bluuebunny/crossref_metadata_2025 huggingface dataset:
Frictional behavior and particle adhesion of abrasive particles during Cu CMP
Authors: Hong Yi-Koan , Han Ja-Hyung , Song Jae-Hoon , Park Jin-Goo
Published: June 2019

The friction behavior and adhesion of abrasive particles were experimentally investigated during Cu CMP process. The highest particle adhesion force was measured in alumina slurry without citric acid. However, the alumina slurry with addition of citric acid had the lowest particle adhesion due to the adsorption of citrate ions on the alumina surfaces. While citrate ions could be easily adsorbed on alumina particles, silica particle showed the least effect on adsorption in citric acid solutions. The magnitude of adsorptions of citrate ions on the particle surfaces had significant effect on frictional behavior as well as adhesion force. Higher particle adhesion force resulted in higher friction, particle contamination and scratches in CMP process. It indicates that the magnitudes of particle adhesions on wafer surfaces in slurries can be directly related to the frictional behavior during CMP process.
Study on the planarization behaviour of copper CMP utilizing a dense pattern and a global step
Authors: Bormann Tilo , Bartha Johann W.
Published: June 2019

The major aim of CMP is not the removal of excess material but the planarization of the surface. Therefore the determination of the planarization length appears to be more important than the removal rate itself. It has been shown, that the planarization length is not a constant process parameter, but is related to the removal respectively to the polish time in a square root behaviour. Founded on models proposed by Boning, Ouma, et. al. we applied a sequential polish on a single quasi infinite step. The resulting profile could be simulated by a sequential convolution of the surface contour with a Gaussian transfer function.

To come closer to the situation on a chip pattern we investigated the planarization behaviour on a specific pattern of the MIT854AZ copper CMP test chip, where a large area of unpatterned surface touches a pattern with a specific constant density.

The 200 mm wafer samples consisted of RIE structured oxide films covered with 850 nm ECD copper. The polish was performed on a standard semiconductor manufacturing tool, using a commercial consumables set. The surface profiles were determined by a high resolution profiler within the polishing sequence. The densely patterned areas are removed within a certain polishing time while the transition point between the unpatterned and patterned area appears as a global step. The deposited copper thickness is sufficient to study the contour evolution in both phases, before and after removal of the dense pattern. The paper presents the experimental results on the contour evolution for the patterned fields as well as the global step.
Effect of Slurry Temperature on Cu Chemical Mechanical Polishing with Different Oxidizing Agents
Authors: Mudhivarthi Subrahmanya , Kumar Ashok
Published: June 2019

Chemical Mechanical Planarization (CMP) has evolved as one of the most critical and significant process in the manufacturing of semiconductor devices. Copper has become the material of choice for the interconnect wiring. The effect of heat generation at the interface is realized to be significant on copper CMP process. The increase in the oxidation rate of the chemical reaction and viscoelastic properties of polishing pads due to increase in temperature at the surface results in increased removal rates and change in planarization performance. In this research, the effect of temperature on the CMP performance using two most common oxidizing agents, KIO3 and H2O2 has been investigated using the CETRTM bench-top CMP tester. The coefficient of friction at the interface of polishing using both the oxidizers at different temperatures was monitored. The generated surface roughness post CMP was evaluated for samples polished using different oxidizing agents at different slurry temperatures. The AFM study was carried out using Digital InstrumentsTM D3100 instrument. This study is aimed at understanding the effect of temperature on the CMP process performance and the relative sensitivity of two most common oxidizers used in copper CMP towards temperature.
Pattern Symmetry and CMP Process Simulation
Authors: Yoshida Takafumi
Published: June 2019

This paper reviews the effect of pattern symmetry on Chemical Mechanical Planarization (CMP) and proposes a methodology to reduce the computational time of the CMP process simulation based on the boundary element method (BEM). We focus on a unit field which generates the symmetry structures and we formulate the BEM field matrix by parallel translations and spin/flip operations. We also discuss the characteristics of the field matrix and demonstrate the applications of the methodology.
In-Situ Chronoamperometry for CMP Slurry Investigations
Authors: Zhang Jian , Grumbine Steven , Carter Phillip W. , Werts Thomas
Published: June 2019

A new application of the electrochemical method, in situ chronoamperometry, has been developed and used as a tool for CMP slurry characterization. The measured current response, i, resulting from a small applied potential is fit to an exponential equation, i = io + A1exp(-t/tD). The fitting parameters serve as a measure of surface reactivity (io), surface film robustness (A1), and the kinetics of film formation (tD). A description of the method and correlations to polishing performance are discussed in this paper.
Strong Synergistic Effects between Ceria and Montmorillonite particles in Glass CMP Slurries
Authors: Fang Mingming , Ianiro Michael , Eisenhour Don , Onge Jason St.
Published: June 2019

