Zhengkui Xu

Thermal stability of electroless-nickel/solder interface: Part A. interfacial chemistry and microstructure

The thermal stability of the interface between the Sn-Pb eutectic alloy and an electroless Ni-P coating was examined by cross-sectional transmission electron microscopy (TEM). The interface was formed by reflowing the eutectic Sn-Pb solder alloy between electroless Ni-P deposits. The microchemistry and microstructure of the interface were analyzed in the as-reflowed, mild-aged, and overaged conditions, by energy-dispersive spectroscopy (EDS), selected-area electron diffraction (SAD) convergentbeam electron diffraction (CBED), and bright- and dark-field imaging. In the as-reflowed condition, the interfacial microstructure consisted of a thin Ni3Sn4 intermetallic layer and a thin P-rich layer. The P-rich layer was composed of two phases, face-centered cubic (fcc) Ni and Ni3P, and the excess P was primarily due to the ejection of P from the electroless Ni-P when it reacted with Sn in the reflow process. Following the mild aging, a trilayer interfacial microstructure was found, including a coarsened Ni3Sn4 layer, a P-rich layer with increased P concentration, and a P-deficient layer. With overaging, a multilayer interfacial microstructure was developed, which consisted of two Ni-Sn intermetallic layers, Ni3Sn4 and Ni3Sn2, and three distinct P-rich layers, Ni12P5, Ni12P5 + Ni3P, and Ni3P + Ni.

ON THE EVOLUTION OF STRUCTURE AND COMPOSITION IN SOL-GEL-DERIVED LEAD ZIRCONATE TITANATE THIN LAYERS

The evolution of structure and chemical distribution in sol-gel derived Pb(Zr{sub 0.53}Ti{sub 0.47})O{sub 3} thin layers was monitored by x-ray diffraction, analytical electron microscopy, and diffuse reflectance Fourier transform infrared spectroscopy. Electron microscopy confirmed the as-deposited coatings were amorphous with short-range order. Medium-range order developed on heat treatment, and chemical heterogeneity was observed at the nanoscale. The extent of compositional heterogeneity decreased with increasing temperature. Above 500 {degree}C, the coatings crystallized into an intermediate phase which converted to the perovskite phase above 600 {degree}C.

High piezoelectric activity in (Na,K)NbO3 based lead-free piezoelectric ceramics: Contribution of nanodomains

The origin of high piezoelectric response in (Na0.52K0.48−x)(Nb0.92−xSb0.08)O3-xLiTaO3 lead-free ceramics was investigated by looking into the composition-dependent domain morphology by means of transmission electron microscopy. Nanodomains of alternate orthorhombic and tetragonal domains with a width of 20–50 nm were surprisingly observed but only for compositions near polymorphic phase coexistence zone. The formation of nanodomain morphology was considered to result from the reduction of the domain wall energy owing to the vanishing of the polar anisotropy, which ultimately induced enhanced piezoelectric properties. The result indicated that nanodomains are not proprietary features of traditional Pb-based piezoelectric ceramics with a morphotropic phase boundary.

Hot-stage transmission electron microscopy studies of phase transformations in tin-modified lead zirconate titanate

The temperature dependence of phase stability for tin‐modified lead zirconate titanate solid solution ceramics Pb(0.98)Nb0.02[(Zr1−x, Snx)1−yTiy]1−zO3 (PZST) was investigated by hot‐stage transmission electron microscopy. Compositions studied included, a material that was antiferroelectric (AFE) at room temperature with x=0.42 and y=0.04, and a material that was ferroelectric (FE) at room temperature with x=0.43 and y=0.08 (abbreviated as PZST 42/4/2 and 43/8/2, respectively). PZST 42/4/2 was found to exhibit a sequence of phase transformations on heating of AFE–multicell cubic (MCC)–simple cubic (SC), whereas PZST 43/8/2 had a sequence of FE‐AFE‐MCC‐SC. Previously referred to F spots (i.e., 1/2[111] superlattice spots) were observed in all four phases. The diffraction intensities for the F spots decreased with increasing temperature, and eventually disappeared above 300 °C. Electron diffraction confirmed the presence of the MCC phase which was characterized by the existence of weak 1/2[110] superlattice ...

Phase transformation and electric field tunable pyroelectric behavior of Pb(Nb,Zr,Sn,Ti)O3 and (Pb,La)(Zr,Sn,Ti)O3 antiferroelectric thin films

Phase transformation and pyroelectric behavior of Pb(Nb,Zr,Sn,Ti)O3 (PNZST) and (Pb,La)(Zr,Sn,Ti)O3 (PLZST) antiferroelectric (AFE) thin films were investigated as a function of temperature and dc bias field. A large pyroelectric coefficient of the order of ∼3×10−7Ccm−2K−1 was realized at the ferroelectric (FE) to AFE and the AFE to FE phase transformations in the PLZST and PNZST films, respectively. The phase transformation temperature could be readily adjusted by dc bias for both films. The large pyroelectric coefficient combined with excellent dc tunability at the phase transformation temperature makes these two systems promising candidates for uncooled tunable pyroelectric thermal sensing applications.

