Volmer model. α parameter is lowest for the MG2 electrode (Pd79Si16Cu5), marking that nonequilibrium conditions change the reaction kinetics. Together with MG2, MG4 shows the lowest Bode magnitude values for hydrogen sorption and evolution regions, indicating that the bonding and release of hydrogen atoms to the electrode is easier. MG4 electrode shows a dramatic decrease of the overpotential after 100 cycles, yielding an increase in hydrogen activity. Besides, MG4 exhibits the sharpest current density drop in the HER region in cyclic voltammetry compared with other MG and Pd electrodes, indicating higher electrocatalytic activity towards hydrogen evolution. The findings highlight the influence of the selected metallic glasses for the design and development of metal catalysts with higher sorption kinetics and/or electrocatalytic turnover.

Effect of N on the microstructure and magnetic properties of FePdSi nanocomposite films

FePdSi and FePdSiN nanocomposite films were fabricated successfully on quartz glass substrate by direct current reactive magnetron sputtering. Under the same preparation parameters, the Fe/Pd ratio of FePdSi and FePdSiN films was unbalanced, which emerged obvious difference in morphology, microstructure and magnetic properties. The addition of N played an important role on the molecular free path and sputtering rate of Fe–Pd–Si film. Preferential combination of N with Si formed an amorphous Si–N matrix between the FePd nanoparticles, which improved the coercivity, maximum magnetic energy product and remanence ratio up to 2.8 kOe, 9.4 MGOe and 0.99 after thermal treatment at 550 °C, respectively. The mechanism was proposed to account for this unique growth technique.

Ultrahigh hydrogen-sorbing palladium metallic-glass nanostructures

The hydrogenation mechanism in a Pd–Si–Cu metallic glass nanofilm, and its post-characterization by chronoamperometry/cyclic voltammetry and HR(S)TEM analyses.

Effects of Pd ion implantation and Si addition on wettability of Al/SiC system

6H-SiC (0001) monocrystal substrates were implanted with 20keV Pd ions at three doses of 5×1015, 5×1016 and 5×1017ions/cm2 at room temperature. The surface characteristics of Pd-implanted SiC substrates were analyzed by Monte Carlo simulation software SRIM-2008 and Raman spectroscopy. The effects of the Pd ion implantation into the monocrystal substrate and of the Si additions (4.8, 11.6, 19.4 and 29.2at.%) into Al on wettability of Al/6H-SiC system were investigated using the sessile drop technique in a high vacuum at 1323K, and the surface and interfacial behaviors were analyzed and discussed. The experimental results showed that the equilibrium contact angle of Al/SiC system increased with increasing Pd implantation dose, which can be mainly attributed to the decreasing interfacial interactions between the Al drop and the SiC substrate. However, the Si concentration had an opposite effect on the wettability of Al/SiC system when the Si concentration was higher or less than the equilibrium Si content of Al-Si/SiC system, which can be related to the variation of σSL of Al-Si/SiC system with the Si content.

A study on PDSI improvement for drought monitoring: focused on the estimation method of potential evapotranspiration

A study on PDSI improvement for drought monitoring: focused on the estimation method of potential evapotranspiration

Pd-on-Si catalysts prepared via galvanic displacement for the selective hydrogenation of para-chloronitrobenzene.

The direct redox reaction (galvanic displacement) between Pd(2+) and substrate Si was used to deposit Pd on Si, and the Pd-Si catalysts enabled a chemoselective hydrogenation of para-chloronitrobenzene with the selectivity for para-chloroaniline higher than 99.9% at complete conversion of para-chloronitrobenzene.

Structural heterogeneity in a binary Pd–Si metallic glass

A Pd81Si19 bulk metallic glassy rod with a diameter of 4.5 mm was produced by water quenching the fluxed alloy. Despite a negative heat of mixing between Pd and Si elements and very simple components constituting the binary Pd–Si glass-forming system, structural heterogeneity was induced either by slow cooling of a liquid or sub-Tg annealing of glassy ribbons. The sub-Tg annealing experiments evidenced that a more ordered amorphous phase emerged from the original glassy matrix. Our work provides an alternative way to tune the microstructure of metallic glasses by subsequent thermal treatment on an as-prepared single glassy phase.