Glass substrate has superior shock resistance and higher stiffness for increased rpm and access time than Al/NiP substrate. The main barrier for the use of glass instead of Al/NiP in desktop computers has been the high cost of the glass substrate, which is mostly caused by its need for a long polishing time. Developing a glass CMP slurry that can deliver a higher polishing rate and fewer surface defects is a continuous challenge in the disk industry. We recently discovered that adding montmorillonite particles into a ceria-based glass CMP slurry can significantly increase the polishing rate without sacrificing the surface smoothness. It is the first time, to our knowledge, that this strong synergistic effect between ceria and montmorillonite particles was reported in the glass CMP area. Several physical properties of the ceria- montmorillonite slurries, such as viscosity, zeta potential, and particle size, are characterized in order to understand the mechanism of the synergistic effect.
The Adsorption Behaviors of Citric Acid on Abrasive Particles in Cu CMP Slurry
Authors: Kang Young-Jae , Hong Yi-Koan , Song Jae-Hoon , Kim In-Kwon , Park Jin-Goo
Published: June 2019

The interaction between Cu surface and abrasive particles in slurry solution was characterized. The adsorption behavior of the citrate ions was dependent on the pH of the slurry and the concentration of the citric acid. The adsorption of citrate ions generated a highly negative charge on the alumina surface and shifted isoelectric point (IEP) to lower pH values. The Cu removal rate of alumina slurry was higher than that of colloidal silica based slurry in the investigated pH ranges. Although lower friction forces of Cu were observed in alumina based slurry of pH 4, 6 and 8, a higher friction force was observed at pH 2. This high friction force was attributed to the positive zeta potential and greater adhesion force of particle. It indicates that the magnitudes of particle adhesions on Cu surfaces in slurries can be directly related to the frictional behavior during CMP process.
Advanced ELID Process Development for Grinding Silicon Wafers
Authors: Islam M. M. , Senthil Kumar A. , Balakumar S. , Lim H. S. , Rahman M.
Published: June 2019

A completely new and novel electrolytic system, Injection Electrode (IE) assisted electrolysis, has been tried here to improve the quality of the silicon wafer surfaces generated by Electrolytic In-process Dressing (ELID) grinding process. The Experiments were conducted by varying the injection flow rate in order to perform extensive analysis on the effect of injection flow rate on the generated wheel surface condition and the consequent grinding performance. Experimental results showed that lower injection flow rate greatly improved the wheel surface condition resulting in superior ground surface qualities.
Technique for Studying Overcurrent Behavior in YBCO Coated Conductors Using a Localized Magnetic Field
Authors: Coulter J. Yates , Ashworth Stephen P. , Dowden Paul C. , Willis Jeffrey O.
Published: June 2019

Over current stabilization of YBa2Cu30x (YBCO) coated conductor high temperature superconductor tape is required in most applications. The conductor must carry currents in excess of the critical current, Ic, without damage during over current events. Conductor damage is the result of joule heating and excessive temperature rise in regions with low Ic. We have developed and applied a measurement technique using a locally applied magnetic field with a high spatial gradient to define a small area over which the Ic is depressed. By measuring the voltage and temperature as a function of current, power dissipation and temperature rise were determined. Unstabilized conductors experienced thermal runaway and are easily damaged. Copper stabilizers applied by electroplating decreased dramatically the temperature rise and increased the level of power dissipation compared with the unstabilized conductor.
Development of Rare Earth Niobate Buffer Layer for YBCO Coated Conductor Using Chemical Solution Deposition Approach
Authors: Bhuiyan M.S. , Paranthaman M. , Sathyamurthy S.
Published: June 2019

We have grown epitaxial rare earth niobate (RE3NbO7; RE – La, Ce, Nd, Sm, Eu, Gd, Ho, Y, and Yb) buffer layers on biaxially textured Ni-W substrates using chemical solution deposition approach. Precursor solutions of 0.4 M total cation concentration were spin coated on short samples of Ni-3 at.%W (Ni-W) substrates and heat-treated at 1050 to 1100°C in a gas mixture of Ar-4%H2 for 15 minutes. Effect of solution chemistry and processing atmosphere on texture and microstructure were studied. Detailed X-Ray studies indicated that all the rare earth niobates were grown epitaxially. There was no significant change observed in texture for different precursor chemistry, however, dramatic effect on surface morphology observed. SEM and AFM investigations of RE3NbO7 films reveal noticeable difference in surface morphology and roughness for various processing atmospheres. Processing under a 20% wet Ar-4% H2 gas mixture was found to be the optimum condition for growing Gd3NbO7 films with high quality microstructure.
Interface Control in All MOD Coated Conductors: Influence on Critical Currents
Authors: Pomar Alberto , Coll Mariona , Cavallaro Andrea , Gàzquez Jaume , Mestres Narcis , Sandiumenge Felip , Puig Teresa , Obradors Xavier
Published: June 2019

In this work we will report our recent progress in the control of the interface quality between buffer layers and YBCO thin films grown by the trifluoroacetates route (TFA) and how it influences the critical current of the coated conductors. We have mainly focused on vacuum and metalorganic deposited (MOD) fluorite-like CeO2 buffer layers and on MOD perovskite SrTiO3 buffer layers. We will show that for vacuum CeO2 buffer layers, microcracks at the surface can be controlled by the means of thermal treatments. Coated conductors TFA-YBCO/CeO2sputt/YSZ/CeO2/Ni with Jc(77K)˜1MA/cm2 can be grown even in the presence of these microcracks. For MOD SrTiO3 we will show that growing the buffer layer at low temperature reduces surface roughness and multilayers with high critical currents can be achieved. An all-chemical coated conductor has been grown TFA-YBCO/SrTiO3MOD/BaZrO3MOD/NiO-SOE/Ni with promising in-plane texture, Δ φYBCO=6.6°. For MOD CeO2 buffer layers, thermal annealings in oxidizing atmospheres lead to atomically flat surfaces that avoid the typical polycrystalline surfaces observed in MOD CeO2 grown in Ar/H2. High Jc multilayers can be achieved and the first all chemical coated conductor in IBAD tapes TFA-YBCO/CeO2MOD/YSZIBAD/SS has been obtained with Jc(60K)=2.3MA/cm2.
High-k Polymerized Dichlorotetramethyldisiloxane Films Deposited by Radio Frequency Pulsed Plasma for Gate Dielectrics in Polymer Field Effect Transistors
Authors: Xu Yifan , Berger Paul R. , Cho Jai , Timmons Richard B.
Published: June 2019

