Micromechanical testing at high strain rates and varying temperatures of 3D-printed polymer structures

Following recent developments, in-situ nanoindenter systems can now perform tests at much faster speeds, enabling us to accomplish micro-compression experiments using strain rates in the range of 103 s-1 for micron-sized specimens. Thus, it has become possible to study deformation processes occurring in the very small scales of all types of materials. Here, we focus on the effect that dynamic testing speeds have on the mechanical behaviour of 3D-printed micron-sized polymer structures. We look at the influence of temperature and compare results obtained from compression and tensile tests. Please click Additional Files below to see the full abstract

Studying deformation mechanisms of nanocrystalline nickel by thermal activation analysis at subambient temperatures and high strain rates

Electrodeposition and magnetron sputtering are promising methods for depositing thin films with nanocrystalline (nc) microstructures. Nc metals are attractive materials, as they show considerably higher mechanical strength compared to their poly- or monocrystalline counterparts. However, they also feature pronounced time- and rate-dependent inelastic behavior and their microstructure may change drastically when exposed to elevated temperatures or ion irradiation. Therefore, in order to assess the mechanical behavior and deformation mechanisms of these materials under controlled conditions and at a constant microstructure, it is desirable to perform thermal activation analysis at subambient temperatures and high strain rates on pristine samples. Large arrays of micropillars were fabricated by electrodeposition of nc Ni into lithography molds by LIGA leading to non-tapered, damage-free microspecimens. X-ray diffraction (XRD) measurements and transmission electron microscopy (TEM) imaging revealed a grain size of approximately 28nm. EDX analysis showed a homogeneous elemental composition and no concentration of impurities at the grain boundaries. A micromechanical testing device was developed that allows performing nanomechanical experiments at sub-ambient temperatures down to 120K in a large range of strain rates between 10-4 and 103s-1. Please click Additional Files below to see the full abstract

Template-assisted electrodeposition of nickel and nickel copper 3D microcomponents

New types of metamaterials and architectured material require metallic materials with precise structural design in the microscale. However, additivemanufacturing of metallic structures in themicroscale has proven difficult, as the demand for the material quality and accuracy of the 3D structure is high. Of themultitude of techniques available, template-assisted electrodeposition (TAE), also known by its German abbreviation, LIGA, has shown excellent results for the production of 2D components. With new lithography techniques, especially two-photon lithography (TPL), TAE shows promise of microscale additive manufacturing for 3D metal structures. This thesis aims to investigate the electrodeposition into a 3D template and discuss the influence of the template on the local current density and the resulting microstructure of the deposit. 3D finite element simulations were used to understand the process of template filling. The electrodeposited structures were investigated towards their shape accuracy as well as their microstructure. Various deposited structures were used for micromechanical investigation with the explicit regard to inspect their properties and performance for future use in MEMS-devices. Simple pillars were investigated at first to inspect themechanical properties and arrangement of the templates in arrays. In this work, subsequently, 3D micro springs were produced and their properties compared with theoretical mechanical simulations. These springs exhibited significant strength, unprecedented in microsprings of this size. Following simple templates, more complex templates with multiple ongoing growth fronts were investigated. Micromechanical shear test specimen were produced and the template design was investigated. These templates were also used to investigate the effectiveness of pulse electrodeposition was investigated to fill corners. Furthermore, the microstructure showed elongated grains along the growth direction allowing insight into the deposits growth. Where to growth fronts meet an accumulation of grain boundaries was visible in the microstructure, but showed no porosity. Building on these results, microlattices with a body center cubic structure were designed and tested. The microlattices showed remarkably high strength as well as excellent promise for energy absorption. Building on the success of the technique and the previous structures, the electrodeposition of Copper Nickel alloys was investigated. A 3D time-dependent pulse electrodeposition simulation was created and the deposition of large 2D components was simulated and compared to experimental values. Overall, trough these specific case studies, it was shown that TAE combined with TPL produces microcomponents with excellent accuracy, homogenous microstructures with no significant defects and excellent mechanical properties. The method has shown reproducibility. The insight gained by the electrodeposition simulation can be used in the future for others to emulate the process or create their electrodeposition simulation

Nanomechanical test specimen preparation techniques by microfabrication and two‐photon lithography to avoid FIB induced Ga implantation damage

Traditional mechanical test specimen preparation methods require a subtractive approach to define the structure out of the bulk material. The most commonly used technology, focused ion beam patterning, leaves a modified specimen surface by gallium implantation resulting in for instance an altered grain structure. In this work, purely chemical approaches and an additive manufacturing technique are introduced to define test specimen. Compression pillars have been fabricated out of single crystal silicon and glass. The developed silicon process is crystalline orientation independent; consists of optical lithography, reactive ion etching via an alternating fluorine plasma and polymer passivation, surface oxidation and HF wet etching for sidewall planarization. High aspect ratio structures are achievable with a sub-50 nm surface roughness and parallel sidewalls. Glass pillar microfabrication requires a hard metallic mask due to the relatively low selectivity of any etchant plasma. Sputtered aluminum is patterned by photolithography and a chlorine based dry etch. The glass is reactively etched in a fluorine based plasma via this mask. The aluminum is then selectively removed by wet etching. The process has been demonstrated on a fused silica substrate with an average of 84º sidewall angle, although a wide variety of glasses may be used. Glass purity influences the sidewall angle. Please click Additional Files below to see the full abstract

