Bioactive ceramic-reinforced composites for bone augmentation

Biomaterials have been used to repair the human body for millennia, but it is only since the 1970s that man-made composites have been used. Hydroxyapatite (HA)-reinforced polyethylene (PE) is the first of the ‘second-generation’ biomaterials that have been developed to be bioactive rather than bioinert. The mechanical properties have been characterized using quasi-static, fatigue, creep and fracture toughness testing, and these studies have allowed optimization of the production method. The in vitro and in vivo biological properties have been investigated with a range of filler content and have shown that the presence of sufficient bioactive filler leads to a bioactive composite. Finally, the material has been applied clinically, initially in the orbital floor and later in the middle ear. From this initial combination of HA in PE other bioactive ceramic polymer composites have been developed

The anisotropic grain size effect on the mechanical response of polycrystals: The role of columnar grain morphology in additively manufactured metals

Additively manufactured (AM) metals exhibit highly complex microstructures, particularly with respect to grain morphology which typically features heterogeneous grain size distribution, anomalous and anisotropic grain shapes, and the so-called columnar grains. In general, the conventional morphological descriptors are not suitable to represent complex and anisotropic grain morphology of AM microstructures. The principal aspect of microstructural grain morphology is the state of grain boundary spacing or grain size whose effect on the mechanical response is known to be crucial. In this paper, we formally introduce the notions of axial grain size and grain size anisotropy as robust morphological descriptors which can concisely represent highly complex grain morphologies. We instantiated a discrete sample of polycrystalline aggregate as a representative volume element (RVE) which has random crystallographic orientation and misorientation distributions. However, the instantiated RVE incorporates the typical morphological features of AM microstructures including distinctive grain size heterogeneity and anisotropic grain size owing to its pronounced columnar grain morphology. We ensured that any anisotropy arising in the macroscopic mechanical response of the instantiated sample is mainly associated with its underlying anisotropic grain size. The RVE was then used for meso-scale full-field crystal plasticity simulations corresponding to uniaxial tensile deformation along different axes via a spectral solver and a physics-based crystal plasticity constitutive model. Through the numerical analyses, we were able to isolate the contribution of anisotropic grain size to the anisotropy in the mechanical response of polycrystalline aggregates, particularly those with the characteristic complex grain morphology of AM metals. Such a contribution can be described by an inverse square relation

Multiscale modelling of delayed hydride cracking

A mechanistic model of delayed hydride cracking (DHC) is crucial to the nuclear industry as a predictive tool for understanding the structural failure of zirconium alloy components that are used to clad fuel pins in water-cooled reactors. Such a model of DHC failure must be both physically accurate and computationally efficient so that it can inform and guide nuclear safety assessments. However, this endeavour has so far proved to be an unsurmountable challenge because of the seemingly intractable multiscale complexity of the DHC phenomenon, which is a manifestation of hydrogen embrittlement that involves the interplay and repetition of three constituent processes: atomic scale diffusion, microscale precipitation and continuum scale fracture. This investigation aims to blueprint a novel multiscale modelling strategy to simulate the early stages of DHC initiation: stress-driven hydrogen diffusion-controlled precipitation of hydrides near loaded flaws in polycrystalline zirconium. Following a careful review of the experimental observations in the literature as well as the standard modelling techniques that are commonplace in nuclear fuel performance codes in the first part of this dissertation, the second and third parts introduce a hybrid multiscale modelling strategy that integrates concepts across a spectrum of length and time scales into one self-consistent framework whilst accounting for the complicated nuances of the zirconium-hydrogen system. In particular, this strategy dissects the DHC mechanism into three interconnected modules: (i) stress analysis, which performs defect micromechanics in hexagonal close-packed zirconium through the application of the mathematical theory of planar elasticity to anisotropic continua; (ii) stress-diffusion analysis, which bridges the classical long-range elastochemical transport with the quantum structure of the hydrogen interstitialcy in the trigonal environment of the tetrahedral site; and (iii) diffusion-precipitation analysis, which translates empirical findings into an optimised algorithm that emulates the thermodynamically favourable spatial assembly of the microscopic hydride needles into macroscopic hydride colonies at prospective nucleation sites. Each module explores several unique mechanistic modelling considerations, including a multipolar expansion of the forces exerted by hydrogen interstitials, a distributed dislocation representation of the hydride platelets, and a stoichiometric hydrogen mass conservation criterion that dictates the lifecycle of hydrides. The investigation proceeds to amalgamate the stress, stress-diffusion and diffusion-precipitation analyses into a unified theory of the mesoscale mechanics that underpin the early stages of DHC failure and a comprehensive simulation of the flaw-tip hydrogen profiles and hydride microstructures. The multiscale theory and simulation are realised within a bespoke software which incorporates computer vision to generate mesoscale micrographs that depict the geometries, morphologies and contours of key metallographic entities: cracks and notches, grains, intergranular and intragranular nucleation sites as well as regions of hydrogen enhancement and complex networks of hydride features. Computer vision mediates the balance between simulation accuracy and simulation efficiency, which is completely novel in the context of DHC research as a paradigm at the intersection of computational science and computer science. Preliminary tests show that the simulation environment of the hybrid model is significantly more accurate and efficient in comparison with the traditional finite element and phase field methodologies. Due to this unprecedented simulation accuracy-efficiency balance, realistic flaw-tip hydrogen profiles and hydride microstructures can be simulated within seconds, which naturally facilitates statistical averaging over ensembles. Such statistical capabilities are highly relevant to nuclear safety assessments and, therefore, a systematic breakdown of the model formulation is presented in the style of a code specification manual so that the bespoke software can be readily adapted within an industrial setting. As the main contribution to DHC research, the proposed multiscale model comprises a state-of-the-art microstructural solver whose unrivalled versatility is demonstrated by showcasing a series of simulated micrographs that are parametrised by flaw acuity, grain size, texture, alloy composition, and histories of thermomechanical cycles. Direct comparisons with experimental micrographs indicate good quantitative agreement and provide some justification to the known qualitative trends. Furthermore, the overall hybrid methodology is proven to scale linearly with the number of hydrides, which is computationally advantageous in its own right because it allows the bespoke software to be extended without compromising its speed. Several possible extensions are outlined which would improve the phenomological accuracy of the multiscale model whilst retaining its efficiency. In its current form, however, this hybrid multiscale model of the early stages of DHC goes far beyond existing methodologies in terms of simulation scope.Open Acces

