John E. Blendell

Phase coexistence and ferroelastic texture in high strain (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 piezoceramics

Dielectric permittivity and x-ray diffraction measurements were used to identify a region of phase coexistence between the rhombohedral and tetragonal phases near the morphotropic phase boundary in (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 (BZT-BCT). This phase coexistence prevails over a considerable composition and temperature range and is bounded by single rhombohedral or tetragonal phases. The maximum piezoelectric response measured in terms of maximum strain divided by maximum electric field, Smax/Emax, is extraordinarily high, with the largest value of 1310 pm/V for x = 0.45. Electrical poling induces ferroelastic domain textures in both the rhombohedral and tetragonal phases simultaneously, which increases the piezoelectric performance significantly. The stability of that ferroelastic texture is limited by the phase transition at the morphotropic phase boundary, suggesting coupling between both coexisting phases and limiting potential applications. The results were confirmed using in situ temperature...

Visualization of a phantom post-yield deformation process in cortical bone

A prominent opacity is evident in the process zone of notched thin wafers of bone loaded in tension. Being recoverable upon unloading, this opaque zone can be stained only when the sample is under load, unlike the classically reported forms of damage which take up the stain in the unloaded state. Furthermore, despite the stain uptake, microcracks are absent in the stained area examined by high magnification optical microscopy and atomic force microscopy (AFM). Therefore, the size scale and the electric charge of the features involved in the process zone were probed at the submicron level by using a wide range of fluorescent dyes of different molecular weights and charges. It was observed that negatively charged dyes penetrate the process zone and that dyes greater than 10 kDa (about 10-20 nm in size) were unable to label the process zone. Digital image correlation (DIC) measurements indicated that the opacity initiates at about 1% principal strain and the strain accumulates up to 14%. While the opacity was largely recoverable upon unloading, the core regions which experienced large strains had permanent residual strains up to 2%, indicating that the observed deformation phenomenon can be interlocked within bone matrix without the formation of microcracks. Based on the similarity of size and their known affinity for negatively charged species, exposure of mineral nanoplatelets is proposed as prime candidates. Therefore, the deformation process reported here may be associated with debonding of mineral crystals from the neighboring collagen molecules. Overall, post-yield deformation of bone at the micron scale takes place by large strain events which are accommodated in bone matrix by the generation of nanoscale positively charged interfaces.

Virtual piezoforce microscopy of polycrystalline ferroelectric films

An innovative methodology is presented that utilizes the experimental results of electron backscattered diffraction to map the crystallographic orientation of each grain, the finite element method to simulate the local grain-grain interactions, and finally piezoforce microscopy to infer the local properties of polycrystalline ferroelectric materials by comparing the output of the numerical calculation(s) with the experimental results. The proposed combined method resolves the local hysteretic and electromechanical interactions in polycrystalline ferroelectric films, thus quantifying the effects of grain corners and boundaries on the polycrystal’s macroscopic response. For a polycrystalline lead zirconate titanate sample, a finite range of crystallographic orientations and epitaxial strains is found to enhance the out-of-plane electrical response of the film with respect to its single-crystal, stress-free counterpart. Results show that {111} oriented grains parallel to the normal of the surface of the film...

Reduction of the piezoelectric performance in lead-free (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 piezoceramics under uniaxial compressive stress

The effect of a uniaxial compressive stress on the properties of BZT-BCT samples across the morphotropic phase boundary (MPB) is investigated using direct piezoelectric coefficient measurements. In contrast to many lead zirconate titanate compositions, the piezoelectric coefficient decreases monotonically with increasing stress and does not show an initial increase or plateau. Electrically softer rhombohedral and MPB compositions are found to be more susceptible to a decrease in piezoelectric coefficient under an increasing pre-stress than tetragonal compositions. Depoling due to ferroelastic domain switching alone, as observed by x-ray diffraction, does not explain this reduction, but instead a decreasing domain wall density is proposed to be responsible for reduced piezoelectric coefficients under increasing compressive stress. The relaxation of the piezoelectric response after complete unloading supports this proposed mechanism.

Defect Morphology and Texture in Sn, Sn–Cu, and Sn–Cu–Pb Electroplated Films

In this paper, the concept of a defect phase diagram is introduced which quantifies the effects of Cu and Pb additions to electrodeposited Sn films on surface defect formation, including but not limited to the formation of Sn whiskers. Transitions were observed in both the defect densities and the morphologies of hillocks and whiskers as Cu and Pb film compositions were systematically varied. Changes in crystallographic texture were also reported for a subset of the Sn-Cu-Pb alloys examined. The transitions between different defect types and the coexistence of certain defect types help to interpret the role of grain boundary pinning in hillock and whisker formation.

