Pb-free System Research Articles

Divalent Lead in Aqueous Solution Changes the Surface Morphology of Dolomite and Inhibits Dissolution

In groundwater systems, heavy metal ions as solutes (e.g., Pb) can adsorb onto the surface of calcite group rocks and influence their dissolution processes. The dolomite surface was examined using field emission scanning electron microscopy (SEM) and various characterization tools and the changes in water chemistry indexes were reviewed throughout the dissolution process. Pb adsorption on the dolomite surface was evident after 15 days of exposure to 1 mg/L or 50 mg/L divalent Pb salt solutions; the Pb surface phase was mainly PbCO3 with an octahedral ligand structure. SEM images show that dolomite in divalent Pb salt solutions can lead to the surface morphology exhibiting curved dissolution steps. In the closed system with Pb (1 mg/L), the total alkalinity and conductivity of the solution were lower than in the Pb-free system, and the pH difference was slight, indicating that the dibasic metal ion Pb inhibited the dolomite dissolution process. Combined with the composition of the final solid phase, it is suggested that the dolomite surface preferentially adsorbs Pb2+ on the active sites and that the newly grown solid phase is PbCO3 possesses an octahedral ligand structure. Part of the surface-active site is occupied, resulting in a change in the dissolution profile, and thus preventing further development of the dissolution profile. Ultimately, the entire dolomite surface area is prevented from dissolution. The results of this study suggest that Pb2+ may be an effective inhibitor of dolomite dissolution and may help to further refine the geological carbon sink.

Evidence for cluster spin glass phase with precursor short-range antiferromagnetic correlations in the B -site disordered Ca(Fe1/2Nb1/2)O3 perovskite

The origin of the spin glass (SG) phase in the well-known multiferroic $\mathrm{Pb}(\mathrm{F}{\mathrm{e}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}){\mathrm{O}}_{3}$ compound remains controversial due to the complications introduced by the coexistence of SG and long-range ordered (LRO) antiferromagnetic (AFM) phases. We have addressed this controversy through a comprehensive study on a Pb-free system $\mathrm{Ca}(\mathrm{F}{\mathrm{e}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}){\mathrm{O}}_{3}$ (CFN) which does not exhibit LRO AFM transition. The SG transition in CFN is confirmed by the appearance of a cusp in the temperature dependence of dc magnetization $M(T)$ with a SG freezing temperature ${T}_{\mathrm{f}}\ensuremath{\sim}25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and bifurcation of the zero-field-cooled and field-cooled magnetization $M(T)$ below the irreversibility temperature ${T}_{\mathrm{irr}}\ensuremath{\sim}27\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Using ac susceptibility $[\ensuremath{\chi}(\ensuremath{\omega},T)]$ measurements, we show that the spin dynamics follows power/Vogel-Fulcher law-type critical dynamics which diverges at ${T}_{\mathrm{SG}}\ensuremath{\sim}24\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ with an attempt time ${\ensuremath{\tau}}_{\mathrm{o}}\ensuremath{\sim}{10}^{\ensuremath{-}6}\phantom{\rule{0.28em}{0ex}}\mathrm{s}$ suggesting cluster spin glass (CSG) behavior. The field dependence of ${T}_{\mathrm{f}}(H)$ and ${T}_{\mathrm{irr}}(H)$ is shown to follow the de Almeida--Thouless line which separates the ergodic and nonergodic phases in the $H\ensuremath{-}T$ plane and gives ${T}_{\mathrm{f}}(H=0)\ensuremath{\sim}25\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, which is in close agreement with ${T}_{\mathrm{SG}}$ obtained from $\ensuremath{\chi}(\ensuremath{\omega},T)$. The existence of the glassy phase below ${T}_{\mathrm{SG}}$ is further confirmed by the observation of slow nonexponential decay of thermoremanent magnetization with time, memory and rejuvenation effects, and unidirectional exchange-bias effect in the $M\ensuremath{-}H$ hysteresis loop of field-cooled samples. The neutron powder-diffraction patterns reveal the absence of any magnetic peak due to LRO AFM phase but show a broad diffuse peak due to the presence of $\ensuremath{\sim}2\ensuremath{-}\mathrm{nm}$-size AFM spin clusters which are responsible for the CSG freezing in CFN.