The interplay of interfaces, supramolecular assembly, and electronics in organic semiconductors

Organic semiconductors, which include a diverse range of carbon-based small molecules and polymers with interesting optoelectronic properties, offer many advantages over conventional inorganic semiconductors such as silicon and are growing in importance in electronic applications. Although these materials are now the basis of a lucrative industry in electronic displays, many promising applications such as photovoltaics remain largely untapped. One major impediment to more rapid development and widespread adoption of organic semiconductor technologies is that device performance is not easily predicted from the chemical structure of the constituent molecules. Fundamentally, this is because organic semiconductor molecules, unlike inorganic materials, interact by weak non-covalent forces, resulting in significant structural disorder that can strongly impact electronic properties. Nevertheless, directional forces between generally anisotropic organic-semiconductor molecules, combined with translational symmetry breaking at interfaces, can be exploited to control supramolecular order and consequent electronic properties in these materials. This review surveys recent advances in understanding of supramolecular assembly at organic-semiconductor interfaces and its impact on device properties in a number of applications, including transistors, light-emitting diodes, and photovoltaics. Recent progress and challenges in computer simulations of supramolecular assembly and orientational anisotropy at these interfaces is also addressed.Belinda J Boehm, Huong TL Nguyen and David M Huan

Processing organic semiconductors

PhDIn recent years, there has been a considerable interest in organic semiconducting materials due to their potential to enable, amongst other things, low-cost flexible opto-electronic applications, such as large-area integrated circuitry boards, light-emitting diodes (OLEDs) and organic photovoltaics (OPVs). Promisingly, improved electronic performance and device structures have been realized with e.g. OLEDs entering the market and organic field-effect transistors (OFETs) reaching the performance of amorphous silicon devices; however, it would be too early to state that the field of organic semiconductors has witnessed the sought-after technological revolution. Initial progress in the field was mostly due to synthetic efforts in the form of enhanced regularity and purity of currently used materials, the creation of new molecular species, etc. In this thesis we show that the advancement of physico-chemical aspects – notably materials processing – and the realisation of increased order and control of the solid state structure is critical to realize the full intrinsic potential that organic semiconductors possess. We first investigated how the bulk charge-transport properties of the liquid-crystalline semiconductor poly(2,5-bis (3-dodecylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTT-C12) can be enhanced by annealing in the mesophase. To this end, temperature treatment of a period of hours was necessary to realize good bulk charge transport in the out-of-plane directions. This behaviour is in strong contrast to in-plane charge transport as measured in thin-film field-effect structures, for which it was shown that annealing times of 10 min and less are often sufficient to enhance device performance. Our observation 4 may aid in future to optimize the use of pBTTT polymers in electronic devices, in which good bulk charge transport is required, such as OPVs. In the second part of thesis, we explored ink-jet printing of pBTTT-C12, in order to realize precise deposition of this material into pre-defined structures. In organic electronic applications this can, amongst other things, enable deposition of different semiconductors or reduction of the unwanted conduction pathways that often result in undesirable parasitic ‘cross-talk’, for instance, between pixels in display products. We demonstrate the integration of ink-jet printed transistors into unipolar digital logic gates that display the highest signal gain reported for unipolar-based logic gates. Finally, recognizing that a broad range of conjugated organic species fall in