Polymerized dichlorotetramethyldisiloxane (DCTMDS) films deposited by radio frequency pulsed plasma polymerization (PPP) demonstrated very high dielectric constants for an organic-based system, in the range of 7 to 10. The high dielectric constants of PPP DCTMDS films are due to the high polarizability of the DCTMDS monomer. The pulsed plasma duty cycle (ON/OFF) resulted in higher dielectric constant DCTMDS films for higher duty cycles. The variation of dielectric constants does not show any trend with varying film thicknesses, indicating that the thickness of the deposited films is not significant for controlling permittivity. Post-deposition annealing in a certain temperature range improves the electrical integrity of PPP DCTMDS films, but temperatures that are too high induce even higher leakage than the samples with no heat treatment. An optimal annealing temperature was identified to be in the range of 150 °C to 200 °C. Samples annealed within this temperature window have low leakage current densities below 0.1 pA/νm2 at 10 V for film thicknesses about 100 nm. The PPP DCTMDS films are resistant to typical chemical solvents, and have withstood conventional photolithographic processing with no observable film shrinkage, warping or peeling. Film adhesion was excellent and withstood the scotch tape test.
Selective Nucleation and Growth of Large Grain Polycrystalline GaAs
Authors: Allen C. G. , Beach J. D. , Khandekar A. A. , Dorr J. C. , Veauvy C. , Collins R. T. , Kuech T.F. , Caputo R. M. , Hollingsworth R. E. , Inoki C. K. , Kuan T. S.
Published: June 2019

A method for depositing large grained polycrystalline GaAs on lattice mismatched substrates through controlled nucleation and selective growth is presented. The process was developed on Si wafers. Nucleation site formation began with nanolithography to create submicron holes in photoresist on Si. Ga metal was electrochemically deposited into the holes. Subsequent arsine anneals converted the gallium deposits into GaAs. Photoluminescence and electron diffraction verified conversion to GaAs. Metal-Organic Chemical Vapor Deposition (MOCVD) enlarged the seed crystals to coalescence without creating additional nucleation sites within the patterned field. Having successfully demonstrated the approach, subsequent work has been directed at lower cost, alternative ways to define initial nucleation sites, such as, microcontact lithography and direct decomposition of triethyl gallium to Ga metal in the MOCVD chamber.
Incorporating Optical Fiber Sensors into Fabrics
Authors: Dhawan A. , Ghosh T. K. , Muth J. F.
Published: June 2019

Optical fiber sensors have many attractive attributes including high sensitivity, environmental robustness, immunity to electromagnetic interference, and the ability to be remotely interrogated. Furthermore, by incorporating optical fibers into woven and nonwoven fabrics these sensors can be distributed across large areas. In woven optical fibers, microscopic bending is an issue due to the fibers going over and under the yarns. Microscopic and macroscopic bending losses are quantified by placing optical fibers on frames of different radii of curvature and measuring the loss of transmitted light. As an example of the non-woven process, electrospinning was used to overlay a net of sub-micron diameter fibers over the optical fibers. This protects the optical fiber, holds it in place, while still permitting flexibility. To form chemical sensors, standard telecommunications grade optical fibers were tapered such that the evanescent wave extended into the environment. Coating the fibers with a thin layer of gold then permits surface plasmon sensors to be formed. However, the resulting sensors were very fragile and hard to place into fabrics. As a result alternative processes were developed that form fiber structures that are robust enough to withstand textile manufacturing processes yet still allow interaction with the environment.
Direct Patterning of Molecular Organic Materials and Metals Using a Micromachined Printhead
Authors: Chen Jianglong , Leblanc Valérie , Kang Sung Hoon , Baldo Marc A. , Benning Paul J. , Schmidt Martin A. , Bulović Vladimir
Published: June 2019

Micron-scale lateral patterning of molecular organic thin films is still one of the most challenging issues in the practical fabrication of pixelated organic light emitting device (OLED) displays. In this work we demonstrate organic and metal thin film patterning using a micromachined printhead that modulates the vapor flux of evaporated materials incident on a substrate. The printhead is integrated with an x-y-z manipulator that facilitates patterning within a vacuum environment at pressure of less than 5×10-6 Torr. This printing scheme enables direct, solvent-free and mask-free patterning of organic optoelectronic devices on diverse substrates. As an example we fabricated an OLED array of 30×30 νm Alq3 (tris(8-hydroxyqunolinato) aluminum) pixels. 30 νm wide silver patterns were also directly written using the same technique. The results show that this printing method is capable of patterning molecular organic materials and metals at high resolution (800 dpi).
Lateral Nonuniformity And Mesoscale Effects in Giant Area Electronics
Authors: Karpov V. G. , Shvydka Diana , Roussillon Yann
Published: June 2019