Understanding the mechanism of thermal-stable high-performance piezoelectricity

Lead zirconate titanate (PZT) ceramics have been widely used because of their large piezoelectric response and good temperature-stability in the vicinity of morphotropic phase boundary (MPB). However, the understanding on the mechanism of temperature-stable piezoelectricity in PZT is still blur and thus needs to explore properly since the urgently-needed Pb-free systems designed by the same MPB method commonly show poor temperature stability. In this paper, we investigate the property-microstructure relationship for the thermal-stable piezoelectricity through comparative studies of the phase diagram, in-situ piezoelectricity and microstructure evolution in both Pb-based and Pb-free systems. Our results show that the piezoelectric thermal-stability is closely related to the verticality of MPB and the doping elements. Tilted MPB shows worse thermal-stability as compared with that of the vertical MPB. While acceptor doped commercial PZTs show better thermal-stability when compared with that of the donor doped PZT. Moreover, our in-situ TEM reveals that it is the thermal-stable fine microstructure (i.e., no degeneration) that corresponds to the thermal-stable high-performance piezoelectricity. Our work provides an in-depth understanding of the relationship between the phase diagram, microstructure and thermal-stable piezoelectricity, which shall benefit the designing of new lead-free temperature-stable piezoelectric materials.

Anomalous atomic displacement parameters and local dynamics in the Curie range of a Pb-free relaxor ferroelectric system (Bi1-xBax)(Fe1-xTix)O3(0.36 ≤ x ≤ 0.50)

We report here the relaxor ferroelectric (RFE) behaviour in a multiferroic solid solution system, (Bi1-xBax)(Fe1-xTix)O3, at a critical disorder level of xC ∼ 0.35 in BiFeO3 and 0.65 (i.e., 1-xC = 0.35) in BaTiO3 similar to the 1:2 ratio of Mg2+ and Nb5+ in the canonical RFE Pb(Mg1/3Nb2/3)O3. This Pb-free system, like canonical Pb-based RFEs, does not exhibit macroscopic symmetry breaking and shows only the signatures of ergodicity breaking at Vogel-Fulcher freezing temperature (TVF). The atomic displacement parameters (ADPs) of Fe3+/Ti4+ and O2−, obtained using high wave vector (Q) and high-resolution synchrotron x-ray diffraction data as a function of temperature, show anomalous diffuse peaks in the Curie range. It is shown that the diffuse peak in ADPs is due to softening of the vibrational frequencies of the B-O chain (B = Fe3+/Ti4+ and O = O2−) below the Burns temperature (TB) followed by hardening below the characteristic temperature (T′m), which corresponds to a peak in the dielectric permittivity (ε′).

Ferroelectric, elastic, piezoelectric, and dielectric properties of Ba(Ti0.7Zr0.3)O3-x(Ba0.82Ca0.18)TiO3 Pb-free ceramics

We designed and synthesized a pseudo-binary Pb-free system, Ba(Ti0.7Zr0.3)O3-x(Ba0.82Ca0.18)TiO3, by combining a rhombohedral end (with only cubic to rhombohedral ferroelectric phase transition) and a tetragonal end (with only cubic to tetragonal ferroelectric phase transition). The established composition-temperature phase diagram is characterized by a tricritical point type morphotropic phase boundary (MPB), and the MPB composition has better ferroelectric, piezoelectric, and dielectric properties than the compositions deviating from MPB. Moreover, a full set of material constants (including elastic stiffness constants, elastic compliance constants, piezoelectric constants, dielectric constants, and electromechanical coupling factors) of the MPB composition are determined using a resonance method. The good piezoelectric performance of the MPB composition can be ascribed to the high dielectric constants, elastic softening, and large electromechanical coupling factor.

