Pb-rich Alloys Research Articles

Chemical Behavior and Local Structure of the Ruddlesden-Popper and Dion-Jacobson Alloyed Pb/Sn Bromide 2D Perovskites.

The alloyed lead/tin (Pb/Sn) halide perovskites have gained significant attention in the development of tandem solar cells and other optoelectronic devices due to their widely tunable absorption edge. To gain a better understanding of the intriguing properties of Pb/Sn perovskites, such as their anomalous bandgap's dependence on stoichiometry, it is important to deepen the understanding of their chemical behavior and local structure. Herein, we investigate a series of two-dimensional Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) phase alloyed Pb/Sn bromide perovskites using butylammonium (BA) and 3-(aminomethyl)pyridinium (3AMPY) as the spacer cations: (BA)2(MA)n-1PbxSnn-xBr3n+1 (n = 1-3) and (3AMPY)(MA)n-1PbxSnn-xBr3n+1 (n = 1-3) through a solution-based approach. Our results show that the ratio and site preference of Pb/Sn atoms are influenced by the layer thickness (n) and spacer cations (A'), as determined by single-crystal X-ray diffraction. Solid-state 1H, 119Sn, and 207Pb NMR spectroscopy analysis shows that the Pb atoms prefer the outer layers in n = 3 members: (BA)2(MA)PbxSnn-xBr10 and (3AMPY)(MA)PbxSnn-xBr10. Layered 2D DJ alloyed Pb/Sn bromide perovskites (3AMPY)(MA)n-1PbxSnn-xBr3n+1 (n = 1-3) demonstrate much narrower optical band gaps, lower energy PL emission peaks, and longer carrier lifetimes compared to those of RP analogs. Density functional theory calculations suggest that Pb-rich alloys (Pb:Sn ∼4:1) for n = 1 compounds are thermodynamically favored over 50:50 (Pb:Sn ∼1:1) compositions. From grazing-incidence wide-angle X-ray scattering (GIWAXS), we see that films in the RP phase orient parallel to the substrate, whereas for DJ cases, random orientations are observed relative to the substrate.

Supergravity-enhanced liquation crystallization for metal recovery from waste printed circuit boards

Waste printed circuit boards (WPCBs) present serious threats to the environment but also the opportunity to recover the valuable metals, motivating a search for processes to allow their cost-effective recycling. In this regard, the advantages afforded by liquation crystallization and supergravity technology were combined herein to develop a new supergravity-enhanced liquation crystallization process for recovering metals from WPCBs. The WPCB particles were fully smelted to obtain WPCB alloy. Fe-rich, Cu–Zn, Cu–Sn, and Pb-rich phases were progressively precipitated from the alloy melt as the temperature decreased and subsequently stratified from top to bottom after supergravity-induced enrichment owing to their different densities. Fe-rich, Cu–Zn, Cu–Sn, and Pb-rich alloys were obtained via supergravity-assisted separation at 1100, 850, and 500 °C, respectively. The mass fractions of Fe in the Fe-rich alloy, (Cu + Zn) in the Cu–Zn alloy, (Cu + Sn) in the Cu–Sn alloy, and Pb in the Pb-rich alloy reached 81.8 wt.%, 92.9 wt.%, 91.2 wt.%, and 94.6 wt.%, respectively, in the three separation steps. The final recoveries of Fe, Cu, Zn, Sn, and Pb after three supergravity-assisted separation stages reached 98.8%, 97.8%, 94.1%, 96.9%, and 97.4%, respectively. This study demonstrates an efficient method for separating metals from WPCBs and improving the grade of the separated metals.

Synthetic 2-D lead tin sulfide nanosheets with tuneable optoelectronic properties from a potentially scalable reaction pathway.

Solventless thermolysis of molecular precursors followed by liquid phase exfoliation allows access to two-dimensional IV-VI semiconductor nanomaterials hitherto unreachable by a scalable processing pathway. Firstly, the use of metal dithiocarbamate precursors to produce bulk alloys in the series Pb1-x Sn x S (0 ≤ x ≤ 1) by thermolysis is demonstrated. The bulk powders are characterised by powder X-ray diffraction (pXRD), Raman spectroscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. It was found that there is a transition from cubic structures for the Pb-rich alloys including the end compound, PbS (0 ≤ x ≤ 0.4) to layered orthorhombic structures for Sn-rich alloys and the end compound SnS (0.5 ≤ x ≤ 1.0). A smooth elemental progression from lead-rich to tin-rich monochalcogenides across the series of materials is observed. Liquid phase exfoliation was applied to produce two dimensional (2D) nanosheets for a mixed Pb1-x Sn x S alloy (where x = 0.8) in 1-methyl-2-pyrrolidone (NMP) using the synthetic bulk powder as starting material. The nanosheet products were characterized by SEM, atomic force microscopy (AFM) and high angle annular dark field scanning transmission electron microscopy (HAADF STEM). First principle calculations of Pb1-x Sn x S alloys show that the Sn content x modifies the size of the band gap by several 100 meV and that x changes the gap type from indirect in SnS to direct in Pb0.2Sn0.8S. These results are supported by UV-Vis spectroscopy of exfoliated Pb0.2Sn0.8S. The method employed demonstrates a new, scalable, processing pathway which can potentially be used to synthesize a range of synthetic layered structures that can be exfoliated to as-yet unaccessed 2D materials with tunable electronic properties.