Domain switching mechanisms in polycrystalline ferroelectrics with asymmetric hysteretic behavior

A numerical method is presented to predict the effect of microstructure on the local polarization switching of bulk ferroelectric ceramics. The model shows that a built-in electromechanical field develops in a ferroelectric material as a result of the spatial coupling of the grains and the direct physical coupling between the thermomechanical and electromechanical properties of a bulk ceramic material. The built-in fields that result from the thermomechanically induced grain-grain electromechanical interactions result in the appearance of four microstructural switching mechanisms: (1) simple switching, where the c-axes of ferroelectric domains will align with the direction of the applied macroscopic electric field by starting from the core of each grain; (2) grain boundary induced switching, where the domain’s switching response will initiate at grain corners and boundaries as a result of the polarization and stress that is locally generated from the strong anisotropy of the dielectric permittivity and th...

Effect of Crystallographic Texture on the Field-Induced-Phase Transformation Behavior of Bi 0.5Na 0.5TiO 3−7BaTiO 3−2K 0.5 Na 0.5NbO 3

The electric-field-induced phase transformation of {001}pc oriented Bi0.5Na 0.5TiO 3−7BaTiO 3−2K 0.5Na 0.5NbO 3 bulk ceramics with a 8.3 multiples of a random distribution fiber texture was investigated using in situ diffraction. Field-dependent diffraction suggests that the high strain is a result of an electric-field-induced pseudo-cubic to tetragonal phase transformation, where the induced tetragonal phase has a strong domain texture. The effect of electric field direction was investigated by applying the electric field parallel and perpendicular to the fiber texture axis. Applied field direction affected the lattice spacing of the induced phase, domain texture, and increased poling field to induce the phase transformation.

Domain evolution in lead-free thin film piezoelectric ceramics

Due to environmental and health concerns lead-free piezoelectric systems are currently being evaluated for use as replacements for lead-based ceramics. (1-x)Na0.5Bi0.5TiO3 – xBaTiO3 (NBT-xBT) is a promising alternative. In order to develop materials with improved performance, it is necessary to understand local structure effects on the piezoelectric response at the grain level. Using piezoresponse force microscopy (PFM), we have studied the domain evolution under locally applied electric fields. Using pulsed laser deposition thin films are deposited on Pt/Ti/SiO2/Si substrates at a temperature of 650 °C in a 150 mTorr O2 environment. In NBT-0.05BT films, domains are clearly visible after the application of a bias field and they relax after a period of time. After the films are exposed to ambient conditions for several weeks, they must be annealed before domains are again visible with PFM. Domain images after poling indicate that the polarization direction is rotating in the plane of the sample.

Whiskers, hillocks, and film stress evolution in electroplated Sn and Sn-Cu films

The growth of surface defects on lead-free tin electroplated films is believed to be a stress relief phenomenon. These observed defects include hillocks and whiskers that grow spontaneously after deposition. Previous work has shown that it is possible to plate pure tin and observe only hillock growth. Whisker growth, however, is observed with the addition of copper contamination to the electrolyte. In this work, pure tin and tin-copper films of increasing copper content were electroplated on phosphor bronze substrates. The stresses associated with plating these films and the evolution of stress over time were measured using cantilever beam deflection. The density and morphologies of hillocks and whiskers were measured as a function of time and are related to cantilever beam stress measurements. Results show an increase in long-term stress with increasing copper content. An initial decrease in the plating stress over the first 24 to 48 hours and an increase in relaxation rate with copper amounts of less than 0.0018 mol / L in the electrolyte was also observed. A transition in film behavior observed in copper concentrations between 0.0018 and 0.0057 mol / L, as characterized by an increasing as- plated plating stress, decreasing relaxation rate, and the appearance of whiskers. A change in hillock appearance was also observed in these concentrations. Results support a growth model where a source feeds material into a defect while the ratio of incoming material to the rate of grain boundary movement determines the types of defects observed.

The ageing and de-ageing behaviour of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 lead-free piezoelectric ceramics

Ageing behaviour usually occurs in acceptor-doped piezoelectric materials (e.g., hard lead zirconate titanate) and exhibits the development of a pinched or shifted hysteresis loop over time. Although no pinched hysteresis loop was observed for lead-free (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 material, this study showed that the piezoelectric properties change over time in the poled state. The shift of the hysteresis loop along the electric field axis and the development of asymmetry in strain and permittivity hysteresis loop were observed during the ageing process. The origin of this ageing behaviour is proposed to be local defect dipoles and the migration of the charged defects to the grain boundaries. The reorientation of the defect dipole contributes to a fast but unstable ageing mechanism in this material while the migration of the charged defects contributes to a slow but more stable mechanism.