Large nonlinear dielectric behavior in BaTi1\u2212xSnxO3

BaTi1−xSnxO3 (BTSn, 0 ≤ x ≤ 0.30) ceramics were prepared by both the conventional sintering (CS) and sparking plasma sintering (SPS). Composition, temperature and grain size dependences of the nonlinear dielectric behaviors were systematically studied. BTSn was found to have especially large tunability (≥90%), which is larger than most other Pb-free systems, and is comparable to Pb-based relaxors. The high dielectric tunability in BTSn is attributed to its specific domain structures. Besides, temperature dependent tunability of BTSn presents a dispersed behavior and the dispersion is enhanced with the increase of Sn4+ concentrations, which is explained by the compositional fluctuation model.

Effect of Copper and Zinc on Microstructures, Melting Points and Corrosion Resistance of Sn-Zn-Cu-Bi Soldering Alloys

Sn-Zn alloy is one of the Pb-free systems that are promising because of its relatively low melting temperature and low cost. However, Zn exhibits poor corrosion and oxidation resistance that hinders its soldering applications. The objective of this research is to study the effect of Zn and Cu alloying contents on the Sn-Zn performance. Four compositions of Sn-Zn alloys were studied in this research including: Sn-7Zn, Sn-9Zn, Sn-9Zn-2Cu-Bi, and Sn-9Zn-4Cu-Bi. The microstructures were studied using Optical Microscope (OM) and Scanning Electron Microscope (SEM). The melting temperatures and corrosion resistance of the alloys were evaluated by Differential Scanning Calorimeter (DSC) and Potentiodynamic Polarization technique, respectively. The results showed that an increase in the Cu-to-Zn ratio led to better corrosion resistance. The selective corrosion of the Zn-rich phase could be visibly observed, with OM, on the post-corrosion samples. With the Cu alloying, the Cu and Zn formed an intermetallic compound resulting in a higher value of Ecorr. However, the higher Cu content caused a significant increase in the liquidus temperature due to the Cu-Zn intermetallic compound, of which the melting temperature is higher than 400 °C, resulting in an incomplete melting at low temperature.

Triple-point-type morphotropic phase boundary based large piezoelectric Pb-free material—Ba(Ti0.8Hf0.2)O3-(Ba0.7Ca0.3)TiO3

We report a large piezoelectric Pb-free system—Ba(Ti0.8Hf0.2)O3-(Ba0.7Ca0.3)TiO3, designed upon a triple-point morphotropic phase boundary (TMPB) idea. The system shows anomalies of both ac (dielectric and piezoelectric) and dc (P-E hysteresis) properties around TMPB, especially the d33 achieving ∼550 pC/N at room temperature. Moreover, the detected non-zero thermal hysteresis around triple point shows a first order transition nature and non-isotropic polarization state, as well as verified by our theoretical Landau-type deduction, thus being considered as an important factor to influence the piezoelectricity along TMPB. Our work may stimulate the study on triple-point-related critical phenomena in other ferroic systems.

A monoclinic-tetragonal ferroelectric phase transition in lead-free (K0.5Na0.5)NbO3-x%LiNbO3 solid solution

A monoclinic ferroelectric phase with space group Pm has been discovered in lead-free (K0.5Na0.5)NbO3-5%LiNbO3 solid solution ceramics by high energy synchrotron x-ray powder diffraction measurements. At ambient temperature, the lattice parameters of this monoclinic structure were (am, bm, cm; β) = (4.015 Å, 3.944 Å, 3.987 Å; 90.34°). This monoclinic phase transformed to a tetragonal (P4mm) one on heating between 340 K and 360 K. The results demonstrate the presence of structurally bridging low symmetry monoclinic phase in (K0.5Na0.5)NbO3-x%LiNbO3 solid solution system: indicating a means to achieve high piezoelectricity in Pb-free systems via domain engineering.