Origin of F spots and stress sensitivity in lanthanum lead zirconate titanate

In this study a systematic microstructural characterization of La‐modified lead zirconate titanate was carried out by hot‐ and cold‐stage transmission electron microscopy and selected area electron diffraction. The existence of 1/2[111] superlattice reflections (F spots) became evident near 300 °C on cooling. The intensity of the F spots was found to increase with decreasing temperature, approaching a near constant intensity below 100 °C. In conjunction with compositional studies, it was proposed that the F spots arise from a doubling of the unit cell associated with a tilting of oxygen octahedra. The presence of F spots at temperatures approaching the onset of local polarization was then interpreted as the existence of a hierarchy of domain states. This hierarchy was proposed to consist of an average‐cubic macroscopic structure containing rhombohedral micropolar regions on the scale of 50–100 A that contain ferroelastic microtwins on the scale of several tens of angstroms.

Ferroelectric properties of PbxSr1−xTiO3 and its compositionally graded thin films grown on the highly oriented LaNiO3 buffered Pt∕Ti∕SiO2∕Si substrates

Thin films of ferroelectric PbxSr1−xTiO3 (PST) with x=0.3–0.7 and graded composition were fabricated on LaNiO3 buffered Pt∕Ti∕SiO2∕Si substrates by a sol-gel deposition method. The thin films crystallized into a single perovskite structure and exhibited highly (100) preferred orientation after postdeposition annealing at 650°C. The grain size of PST thin films systematically decreased with the increase of Sr content. Dielectric and ferroelectric properties were investigated as a function of temperature, frequency, and dc applied field. Pb0.6Sr0.4TiO3 films showed a dominant voltage dependence of dielectric constant with a high tunability in a temperature range of 25–230°C. The compositionally graded PST thin films with x=0.3–0.6 also showed the high tunability. The graded thin films exhibited a diffused phase transition accompanied by a diffused peak in the temperature variations of dielectric constants. This kind of thin films has a potential in a fabrication of a temperature stable tunable device.

Uncooled tunable pyroelectric response of antiferroelectric Pb0.97La0.02(Zr0.65Sn0.22Ti0.13)O3 perovskite

Pb0.97La0.02(Zr0.65Sn0.22Ti0.13)O3 ceramic was confirmed to be in an antiferroelectric (AFE) phase at T<170°C by macroscopic and microscopic studies. The electric-field-induced ferroelectric (FE) state was found to exhibit a much longer lifetime than the reasonable laboratory measuring time scale at T<60°C, which has led to the frequent misinterpretation of this material system as FE. The dc bias dependence of the dielectric and pyroelectric properties was studied as a function of temperature. The poled sample exhibited a low dissipation factor (tanδ∼0.03), a large transient pyroelectric coefficient of the order of 10−3–10−2Cm−2K−1, and excellent dc tunability at the FE-to-AFE transition temperature (39°C∕kV∕mm). The possible application of this material to uncooled tunable pyroelectric thermal sensing is also discussed.

In situ transmission electron microscopy observations of electric-field-induced domain switching and microcracking in ferroelectric ceramics

In situ transmission electron microscopy (TEM) technique was developed to examine micromechanisms of the electric fatigue in ferroelectric ceramics. The technique was based on a specially designed specimen connected to a modified TEM heating stage. With this technique, domain switching and nanodomain alignment near crack-like flaws were observed under cyclic electric fields. Following repeated electric cycles, microcracks were found to develop along domain and grain boundaries.

In situ TEM study of electric field-induced microcracking in piezoelectric single crystals

Abstract Pb(Mg 1/3 Nb2 /3 )O3–PbTiO3 (PMN–PT) single crystals exhibit ultrahigh piezoelectric coefficients and are the most promising candidates for the next generation of transducers, sensors and actuators. One critical problem that limits the device performance using these crystals is the fatigue degradation associated with the electric cycling. Microcracking is the most serious degradation phenomenon in these piezoelectric materials. In this work, in situ transmission electron microscopy (TEM) was used to investigate electric field-induced microcracking in PMN–PT single crystals. Microcrack initiation from a fine pore under a cyclic field and field-induced ferroelectric domain boundary cracking were directly observed in the piezoelectric single crystals.