the category of “plastic crystals”, we explored the option to process this class of materials in the solid state. We find that solid-state compression moulding indeed can effectively be applied to a wide spectrum of organic small molecular and polymeric semiconductors without affecting adversely the intrinsic favourable electronic characteristics of these materials. To the contrary, we often observe significantly enhanced [bulk] charge transport and essentially identical field-effect transistor performance when compared with solution- or melt-processed equivalents. We thus illustrate that fabrication of functional organic structures does not necessitate the use of solution processing methods, which often require removal of 99 wt% or more of solvent, or precursor side-products, nor application of cumbersome vapour deposition technologies

Magnetic Hybrid-Materials

Externally tunable properties allow for new applications of suspensions of micro- and nanoparticles in sensors and actuators in technical and medical applications. By means of easy to generate and control magnetic fields, fluids inside of matrices are studied. This monnograph delivers the latest insigths into multi-scale modelling, manufacturing and application of those magnetic hybrid materials

Magnetic Hybrid-Materials

Externally tunable properties allow for new applications of suspensions of micro- and nanoparticles in sensors and actuators in technical and medical applications. By means of easy to generate and control magnetic fields, fluids inside of matrices are studied. This monnograph delivers the latest insigths into multi-scale modelling, manufacturing and application of those magnetic hybrid materials

Investigating the Roles of Matrix Nanotopography and Elasticity in the Osteogenic Differentiation of Mesenchymal Stem Cells.

We used substrates of poly(methyl methacrylate) (PMMA) to investigate the influence of nanotopography on the osteogenic phenotype of mesenchymal stem cells (MSCs), focusing on their ability to produce mineral similar to bone. Topography induced anisotropy in contact angles and surface free energy (SFE). Smooth PMMA had an SFE of 40 mN/m. Topographic surfaces had elevated and reduced SFEs when measuring parallel (up to ~45 mN/m ) or perpendicular to the gratings (as low as ~31 mN/m). Topography influenced alignment and enhanced MSC proliferation after 14 days of culture. Focal adhesion size was influenced after 7 days. Calcium deposition was not increased after 21 days. Ca: P ratios were similar to native mouse bone on films with gratings of 415 nm width and 200 nm depth (G415) and 303 nm width and 190 nm depth (G303). Notably, all surfaces had Ca:P ratios significantly lower than G415 films (less than 1.39). We used thin films of poly(ethylene glycol diacrylate) (PEGDA) with nanoscale gratings and tunable elasticity to investigate the potential synergistic effect on the osteogenic phenotype of MSCs. Three distinct moduli were used having a shear storage modulus of ~64, 300, and 530 kPa respectively. Topography did not influence cell alignment nor did the combination of matrix topography and elasticity enhance the calcium deposition or Ca:P ratios. Collagen I density (5 or 50 µg/mL) did not significantly influence calcium deposition. These data demonstrate that nanotopographic PMMA films, mimicking the architecture of bone, do not enhance calcium levels in mineral deposited by hMSCs. We showed that increased focal adhesion size on PMMA nanotopography is insufficient by itself to drive increased calcium deposition. Reports in literature rarely quantify changes in focal adhesions with changes in mineral quantity and composition. Similarly, an in vivo report suggests that topography does not enhance osteogenesis. In the first known attempt to combine adhesion ligand concentration (Col I) to nanogratings on surfaces with varied mechanical properties, these inputs were insufficient to synergistically enhance an osteogenic phenotype in MSCs. We highlight the importance of studying functional output rather than minor markers as function is a greater indicator of differentiation.PhDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/107156/1/ijanson_1.pd

Multiscale Calculations of Intrinsic and Extrinsic Properties of Permanent Magnets