Large piezoelectric effect in Pb-free Ba(Ti,Sn)O3-x(Ba,Ca)TiO3 ceramics

We designed a Pb-free pseudo-binary system, Ba(Sn0.12Ti0.88)O3-x(Ba0.7Ca0.3)O3 (BTS-xBCT), characterized by a phase boundary starting from a tricritical triple point of a paraelectric cubic phase, ferroelectric rhombohedral, and tetragonal phases. The optimal composition BTS-30BCT exhibits a high piezoelectric coefficient d33 ∼ 530 pC/N at room temperature. In view of the recent report of high piezoelectricity in another Pb-free system BZT-BCT (Liu and Ren, Phys. Rev. Lett. 103, 257602 (2009)), which possesses a similar tricritical triple point in the phase diagram, it seems that forming a suitable phase boundary starting from a tricritical triple point could be an effective way to develop high-performance Pb-free piezoelectrics.

Monoclinic MC phase in (001) field cooled BaTiO3 single crystals

We report here the finding of a new phase in BaTiO3 single crystals. High precision x-ray diffraction investigations of (001) field cooled crystals have shown the presence of a monoclinic MC phase for temperatures below 300 K. This MC phase is shown to be stable upon removal of the electric field. The results demonstrate the presence of structurally bridging low symmetry phases that lie between tetragonal and rhombohedral ones in this classic perovskite system: indicating a means to achieve high piezoelectricity in Pb-free systems via domain engineering.

A comparative study of EDTA-gel derived BSCCO and Pb-BSCCO systems by thermoanalytical and X-ray diffraction techniques

Superconducting phases in the BSCCO and Pb-BSCCO systems have been produced by ethylenediaminetetraacetic acid (EDTA) gel derived route using nitrates of metal salts. Thermal decomposition and formation of various phases of gel and calcined powder of both BSCCO and Pb-BSCCO have been investigated by thermoanalytical and XRD techniques. It has been observed that Pb-containing system shows high volume of high Tc phase as compared to Pb-free system at the same sintering temperature. Thermal behavior (TG) indicates that sample doped with Pb decomposed in a series of steps when compared with Pb-free sample, which decomposed in a single step. The effect of Pb is shown to lower the sintering temperature (845°C) of formation of high Tc phase (2223). However, in the Pb-free sample the high Tc phase produced at higher temperature (860°C).

Interfacial microstructure evolution in Pb-free solder systems

The Sn-3.5Ag and Sn-3.0Ag-0.5Cu ball-grid-array solder balls bonded onto Ni/Au metallization exhibited different interfacial morphology after both wetting and solid-state reactions. In contrast to the eutectic-SnPb solder system, both Pb-free systems showed higher solder-ball shear strength after annealing. Reprecipitation of Au as (Au,Ni)Sn4 at the interface, as shown in the eutectic-SnPb solder system, was not observed in both Pb-free solder systems. Instead, Ni3Sn4 and Cu-Sn-Ni-Au intermetallic compounds (IMCs) were found in the SnAg and SnAgCu systems, respectively. In the SnAgCu system, a thick, acicular-Cu-Sn-Ni IMC formed after wetting, but a faceted-Cu-Sn-Ni-Au phase was found with longer annealing. The growth of this interfacial phase in the Sn-3.0Ag-0.5Cu solder system was also slightly inhibited by the addition of Cu, with a formation energy of about 200 kJ/mol.

Studies of the formation mechanism of the phase

The formation mechanism of the phase has been studied starting from calcined precursor powders in the Pb-containing as well as in the Pb-free system. In both cases, a nucleation and growth model was found to be suitable to describe the transformations. The formation of the phase inside Ag-sheathed tapes was also investigated. Indications for a nucleation and growth mechanism were found in this case as well.

The kinetic relationships and activation energy of Bi-based superconducting phase formation

In this paper, the kinetic relationships with regard to the informations of 2212 and 2223 superconducting phases have been studied for a Bi-based superconductor with 2223 nominal composition. The formation of the Pb-doped 2223 phase and the 2212 phase with or without Pb obey the nucleation and crystal growing model, whose kinetic equations are Q i =1− exp (− K i t n ), whereas the formation of 2223 phase in the Pb-free system belongs to the nucleus boundary shrinking model, and the equation is 1−(1−Q 2223) 1 3 =K 2223t . It is proved that the factor controlling phase formation is the activation energy of the overall reaction. The optimum sintering temperature range for the formation and the transformation relationships between the high-temperature phase and low-temperature phase are discussed.