The detrimental effect of flux-induced boron alloying in Pd–Si–Cu bulk metallic glasses

We report on advanced insights into the fluxing of Pd–Si–Cu bulk metallic glasses. Flux-induced boron alloying and trapping of oxides are found to be associated with the employed boron oxide fluxing agent, and both influence the attainable glass-forming ability (GFA) in opposite ways. Incorporated boron strongly deteriorates the GFA due to a rising liquidus temperature, while the oxygen reduction improves it. Thus, proper fine-tuning of the fluxing time and overheating characteristics leads to an enhancement of GFA. In the current case, the critical diameter of Pd77.5Si16.5Cu6 bulk metallic glasses can be increased to 15 mm, as compared to 3 mm in the unfluxed case. Based on these results, we illustrate that the development of further fluxing agents is crucial for enhancement of the key properties of bulk metallic glasses.

Solution synthesis of metal silicide nanoparticles.

Transition-metal silicides are part of an important family of intermetallic compounds, but the high-temperature reactions that are generally required to synthesize them preclude the formation of colloidal nanoparticles. Here, we show that palladium, copper, and nickel nanoparticles react with monophenylsilane in trioctylamine and squalane at 375 °C to form colloidal Pd(2)Si, Cu(3)Si, and Ni(2)Si nanoparticles, respectively. These metal silicide nanoparticles were screened as electrocatalysts for the hydrogen evolution reaction, and Pd(2)Si and Ni(2)Si were identified as active catalysts that require overpotentials of -192 and -243 mV, respectively, to produce cathodic current densities of -10 mA cm(-2).

First-principles study of the Pd–Si system and Pd(0 0 1)/SiC(0 0 1) hetero-structure

First-principles molecular dynamics simulations of the Pd(001)/3C–SiC(001) nano-layered structure were carried out at different temperatures ranging from 300 to 2100K. Various PdSi (Pnma, Fm3¯m, P6¯m2, Pm3¯m), Pd2Si (P6¯2m, P63/mmc, P3¯m1, P3¯1m) and Pd3Si (Pnma, P6322, Pm3¯m, I4/mmm) structures under pressure were studied to identify the structure of the Pd/Si and Pd/C interfaces in the Pd/SiC systems at high temperatures. It was found that a large atomic mixing at the Pd/Si interface occurred at 1500–1800K, whereas the Pd/C interface remained sharp even at the highest temperature of 2100K. At the Pd/C interface, voids and a graphite-like clustering were detected. Palladium and silicon atoms interact at the Pd/Si interface to mostly form C22-Pd2Si and D011-Pd3Si fragments, in agreement with experiment.

The Sr-poor part of the Sr–{Pd,Pt}–{Si,Ge} systems: Phase equilibria and crystal structure of ternary phases

Phase relations have been established by electron probe microanalysis (EPMA) and X-ray powder diffraction (XPD) for the Sr-poor part of four ternary systems: Sr–{Pd,Pt}–Si at 900°C and Sr–{Pd,Pt}–Ge at 700°C. In the Sr–Pd–Si system the formation of the silicide SrPdSi3 (BaNiSn3-type) was confirmed and a small homogeneity region was found. Furthermore, two novel compounds were detected and their crystal structure was refined from X-ray powder patterns: SrPd0.3Si1.7 (AlB2-type) and SrPd5.9Si6.1 (own-type). In the Sr–Pt–Si ternary system a novel compound with AlB2-type was discovered (SrPt0.3Si1.7), whereas SrPtSi3 with the BaNiSn3-type was confirmed. Two more compounds were detected by EPMA, but their crystal structure remains unknown. In the Sr–{Pd,Pt}–Ge systems no new compounds were observed, but the existence of SrPdGe3 and SrPtGe3 (both adopt the BaNiSn3 structure type), and SrPt4Ge12, crystallizing in the LaFe4Sb12 structure type, was corroborated. For selected ternary silicides the electronic structure was evaluated by DFT calculations.

Formation and properties of P-free Pd-based metallic glasses with high glass-forming ability

New P-free Pd-based bulk metallic glasses (BMGs) with high glass-forming ability (GFA) were synthesized in Pd–Si–Ag–Cu alloy system, based on a binary Pd83Si17 alloy. Combined addition of Ag and Cu greatly enhances the stability of supercooled liquid and GFA. The Pd-based BMGs have large supercooled liquid region of 66–77K. The best GFA was obtained for Pd75Si15Ag3Cu7 alloy, and the glassy rods with diameters of up to 10mm were fabricated by copper mold casting method without fluxing treatment. These BMGs also exhibit low viscosity of ∼2×107Pas in the supercooled liquid state, excellent thermoplastic formability, high compressive yield strength of ∼1626MPa, and large plastic strain of ∼0.063.