Effect of compliant layers within piezoelectric composites on power generation providing electrical stimulation in low frequency applications.

For patients that use tobacco or have diabetes, bone healing after orthopedic procedures is challenging. Direct current electrical stimulation has shown success clinically to significantly improve bone healing in these difficult-to-fuse populations. Energy harvesting with piezoelectric material has gained popularity in the last decade, but is challenging at low frequencies due to material properties that limit total power generation at these frequencies. Stacked generators have been used to increase power generation at lower voltage levels but have not been widely explored as a load-bearing biomaterial to provide DC stimulation. To match structural compliance levels and increase efficiency of power generation at low frequencies, the effect of compliant layers between piezoelectric discs was investigated. Compliant Layer Adaptive Composite Stacks (CLACS) were manufactured using five PZT discs connected electrically in parallel and stacked mechanically in series with a layer of low modulus epoxy between each disc. The stacks were encapsulated, keeping PZT and overall volume constant. Each stack was electromechanically tested by varying load, frequency, and resistance. As compliant layer thickness increased, power generation increased significantly across all loads, frequencies, and resistances measured. As expected, increase in frequency significantly increased power output for all groups. Similarly, an increase applied peak-to-peak mechanical load also significantly increased power output. The novel use of CLACS for power generation under load and frequencies experienced by typical orthopedic implants could provide an effective method to harvest energy and provide power without the use of a battery in multiple low frequency applications.

Role of the resistivity of insulating field emitters on the energy of field-ionised and field-evaporated atoms.

In order to improve the accuracy of laser atom probe analyses, it is important to understand all the physical processes induced by the combination of the high electrical field and the femtosecond laser beam during field evaporation. New information can be accessed from the energy of evaporated surface atoms or field-ionised atoms of an imaging gas. In order to study the ions energy, we combine La-APT and FIM analyses in a new experimental setup equipped with electrostatic lenses. We report measurements for semiconductors and oxides and we study the influence of the illumination conditions (laser power and wavelength), the evaporation rate, the sample geometry and the tip preparation processes. The results are discussed taking into account the resistive properties of non-metallic samples and the photo-stimulated conductivity. This work clarifies the role of the laser and DC field in the energy deficit of field evaporated ions.

3D analysis of advanced nano-devices using electron and atom probe tomography.

The structural and chemical properties of advanced nano-devices with a three-dimensional (3D) architecture have been studied at the nanometre scale. An original method has been used to characterize gate-all-around and tri-gate silicon nanowire transistor by combining electron tomography and atom probe tomography (APT). Results show that electron tomography is a well suited method to determine the morphological structure and the dimension variations of devices provided that the atomic number contrast is sufficient but without an absolute chemical identification. APT can map the 3D chemical distribution of the atoms in devices but suffers from strong distortions in the dimensions of the reconstructed volume. These may be corrected using a simple method based on atomic density correction and electron tomography data. Moreover, this combination is particularly useful in helping to understand the evaporation mechanisms and improve APT reconstructions. This paper demonstrated that a full 3D characterization of nano-devices requires the combination of both tomography techniques.

Atomic-scale redistribution of Pt during reactive diffusion in Ni (5% Pt)-Si contacts.

The NiSi silicide that forms by reactive diffusion between Ni and Si active regions of nanotransistors is used nowadays as contacts in nanoelectronics because of its low resistivity. Pt is added to the Ni film in order to stabilise the NiSi phase against the formation of the high-resistivity NiSi(2) phase and agglomeration. In situ X-ray diffraction (XRD) experiments performed on material aged at 350 degrees C (under vacuum) showed the complete consumption of the Ni (5 at% Pt) phase, the regression of Ni(2)Si phase as well as the growth of the NiSi phase after 48 min. Pt distribution for this heat treatment has been analysed by laser-assisted tomographic atom probe (LATAP). An enrichment of platinum in the middle of the NiSi phase suggests that Pt is almost immobile during the growth of NiSi at the two interfaces: Ni(2)Si/NiSi and NiSi/Si. In the peak, platinum was found to substitute for Ni in the NiSi phase. Very small amounts of Pt were also found in the Ni(2)Si phase close to the surface and at the NiSi/Si interface.