The recently developed physics of thin-film photovoltaics is suggested to be representative of other giant area electronics. We show that (i) giant-area devices are intrinsically nonuniform in the lateral directions, (ii) the nonuniformity spans length scales from millimeters to meters depending on external drivers such as light intensity and bias, and (iii) it significantly impacts the device performance. We derive a fundamental length scale that discriminates between the cases of small and large-area devices, and beyond which a new physics emerges. In addition, we present a practical method of mitigating the nonuniformity effects.
A novel patterning technique for high-resolution RGB-OLED-displays: Laser induced local transfer (LILT)
Authors: Kröger Michael , Hüske Marc , Dobbertin Thomas , Meyer Jens , Krautwald Henning , Riedl Thomas , Johannes Hans-Hermann , Kowalsky Wolfgang
Published: June 2019

A novel patterning technique for high-resolution full-color OLED-displays will be discussed. Currently applied production systems for OLED-displays incorporate a shadow masking system for patterning of single red, green and blue pixels. Due to its limited scalability, alternative techniques, which can be applied to larger substrate sizes, have to be developed. One approach can be the laser induced local transfer of organic materials.

An infrared absorbing substrate (target) is coated with either a red, green or blue light-emitting organic material and placed in a short distance (below 50 νm) of the OLED-substrate onto which the organic material is to be patterned. The laser beam is deflected by a scanner onto the target in single lines. If the scanning speed and the laser power are adjusted properly, the target locally heats up to a temperature at which the organic material sublimes and condenses on the opposing OLED-substrate. By repeating this process for each colour red, green and blue stripes can be deposited. Line widths below 70 νm have been achieved.
Effect of Age-Induced Transparency on the Mechanical Properties of Human Dentin
Authors: Nalla Ravi K. , Kinney John H. , Pople John A. , Breunig Thomas M. , Tomsia Antoni P. , Ritchie Robert O.
Published: June 2019

Most non-traumatic fractures occur in teeth that have been treated, for example restored or endodontically repaired. It is therefore essential to evaluate the structure and mechanical properties of altered forms of dentin. One such altered dentin is transparent (sclerotic) dentin, which forms gradually with aging. Accordingly, in the present study, we seek to study differences in the structure, i.e., dentinal mineral concentration, mineral crystallite size, and the mechanical properties, i.e., elastic moduli, fracture toughness and fatigue behavior, of normal and transparent root dentin. The mineral concentration, measured by x-ray computed tomography, was found to be significantly higher in transparent dentin, with the majority of the increase being due to the closure of the tubule lumens. Crystallite size, as measured by small angle x-ray scattering, appeared to be slightly reduced in transparent dentin, although the difference was not statistically significant. The elastic properties remained unchanged, although transparent dentin showed almost no yield/post-yield behavior. The fracture toughness was lowered by roughly 20%, while the fatigue resistance was deleteriously affected at high stress levels. These results are discussed in terms of the altered microstructure of transparent dentin.
Quantitatively Studying Nano-mechanical Properties within the Prism and Organic Sheath of Enamel
Authors: Cui Fuzhai , Ge Jun , Wang Xiumei
Published: June 2019

Enamel is made up of enamel prisms separated by thin layer of organic sheaths. The mechanical properties of the prisms and the organic sheaths are obviously different from each other due to different compositions and microstructures. However, quantitative measurements of such differences have been a challenge in the past. The objective of this study is to accurately study the mechanical properties in the isolated domains within single enamel prism. The technique of nanoindentation combined with Atomic Force Microscopy (AFM) was employed to test the enamel specimens from mature human maxillary third molar. It was revealed that the nanohardness and elastic modulus of the sheaths were about 73.6% and 52.7% lower than those of the prisms. AFM topographies of the residual indent impressions also visually confirmed the differences. In addition to nanoindentation tests, the microstructures of enamel were carefully investigated in terms of hierarchical levels of organization to understand the structural reasons of the mechanical differences. We found a close relation between the variations of mechanical properties of enamel and its hierarchical structure. The analysis of the mechanical properties within enamel upon hierarchy is not only helpful to understand its unique property, but may also inspire ideas for the design of novel synthetic materials.
Protein Forced Unfolding and Its Effects on the Finite Deformation Stress-Strain Behavior of Biomacromolecular Solids
Authors: Jerry Qi H. , Ortiz Christine , Boyce Mary C.
Published: June 2019

Many proteins have been experimentally observed to exhibit a force-extension behavior with a characteristic repeating pattern of a nonlinear rise in force with imposed displacement to a peak, followed by a significant force drop upon reaching the peak (a “saw-tooth” profile) due to successive unfolding of modules during extension. This behavior is speculated to play a governing role in biological and mechanical functions of natural materials and biological networks composed of assemblies of such protein molecules. In this paper, a constitutive model for the finite deformation stress-strain behavior of crosslinked networks of modular macromolecules is developed. The force-extension behavior of the individual modular macromolecule is represented using the Freely Jointed Chain (FJC) statistical mechanics model together with a two-state theory to capture unfolding. The single molecule behavior is then incorporated into a formal continuum mechanics framework to construct a constitutive model. Simulations illustrate a relatively smooth “yield”-like stress-strain behavior of these materials due to activate unfolding in these microstructures.
Raman Spectroscopic Evidence for Side-Chain Unfolding in Spider Dragline Silk under Tensile Deformation
Authors: He Xiaojun , Ellison Michael S. , Palmer Jacqueline M.
Published: June 2019