Management of hepatitis C in opioid agonist therapy patients of the Swiss canton Aargau within and outside the cohort study

BACKGROUND Hepatitis C virus (HCV) treatment reduces hepatic and extrahepatic morbidity and mortality and prevents further transmissions. Since October 2017, direct-acting antivirals (DAAs) have been reimbursed in Switzerland for all patients. Intravenous drug use accounts for the majority of HCV infections in Switzerland. Between July 2013 and July 2015, 205 of the 631 opioid agonist therapy (OAT) patients in the Swiss canton Aargau were enrolled into a cohort study, the Argovian OAT cohort study. In March 2019, the Federal Office of Public Health (FOPH) published guidelines for the HCV management in drug users. AIM To describe current HCV management in OAT patients of the Swiss canton Aargau in view of the FOPH guidelines and to compare the management of patients within and outside the cohort study. METHODS Between July 2013 and August 2018, 330 patients were enrolled into the Argovian OAT cohort study offering human immunodeficiency virus (HIV)/HCV antibody rapid testing, noninvasive liver fibrosis assessment (Fibroscan®) and, since August 2017, capillary HCV RNA rapid testing with the GeneXpert®. To assess HCV management, all information available before 1 September 2018 was considered. In September 2018, 592 of the then 809 OAT patients were not yet enrolled into the cohort study. For them, the cantonal physician sent a questionnaire regarding HCV, HIV, and hepatitis A and B viruses (HAV and HBV) to the OAT prescriber. Up to September 2019, we had received 182 (31%) questionnaires; 160 were eligible for analysis. RESULTS In the HCV cascade, the four diagnostic gaps, but not the two treatment-related gaps, were significantly larger in non-cohort compared with cohort patients: (1) never HCV antibody screened: 14% (22/160) versus 0.3% (1/330); (2) no HCV RNA test, if HCV antibody positive: 36% (21/58) versus 11% (19/167) if ever chronically infected; (3) liver fibrosis stage unknown: 51% (19/37) versus 3% (4/120); (4) HCV genotype unknown: 41% (15/37) versus 18% (21/120); (5) never received HCV treatment: 24% (9/37) versus 30% (36/120); (6) no treatment success, if treated and outcome known: 7% (1/14) versus 6% (5/84). HCV treatment outcome was unknown by the OAT prescriber in 50% of non-cohort patients. Adequate HCV management (HCV antibody test ≤1 year ago if HCV antibody negative or last HCV RNA test negative, and ≤1 year ago if HCV antibody positive) was less frequent in non-cohort than in cohort patients: 28% (44/160) versus 69% (229/330). CONCLUSION With regard to HCV elimination in OAT patients by 2030, case finding and regular screening for new and re-infections remain a challenge, especially for non-cohort patients in a decentralised setting. Documentation of the HCV sero- and RNA status of each OAT patient by the cantonal physician and a yearly HCV screening reminder sent to the OAT prescriber combined with capillary HCV antibody and HCV RNA testing by the OAT prescriber, general practitioner or the pharmacy might facilitate the implementation of the FOPH guidelines. DAA prescription directly by the OAT prescriber could increase awareness and improve linkage to care

Dynamic cryo-mechanical properties of additively manufactured nanocrystalline nickel 3D microarchitectures

Template-assisted electrodeposition is a promising microscale additive manufacturing technique allowing to deposit pure metals with high resolution. To allow the application-relevant design of metamaterials, it is necessary to establish microstructure-mechanical property relationships under extreme conditions. In this work, a novel process based on two-photon lithography was used to synthesize arrays of nanocrystalline nickel micropillars and complex microlattices. This allowed high throughput mechanical testing using a newly developed in situ nanoindenter at unprecedented combination of cryogenic temperatures (160 to 300 K) and strain rates (0.001 to 500 s−1). Strain rate sensitivity was found to increase from ∼ 0.004 at 300 K to ∼ 0.008 at 160 K. Thermal activation analysis showed a decrease in activation volume from 122b3 at 300 K to 45b3 at 160 K and an activation energy of 0.59 eV in line with collective dislocation nucleation as the rate limiting mechanism. Transmission Kikuchi Diffraction allowed quantifying microstructural changes during deformation. As such, a deformation map along with the responsible deformation mechanisms has been ascertained for additively micromanufactured nanocrystalline nickel at unique combinations of extreme temperatures and strain rates. Further, rate-dependent compression of microlattices and complementary finite element simulations using the results from micropillars as constitutive models exemplified the promise of such metal microarchitectures in space and aviation applications