Identification of phases in gas-atomised droplets by combination of neutron and X-ray diffraction techniques with atom probe tomography.

Powders of Al(68.5)Ni(31.5) alloy have been produced by gas atomisation and sieved in different grain size families. The resulting families have been analysed by combined neutron and X-ray diffraction in order to investigate the structure and identify the existing phases at the surface and in the bulk of the grains. The weight fraction of the identified phases (Al(3)Ni(2), Al(3)Ni and Al) has been estimated from a profile refinement with the FULLPROF computer codes. An additional phase was observed but could not be identified in the diffraction patterns. Starting from grains less than 5mum in diameter, samples have been shaped by annular focused ion beam into needles that were suitable for atom probe investigations. The structure and morphologies observed by different techniques are compared and discussed. It has also been possible to estimate the crystallite sizes and the strains corresponding to the different phases present in the powders from the refinement of the ND patterns. In addition to Al(3)Ni(2) and Al(3)Ni, a phase of composition close to the nominal one of the alloy was observed in the atom probe measurements. This phase could be one of the decagonal ones referred to in the literature. Small particles of composition close to Al(82)Ni(18) are attributed to the metastable Al(9)Ni(2) phase. The achieved conclusions demonstrate the complementarity of X-ray and neutron diffraction techniques and atom probe tomography to analyse complex structures.

Three-dimensional atom mapping of boron in implanted silicon.

The redistribution of boron in highly implanted 100 silicon (10keV; 5x10(15)at/cm(2)) annealed at 600 degrees C for 1h was studied using both laser-assisted wide-angle atom probe (LaWaTAP) and secondary ion mass spectrometry (SIMS). As expected, the concentration was found to increase steeply to 10(21) boron atoms/cm(3) at a distance close to 35nm and to decrease slowly to 10(19)/cm(3), a value close to the boron level of the silicon substrate. For depth under 75nm, the implantation profile of boron as given by LaWaTAP was found very close to that given by SIMS investigations without any calibration of the LaWaTAP data. For larger depth, the LaWaTAP profile is observed above that of SIMS. Detection limits of LaWaTAP for low dopant concentrations are discussed. The contribution of the background noise in the spectrum and sampling errors are considered. Fine-scale fluctuations not detected in SIMS profile and related to clustering were evidenced in LaWaTAP maps and profiles. Numerous boron clusters lying on {001} planes parallel to the implanted surface, a few nanometer in size, were identified and interpreted as boron interstitial clusters (BICs), in agreement with Cristiano et al. observations. They contained between 50 and 300 atoms (Si and B). This is much higher than that generally assumed in particular in ab-initio modelling where a few atoms BICs are considered. These clusters contained 7at% of boron in average.

The spatial resolution of 3D atom probe in the investigation of single-phase materials

The resolution of three-dimensional atom probe (3DAP) is known to be mainly controlled by the aberrations of the ion trajectories near the surface of the specimen. A model has been developed to compute the ion trajectories in 3D near a sharp hemispherical electrode defined at the atomic scale. Simulations were applied on one-phase binary alloys. The influence of the evaporation fields of chemical species is studied. Simulated desorption images are consistent with experiments in both ordered alloys and random solid solution. An extra loss in the lateral resolution is observed in disordered alloys as compared to pure metals. The predicted order of evaporation provided by this model is in excellent agreement with experiments. The stacking sequence of atomic planes reconstructed from simulated data is shown to be disturbed in a similar way as observed in real experiments with 3DAP.

Three-dimensional atomic-scale imaging of impurity segregation to line defects

Clouds of impurity atoms near line defects are believed to affect the plastic deformation of alloys. Three-dimensional atom probe techniques were used to image these so-called Cottrell atmospheres directly. Ordered iron-aluminum alloys (40 atomic percent aluminum) doped with boron (400 atomic parts per million) were investigated on an atomic scale along the <001> direction. A boron enrichment was observed in the vicinity of an <001> edge dislocation. The enriched region appeared as a three-dimensional pipe 5 nanometers in diameter, tangent to the dislocation line. The dislocation was found to be boron-enriched by a factor of 50 (2 atomic percent) relative to the bulk. The local boron enrichment is accompanied by a strong aluminum depletion of 20 atomic percent.