In-situ Ramanspectra were collected on the N. clavipes spider dragline silk under a tensile deformation rate of 15mm/min. The most prominent features on the spectra were due to those bands near 1100 cm-1, which present as a sensitive probe to structural changes associated with side-chains of silk peptide. A downshift of Raman bands at 1095 cm-1 and 1089 cm-1 was detected with increasing strain. Furthermore, an increase in the intensity of the Raman band at 1062 cm-1 due to the vibration of trans structure without lateral coupling was prominent at certain strain levels. This was interpreted in terms of a morphology transition from the random configuration to the trans conformation modulated by the reorganization of the hydrogen bonding among the side-chain.
Modeling and characterization of the amino propeptide of collagenα1(XI), a regulatory domain in collagen fibrillar architecture.
Authors: Warner Lisa R. , Fallahi Arzhang , Kroll Becky , Irwin Katey M. , Yingst Sorcha , Mercer Linda M. , Shadle Susan E. , Oxford Julia Thom
Published: June 2019

Connective tissues such as cartilage, tendon, skin, bone, and arteries are composite bio-materials that contain predominantly water, collagen, proteoglycans and hyaluronic acid. Like any composite material, the components themselves and their interactions dictate the properties of the material. Fibrillar collagens are the principal structural molecules of the connective tissues and require regulated assembly and growth. Previous work from our lab indicates that the amino propeptide (Npp) domain of collagen type XI α1 chain regulates fibril diameter growth. Npp is a globular domain that is thought to sterically hinder the dense packing assembly of collagen molecules in fibrils. This mechanism of regulating collagen fibril assembly may be more complex than steric hindrance. We hypothesize that the Npp domain has a more dynamic role in establishing the structure/function relationship of collagen fibrils in connective tissues. In this study, the molecular structure of Npp was predicted by modeling. The model predicted putative binding sites for heparan sulfate and divalent cations. These predicted binding sites were evaluated empirically by fluorescence spectroscopy and surface plasmon resonance.
Structure and Properties of Murine and Human Dentin
Authors: Habelitz Stefan , Zartoshtimanesh Shabnam , Balooch Mehdi , Marshall Sally J. , Marshall Grayson W. , DenBesten Pamela K.
Published: June 2019

Mice are commonly considered the model mammal for many biomedical studies. In this work, mouse and human dentin were compared to specify structural and mechanical differences to establish a baseline for comparison of dental tissues between these species. Atomic force microscopy revealed tubules of about 1.0 to 1.6 μm in diameter as the main structural feature in dentin of both species. Nanoindentation yielded the elastic modulus about 15% lower in murine intertubular dentin while the hardness was almost equal. Dynamic stiffness mapping confirmed the lower elastic properties and also revealed that the peritubular region of increased mineralization around tubules is drastically reduced or maybe absent in murine dentin of this age. This study suggests that structural and mechanical differences need to be considered when murine dentin is used as a model system.
Mesostructure of the Exoskeleton of the Lobster Homarus Americanus
Authors: Raabe D. , Romano P. , Al-Sawalmih A. , Sachs C. , Servos G. , Hartwig H.G.
Published: June 2019

The exoskeleton of the lobsterHomarus americanusis a multiphase bio-composite which consists of a fibrous organic matrix (crystalline α-chitin and various types of non-crystalline proteins) and embedded biominerals (mainly calcite). In this study we present experimental data about the microscopic and mesoscopic structure of this material.
Influence of Cross-linking and Oxidation on the Microstructural Mechanical Properties of UHMWPE
Authors: Roy Marcel E.
Published: June 2019

Ultra-high molecular weight polyethylene (UHMWPE) is commonly used as a bearing surface in total joint replacements. This study examined the effects of cross-linking and oxidation on the mechanical properties of UHMWPE at the microstructural level, rather than at the bulk level. Nanoindentation tests were performed on the articulating surfaces of two freshly machined UHMWPE acetabular inserts, one of which was cross-linked, and also performed along a freshly cut cross-section of a UHMWPE tibial insert that had been sealed in its original packaging and shelf-aged for at least five years. The reduced modulus of cross-linked UHMWPE was significantly lower than native UHMWPE. Tests on the cross-section of the shelf-aged specimen revealed a reduced modulus profile with a subsurface peak that was similar to the oxidation profiles of aged UHMWPE observed by others. Future work should examine the microstructural mechanical properties of UHMWPE using methods that more accurately account for its viscoelastic nature, and should directly correlate microstructural properties with oxidation.
Three-Dimensional PEG Hydrogel Construct Fabrication using Stereolithography
Authors: Arcaute Karina , Ochoa Luis , Medina Frank , Elkins Chris , Mann Brenda , Wicker Ryan
Published: June 2019