Interdiffusion of Sn and Pb in liquid Pb-Sn alloys

Coefficients for the interdiffusion of Sn in Pb-rich alloys and Pb in Sn-rich alloys were established using 1.5-mm-diameter capillaries and the semi-infinite rod technique. Interdiffusion coefficients are presented for the entire concentration range from pure Pb to pure Sn, for temperatures from 668 to 1031 K. The concentration dependence of the interdiffusion coefficients was determined by establishing the concentration along the length of the capillaries and calculating the coefficients using a finite-difference technique. The interdiffusion of Sn in Pb, extrapolated to 0 at, pct Sn, is given by $$D = 8.8 \times 10^{ - 8} \exp - (22,600/RT)m^2 /s$$ and that for Pb in Sn, extrapolated to 0 at. pct Pb, by $$D = 2.4 \times 10^{ - 8} \exp - (19,300/RT)m^2 /s$$ The “average” value for the interdiffusion of Sn in Pb, for the concentration range from 0 to 74 at. pct Sn, is given by $$D = 1.1 \times 10^{ - 7} \exp - (25,200/RT)m^2 /s$$ and the average value for the interdiffusion of Pb in Sn, for the concentration range from 0 to 26 at. pct Pb, is given by $$D = 1.3 \times 10^{ - 8} \exp - (22,600/RT)m^2 /s$$ The values obtained for the coefficients agree reasonably well with previous results for the diffusion of Sn in Pb-rich alloys and are consistent with solvent self-diffusion coefficients for pure Pb and pure Sn. However, while the diffusion coefficients obtained from these Arrhenius equations are likely of the right order of magnitude, it is concluded that the results are affected by fluid flow in the capillaries, resulting in higher than actual activation energies. It is suggested that, for the capillary-reservoir technique, convective flow in the reservoir across the open end of the capillaries induces “lid-driven” flow in the upper portions of the capillaries, resulting in higher than actual diffusion coefficients, particularly for the Sn-rich alloys, since the Sn-rich end of the capillaries was open to the reservoir. Because of fluid motion induced in the capillaries, all of the results for solute and self-diffusion in Pb, both present and previous, are likely erroneous because they were obtained using the capillary-reservoir technique.

Gravitational macrosegregation in unidirectionally solidified Lead-tin alloy

Small laboratory samples of binary lead-tin alloys, about 20 mm diameter, 50 mm high, and weighing about 180 grams, were solidified unidirectionally upward, with both cooling rate and thermal gradient being closely controlled. A total of nine ingots were produced; six of these ingots had a nominal composition of Pb-15 wt pct Sn, and the remaining three were Sn-15 wt pct Pb. Detailed thermal measurements, chemical composition measurements (dendrite composition, fraction of interdendritic eutectic formed), and microstructural characterization (primary and secondary dendrite arm spacing measurements) were carried out. Normal macrosegregation was observed in the Pb-rich alloys, with Sn content being highest at the top of the ingot. In the Pb-15 wt pct Sn alloys, macrosegregation was found to increase with increasing thermal gradients, and also with decreasing cooling rates. In some of our experiments with Pb-rich alloys it was possible to induce severe segregation, in the form of deep freckles. The freckles were in the form of a column of tin-rich material which extended over the upper half of the ingot. No significant macrosegregation was evident in the Sn-rich alloy for the growth conditions employed here. Segregation behavior of all ingots was correlated with the measured thermal history, compositional data, and the fraction eutectic phase in the finally solidified samples.

Electronic structure ofPb1−xSrxS. Application of the Haydock recursion method

The Haydock recursion method is used to study the electronic structure of the pseudobinary alloy semiconductor ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{S}$. Density of states and $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$-dependent spectral weight functions are calculated for a tight-binding model. Large deviations from virtual-crystal behavior are found. The density of cation $s$ states decreases below the valence band and increases above the gap as $x$ increases from 0 to 1. For Pb-rich alloys, the absorption edge (due to the direct gap at $L$) goes from the infrared to the visible as Sr is added, in agreement with experiment. Near $x=1$ (Sr rich), the direct gap at $X$ increases slightly as the Pb concentration is increased. The gap in the density of states and the optical-absorption edge decreases, however, due to the formation on an electron-volt scale of an impurity band comprised mostly of Pb $p$ states. The advantages of the recursion method relative to the coherent-potential approximation for calculating the electronic structure of alloys are discussed. It is concluded that the recursion method represents a viable alternative for real materials.

Solidification of Undercooled Monotectic Alloys

ABSTRACTDuring the processing of monotectic alloys large compositional segregation of the liquid can precede final solidification. Under normal treatment a coarse scale phase distribution is obtained as a result of convection and sedimentation effects which may be minimized by microgravity processing. In undercooled droplets (5–20μ) of Bi-Ga alloys with compositions near the monotectic point, a finescale segregation has been observed and is similar to that formed during a microgravity treatment of bulk samples. Near the critical point of the miscibility gap, Bi-rich alloys exhibit an undercooling prior to phase separation, while in Ga-rich alloys the onset of phase separation is at the miscibility gap boundary. Similarly, in Cu-Pb alloys an undercooling below the miscibility gap prior to phase separation is observed for Cu-rich alloys, but not for Pb-rich alloys near the critical point. These observations are consistent with the operation of a critical point wetting behavior which can modify the liquid segregation pattern during microgravity treatment.

Two valence band evidence and thermal energy gap in Pb 1− xSn x Te

Hall coefficient behavior inferring the existence of two valence bands is reported. Thermal energy gap results in Pb-rich alloys are discussed, suggesting an increase of the valence band energy separation with increasing Sn content.