Permanent magnets with high coercivity Hc and maximum energy product (BH)max are indispensible for the modern technologies in which electric energy is efficiently converted to motion, or vice versa. Modelling and simulation play an important role in mechanism understanding and optimization of Hc and (BH)max and uncovering the associated coercivity mechanism. However, both Hc and (BH)max are extrinsic properties, i.e., they depend on not only the intrinsic magnetic properties of the constituent phases but also the microstructures across scales. Therefore, multiscale simulations are desirable for a mechanistic and predictive calculation of permanent magnets. In this thesis, a multiscale simulation framework combining first-principles calculations, atomistic spin model (ASM) simulations, and micromagnetic simulations is demonstrated for the prediction of temperature-dependent intrinsic magnetic properties as well as the microstructure-related extrinsic properties in permanent magnets, with a focus on Nd-Fe-B and rare-earth free exchange-spring magnets. The main contents and results are summarized in the following. (1) The intrinsic temperature-dependent magnetic properties of the main phase Nd2Fe14B in Nd-Fe-B permanent magnets are calculated by ab-initio informed ASM simulations. The ASM Hamiltonian for Nd2Fe14B is constructed by using the Heisenberg exchange of Fe–Fe and Fe–Nd atomic pairs, the uniaxial single-ion anisotropy of Fe atoms, and the Nd ion crystal-field energy. The calculated temperature-dependent saturation magnetization Ms(T ), effective magnetic anisotropy constants Keff i (T ) (i = 1, 2, 3), domain-wall width δw(T ), and exchange stiffness constant Ae(T) are found to agree well with the experimental results. This calculation framework enables a scale bridge between first-principles calculations and temperature-dependent micromagnetic simulations of permanent magnets. (2) The intrinsic bulk exchange stiffness Ae in Nd2Fe14B and the extrinsic interface exchange coupling strength Jint between Nd2Fe14B and grain boundary (GB), as well as their influences on Hc, are explored by combining the first-principles calculations, ASM simulations, and micromagnetic simulations. Both Ae and Jint are found to be anisotropic. Ae is larger along crystallographic a/b axis than along c axis of Nd2Fe14B. "Double anisotropy" phenomenon regarding to GB is discovered, i.e., in addition to GB magnetization anisotropy, Jint is also strongly anisotropic even when GB possesses the same magnetization. It is found that Jint for (100) interface is much higher than that for (001) interface. The discovered anisotropic exchange is shown to have profound influence on Hc. These findings allow new possibilities in designing Nd-Fe-B magnets by tuning exchange. (3) Hc of Nd-Fe-B permanent magnets with featured microstructure are calculated by combining ASM and micromagnetic simulations. With the intrinsic properties from ASM results as input, finite-temperature micromagnetic simulations are performed to calculate the magnetic reversal and Hc at high temperatures. It is found that apart from the decrease of anisotropy field with increasing temperature, thermal fluctuations further reduce Hc by 5–10% and β (temperature coefficient of Hc) by 0.02–0.1% K−1 when a defect layer exists. Both Hc and β can be enhanced by adding the Dy-rich shell, but they saturate at a shell thickness (tsh) around 6–8 nm after which further increasing tsh or adding Dy into the core is not essential. (4) The microstructural influence in rare-earth free permanent magnet candidates, in particular the α′′-Fe16N2/SrAl2Fe10O19 composite and MnBi/FexCo1−x bilayer are investigated in collaboration with the experimental and theoretical partners. For the former, pure micromagnetic simulations show that the design criterion for the magnetically hard/softphase composite is invalid for the hard/semi-hard-phase composite. α′′-Fe16N2 nanoparticle diameter less than 50 nm and an interface exchange in the order of 0.01–0.1 pJ/m enable the Hc enhancement, while less surface oxides and higher volume fraction of α′′-Fe16N2 nanoparticles are decisive for enhancing the composite’s (BH)max. For the latter, DFTinformed micromagnetic simulations show that the interface roughness could deteriorate the interface exchange coupling and induce premature magnetic reversal in FeCo layer. A 1-nm thick FeCo layer and an interface exchange parameter around 2 pJ/m could improve (BH)max by 10% when compared to the pure MnBi layer. The presented multiscale simulation framework across scales from the electronic level, atomistic classic spin to microstructure in this thesis is demonstrated to be of the capability towards a powerful and predicative computational design of high-performance permanent magnets, even though there is still a long way to go for its direct application to the real product design