Layered manufacturing (LM) using stereolithography (SL) of aqueous polymer solutions was accomplished so three-dimensional (3D) tissue engineered scaffolds with complex distributions of bioactive agents could be produced. Successful LM with embedded channel architectures required investigation of hydrogel thickness or cure depth as a function of photoinitiator type and concentration, energy dosage, and polymer concentration in solution. Poly(ethylene glycol) dimethacrylate (PEG-dma) with an average molecular weight of 1000 in quantities of 20% and 30% (w/v) was prepared in distilled water. Different concentrations of two photoinitiators (PIs), Sarcure1121 (2-hydroxy-2-methyl-1-phenyl-1-propanone) and Irgacure 2959 (2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone), were used to vary gel thickness at select energy dosages by controlling the scan speed of the SL machine's ultraviolet scanning system. Gel thickness was a strong function of PI type and concentration, energy dosage, and PEG-dma concentration, especially at the low PI concentrations required for implantation. The gel thickness curves were utilized to demonstrate LM for two construct geometries where different layer thicknesses were required to successfully fabricate the constructs. This work demonstrates the effective use of SL as a processing technique for complex 3D tissue scaffolds and addresses some practical considerations associated with the use of hydrogels in LM.
Investigations on Multi-axial Domain Switching Criteria for Piezoceramics
Authors: Laskewitz Bernd , Zhou Dayu , Kamlah Marc
Published: June 2019

Initially unpoled soft PZT was subjected to a proportional, coaxial electromechanical loading. The ratio of compressive stress to electric field was changed between the experiments. From this series of nonlinear polarization and strain responses were obtained. Based on an offset method, initial domain switching states in the two-dimensional stress-electric field space were determined. In continuum mechanics, thin walled tubes are used to investigate multi-axial stress states. In this context, thin walled means a ratio of wall thickness to radius of 1:10 or thinner. However, simple linear dielectric analysis indicates an inhomogeneous electric field distribution in such geometries.

Therefore, the suitability of hollow cylinders (in the range from thick to thin walled tubes) for multi-axial electromechanical experiments has to be investigated. Simulations with a finite element tool based on a phenomenological constitutive model for ferroelectric and ferroelastic hysteresis behavior were performed. The results confirm inhomogeneous distributions of electric fields and stresses after poling. A geometry variation is discussed to minimize these effects.
A Free Energy Model for the Inner Loop Behavior of Pseudoelastic Shape Memory Alloys
Authors: Heintze Olaf , Seelecke Stefan
Published: June 2019

The paper presents a free energy model for the pseudoelastic behavior of shape memory alloys. It is based on a stochastic homogenization process, which uses distributions in energy barriers and internal stresses to represent effects typically encountered in polycrystalline materials. This concept leads to a realistic desription of the rate-dependent inner loop behavior, but is characterized by rather long computation times. This is prohibitive in regard to a potential implementation into other numerical codes, such as finite element or optimal control programs or a Matlab/Simulink environment. For this purpose a parameterization method is introduced, which is derived from the concept of a representative single crystal. The approach preserves the desirable properties of the original formulation, at the same time reducing the numerical effort significantly. Finally, we show that the method can reproduce the experimentally observed behavior accurately over a large range of strain rates including the minor loop behavior.
A Stress-Dependent Hysteresis Model for PZT
Authors: Ball Brian L. , Smith Ralph C. , Kim Sang-Joo , Seelecke Stefan
Published: June 2019

This paper addresses the development of an energy-based characterization framework which quantifies stress-induced dipole switching in ferroelectric materials. Helmholtz and Gibbs energy relations that accommodate 90° and 180° dipole orientations as equilibrium states are constructed at the lattice level. For regimes in which thermal relaxation mech- anisms are negligible, minimization of the Gibbs relations provides local polarization and strain relations. Alternatively, behavior such as creep or thermal relaxation can be incorpo- rated by balancing Gibbs and relative thermal energies through Boltzmann principles. In the nal step of the development, stochastic homogenization techniques based on the assump- tion that parameters such as coercive and induced elds are manifestations of underlying distributions are employed to construct macroscopic models suitable for nonhomogeneous polycrystalline compounds. Attributes and limitations of the model are illustrated through comparison with experimental PLZT data.
Experimental Investigation of Rate-Dependent Inner Hysteresis Loops in PZT
Authors: York Alexander , Seelecke Stefan
Published: June 2019

The rate-dependence of piezoelectric materials resulting from the kinetics of domain switching is an important factor that needs to be included in realistic modeling attempts. This paper provides a systematic study of the rate-dependent hysteresis behavior of a commercially available PZT stack actuator. Experiments covering full as well as minor loops are conducted at different loading rates with polarization and strain recorded. In addition, the creep behavior at different constant levels of the electric field is observed. This provides evidence of kinetics being characterized by strongly varying relaxation times that can be associated with different switching mechanisms.
Effective Medium Approach For Calculation of Linear and Nonlinear Properties of Porous Semiconductor Composites.
Authors: Kochergin Vladimir , Föll Helmut
Published: June 2019

The general methodology of calculating linear and nonlinear properties of nanoporous and nanostructured semiconductor materials and composites is presented. A Maxwell-Garnett approach is generalized for the case of porous semiconductor materials composed of a number of differently oriented pore lattices. Specifically, the cases of electrochemically etched mesoporous silicon on (110)-oriented substrate and electrochemically-etched porous InP and GaAs materials on (100) substrates are considered. The observed optical anisotropy of mesoporous Si is explained. A biaxial anisotropy of the porous InP or GaAs material with crystallographic pores is predicted.
Finite Element Analysis of the non Linear Behavior of a Multilayer Piezoelectric Actuator
Authors: Elhadrouz M. , Zineb T. Ben , Patoor E.
Published: June 2019

A constitutive law for ferroelectric and ferroelastic piezoceramics is implemented in ABAQUS Standard using the subroutine user element. A linear solid element is defined: it is an eight-node hexahedron having the mechanical displacement components and the electric potential as degrees of freedom for each node. The element is formulated for static analysis and it needs the definition of the contribution of this element to the Jacobian (stiffness) and the definition of an array containing the contributions of this element to the right-hand-side vectors of the overall system of equations The subroutine is called for each element that is of a user-defined element type each time element calculations are required. As an example, the element is used for the simulation of a multilayer actuator made of piezoceramics. In this case, the piezoelectric equations are not valid since the electric loading induces non linear phenomena, which are captured through the constitutive law implemented in the user element.
Micromechanical Modelling of Ferroelectric Films
Authors: Huber J. E.
Published: June 2019

Ferroelectric films are growing in significance as non-volatile memory devices, sensors and microactuators. The stress state of the film, induced by processing or constraints such as the substrate, strongly affects device behaviour. Thus it is important to be able to model the coupled and constrained behaviour of film material. This work presents a preliminary study of the application of micromechanical modelling to ferroelectric films. A self-consistent micromechanics model developed for bulk ferroelectrics is adapted for thin film behaviour by incorporating features such as grain structure, mechanical clamping by the substrate, residual stresses, and crystallographic orientation of the film.
Practical Sorption Kinetics of TiCl3 Catalyzed NaAlH4
Authors: Tang Xia , Mosher Daniel A , Anton Donald L
Published: June 2019

Sodium alanate has been studied as a promising candidate material for reversible hydrogen storage due to its intermediate temperature range and relatively high storage capacity. Its rates of desorption and absorption of hydrogen have been shown to be enhanced by the addition of Ti in various compounds. To date, the sorption kinetics, especially absorption kinetics, is not well understood. In this study, a practical sorption kinetics model for TiCl3 catalyzed NaAlH4 has been developed to assist in the engineering design and evaluation of a prototype hydrogen storage system.
Entropic Elasticity Controls Nanomechanics of Single Tropocollagen Molecules
Authors: Buehler Markus J. , Wong Sophie
Published: June 2019

We report molecular modeling of stretching single molecules of tropocollagen, the building block of collagen fibrils and fibers that provide mechanical support in connective tissues. For small deformation, we observe a dominance of entropic elasticity. At larger deformation, we find a transition to energetic elasticity, which is characterized by first stretching and breaking of hydrogen bonds, followed by deformation of covalent bonds in the protein backbone, eventually leading to molecular fracture. Our force-displacement curves show excellent quantitative agreement with optical tweezer experiments, suggesting a persistence length of approximately 16 nm. We demonstrate that assembly of single TC molecules into fibrils significantly decreases their flexibility, leading to decreased contributions of entropic effects during deformation. We develop a simple continuum model capable of describing entire deformation range of TC molecules.
Time Dependent Nanomechanical Response of Nacre
Authors: Mohanty Bedabibhas , Verma Devendra , Katti Kalpana S , Katti Dinesh R
Published: June 2019

Nacre, the shiny inner layer of seashells is a model biomimetic system composed of 95% of inorganic (aragonite) phase and 5% of organic phase (mainly proteins and polysaccharides). Nacre exhibits an interlocked layered “brick and mortar” structure where the bricks are made up of aragonite and mortar is the organic phase. We have performed nanoindentation and dynamic nanoindentation tests to study the nanomechanical and dynamic nanomechanical response of nacre. The indentation experiments performed at low loads indicate an elastic modulus of about 15 GPa for the organic phase. The low load, low penetration experiments appear to be better indicators of nanomechanical behavior. Dynamic nanomechanical response of nacre was studied using dynamic nanoindentation (nano-DMA). Significant increase in the values of tan δ was observed with increase in frequency. Also the dynamic nanoindentation experiments indicate that nacre exhibits viscoelastic behavior. Further, fourier transform spectroscopy experiments of nacre in innate and undisturbed state indicate the presence of water in nacre. The nanograin structure of nacre platelets, as well as the entrapped and adsorbed water, is two important contributors to the viscoelastic response of nacre. Atomic force microscopy experiments also indicate a very high force to remove organic from the aragonite in nacre. These experiments provide important insight into nanomechanical response of nacre, its constituents and also interfaces.
Ultrastructural Mechanical and Material Characterization of Fossilized Bone
Authors: Olesiak Sara Elizabeth , Oyen Michelle , Sponheimer Matthew , Eberle Jaelyn J. , Ferguson Virginia L.
Published: June 2019

Bone plays a key role in the paleontological and archeological records and can provide insight into the biology, ecology and the environment of ancient vertebrates. Examination of bone at the tissue level reveals a definitive relationship between nanomechanical properties and the local organic content, mineral content, and microstructural organization. However, it is unclear as to how these properties change following fossilization, or diagenesis, where the organic phase is rapidly removed and the remaining mineral phase is reinforced by the deposition of apatites, calcites, and other minerals. While the process of diagenesis is poorly understood, its outcome clearly results in the potential for dramatic alteration of the mechanical response of biological tissues. In this study, fossilized specimens of mammalian long bones, collected from Colorado and Wyoming, were studied for mechanical variations. Nanoindentation performed in both longitudinal and transverse directions revealed preservation of bone's natural anisotropy as transverse modulus values were consistently smaller than longitudinal values. Additionally modulus values of fossilized bone from 35.0 to 89.1 GPa increased linearly with logarithm of the sample's age. Future studies will aim to clarify what mechanical and material elements of bone are retained during diagenesis as bone becomes part of the geologic milieu.
Mechanical Properties and Microarchitecture of Nanoporous Hydroxyapatite Bioceramic Nanoparticle Coatings on Ti and TiN
Authors: Stanishevsky Andrei , Chowdhury Shafiul , Greenstein Nathaniel , Yockell-Lelievre Helene , Koskinen Jari
Published: June 2019

The hydroxyapatite (HA) based bioceramic materials are usually prepared at high sintering temperatures to attain suitable mechanical properties. The sintering process usually results in a material which is compositionally and morphologically different from nonstoichiometric nano-crystalline HA phase of hard tissue. At the same time, HA particulates used as precursors in ceramic manufacturing are often very similar to the natural HA nanocrystals. It has been shown that synthetic nanoparticle HA (nanoHA) based materials improve the biological response in vitro and in vivo, but the information on mechanical properties of these materials is scarce.

In this work we studied the HA nanoparticle (10 – 80 nm mean size) coatings with 30 – 70% porosity prepared by a dip-coating technique on Ti and TiN substrates. It has been found that the mechanical properties of HA nanoparticle coatings are strongly influenced by the initial size, morphology, and surface treatment of nanoparticles. The nanoindentation Young's modulus and hardness of as–deposited nanoHA coatings were in the range of 2.5 – 6.9 GPa and 80 – 230 MPa, respectively. The coatings were stable after annealing up to at least 600 °C, reaching the Young's modulus up to 23 GPa and hardness up to 540 MPa, as well as in simulated body fluids.
How Baby Plants Avoid Getting Hurt and Blossom into Adulthood: The Story of a Tropical Seed
Authors: Lucas Peter W. , Cook Robert , Lowrey Timothy K.
Published: June 2019

The seed shell of Mezzettia parviflora (Annonaceae), an understory tree from the lowland tropical rain forest of Malaysia, has an extraordinarily sophisticated structure. The shell is entirely made of sclerenchyma (mechanical support tissue) that is in all parts equally dense (about 95% of volume occupied by cell wall). Yet, differences in cell shape and orientation have a strong influence on fracture resistance. The structure appears designed to allow the seed to open for germination, but to limit predation by large animals.
Universal Microstructural Patterns in Bone: Micromechanics-Based Prediction of Anisotropic Material Behavior
Authors: Fritsch Andreas , Hellmich Christian
Published: June 2019

Bone materials are characterized by an astonishing variability and diversity. Still, because of ‘architectural constraints’, their fundamental hierarchical organization or basic building plans remain largely unchanged during biological evolution. These building plans govern the mechanical interaction of the elementary components of bone (hydroxyapatite, collagen, water; with directly measurable tissue-independent elastic properties), which are here quantified through a multiscale homogenization scheme delivering effective elastic properties of bone materials: At a scale of 10 nm, long cylindrical collagen molecules, attached to each other at their ends by ∼1.5 nm long crosslinks and hosting intermolecular water inbetween, form a contiguous matrix called wet collagen. At a scale of several hundred nanometers, wet collagen and mineral crystal agglomerations interpenetrate each other, forming the mineralized fibril. At a scale of 5 microns, the solid bone matrix is represented as collagen fibril inclusions embedded in a foam of largely disordered (extrafibrillar) mineral crystals. Remarkably, needle and sphere type representations of disordered minerals deliver quasi-identical mechanical behavior of such extrafibrillar porous polycrystals. At a scale above the ultrastructure lacunae are embedded in extracellular bone matrix, forming the extravascular bone material. Model estimates predicted from tissue-specific composition data agree remarkably well with corresponding stiffness experiments across cortical and trabecular materials, which opens new possibilities in the exploitation of computer tomographic data for nano-to-macro mechanics of bone organs, especially in combination with currently investigated extensions towards damage and failure.
Modeling Receptor-Ligand Mediated Adhesion in Nanoindentation of Cells
Authors: Chunyu Zhang , Yongwei Zhang
Published: June 2019

The specific adhesion mediated by receptor-ligand binding in the nanoindentation of cells was studied by using a continuum-kinetics approach. It was found that the adhesion force only causes a shift to the indentation loading curve whereas the unloading curve is tilted downward by the adhesion force, resulting in a hysteresis. Parametric studies were conducted to investigate how experimental conditions influence the adhesion force. It was found that the maximum adhesion force is sensitive to the geometry of the indenter but insensitive to the indentation depth and the mechanical properties of cells. It was also shown that the dependence of the adhesion force on the loading and unloading rates is controlled by the association rate of the receptor-ligand binding, leading to three well-defined regimes.