Molecular Memory with Atomically-Smooth Graphene Contacts

We report the use of bilayer graphene as an atomically-smooth contact for nanoscale devices. A two-terminal Bucky ball (C60) based molecular memory is fabricated with bilayer graphene as a contact on the polycrystalline nickel electrode. Graphene provides an atomically-smooth covering over an otherwise rough metal surface. The use of graphene additionally prohibits the electromigration of nickel atoms into the C60 layer. The devices exhibit a low-resistance state in the first sweep cycle and irreversibly switch to a high resistance state at 0.8-1.2 V bias. The reverse sweep has a hysteresis behavior as well. In the subsequent cycles, the devices retain the high-resistance state, thus making it write-once read-many memory (WORM). The ratio of current in low-resistance to high-resistance state is lying in 20-40 range for various devices with excellent retention characteristics. Control sample without the bilayer graphene shows random hysteresis and switching.Comment: 13 pages and 4 figure

More on Comparison Between First Geometric-Arithmetic Index and Atom-Bond Connectivity Index

The first geometric-arithmetic (GA) index and atom-bond connectivity (ABC) index are molecular structure descriptors which play a significant role in quantitative structure-property relationship (QSPR) and quantitative structure-activity relationship (QSAR) studies. Das and Trinajsti\'{c} [\textit{Chem. Phys. Lett.} \textbf{497} (2010) 149-151] showed that $GA$ index is greater than $ABC$ index for all those graphs (except $K_{1,4}$ and $T^{*}$, see Figure 1) in which the difference between maximum and minimum degree is less than or equal to 3. In this note, it is proved that $GA$ index is greater than $ABC$ index for line graphs of molecular graphs, for general graphs in which the difference between maximum and minimum degree is less than or equal to $(2\delta-1)^{2}$ (where $\delta$ is the minimum degree and $\delta\geq2$) and for some families of trees. Thereby, a partial solution to an open problem proposed by Das and Trinajsti\'{c} is given.Comment: 10 pages, 2 tables, 1 figure, revised versio

Powder degradation during powder bed fusion processing: impact of processing conditions and alloy composition

In the recent decade, powder bed fusion (PBF) metal additive manufacturing (AM) has attracted huge attention both from the industry and the research community. This effort has helped mature PBF technology as a potential alternative to conventional manufacturing processes as casting, machining, forging, etc. However, there remain various challenges hindering the path of large-scale adoption of these techniques in the manufacturing industry. One such challenge affecting the cost, reproducibility of the products and sustainability of the process, is the reusability of unconsumed powder after each build job. The issue during powder reusability is the likelihood of degraded quality of the reused powder compared to virgin powder either by oxidation during exposure to the atmosphere, or accumulation of process byproducts, referred to as spatters, during processing. The quality degradation of the feedstock powder can lead to an increased number of defects in the produced products and affect the robustness and reproducibility of the PBF process. This thesis is focused on determining the dominant powder degradation mechanisms in powder bed fusion laser-beam (PBF-LB) and powder bed fusion electron-beam (PBF-EB) processes. Here, the approach to investigate the degradation of reused powder is based on the dedicated analysis of changes in powder surface chemistry, analysis of oxygen pick-up, and variation in surface morphology. During the analysis of the powders during the PBF-LB process, three different alloy systems were studied, namely aluminum alloys (AlSi10Mg), nickel-based superalloys (Alloy 718 and Hastelloy X (HX)), and titanium alloys (TiAl6V4). The assessment of powder degradation was initiated with the investigation of AlSi10Mg powder reused for over 30 months. The analysis showed that the powder degradation is mainly triggered by the accumulation of highly oxidized spatter particles in the powder, characterized by the overall greater oxide layer thickness (~75-125 nm) on the surface of powder. These oxidized spatter particles are contributing towards increasing the oxygen content and number of defects in the as-printed components. Analysis of the surface oxide state of spatter particles, generated during the processing of Alloy 718, HX alloy, and TiAl6V4 revealed that the extent of oxidation of spatters from different alloy systems is dependent on the content of oxidation-sensitive elements e.g., Al. Ti, Cr, etc. The impact of the part design in terms of surface to volume ratio of the part on the spatter generation and accumulation was also shown. Results also show an increasing amount of spatter formation with increasing layer thickness per layer deposited. However, the total amount of spatter generated per build job is lower when a higher layer thickness was applied. The results have shown that by employing appropriate processing gas composition containing He the generation of spatter can be reduced. Furthermore, by reducing residual oxygen content in the build chamber, the extent of spatter oxidation can be reduced. Finally, the effect of powder degradation on the quality of fabricated parts was analyzed where the accumulation and redeposition of spatters on the powder bed resulted in a lack of fusion defects, higher porosity, and a decrease in the strength of fabricated parts. In the PBF-EB process, powder oxidation and sublimation of volatile elements during the processing of Alloy 718 have been investigated. The results have identified powder oxidation during PBF-EB processing, due to the long-term powder exposure to high temperature, as the dominant powder degradation mechanism. Furthermore, the sublimation of the alloying elements such as Al and Cr in the case of PBF-EB processing of Alloy 718 was detected

Powder degradation during powder bed fusion processing

Powder bed fusion (PBF) techniques, including laser-based powder bed fusion (LB-PBF) and electron beam powder bed fusion (EB-PBF), are two rapidly growing additive manufacturing (AM) processes due to their ability to produce complex geometries in near-net shapes. To attain reproducibility and repeatability in PBF processes, a consistent set of powder properties is vital. This is achievable by using virgin powder in every new build cycle. However, considering the amount of unconsumed powder after a build cycle in PBF techniques, reusability of unconsumed powder is imperative to reduce the cost and increase the sustainability of the process. Still, upon reuse, the quality of the processed powder gets degraded by surface oxidation or accumulation of by-products often referred to as spatters. The increase in impurities in the powder feedstock can lead to deviation of the powder quality from the initial state and cause stochastic flaws in the produced components such as inclusions and porosity. Therefore, it is important to study the powder degradation mechanisms and extent of degradation upon processing to track the changes in quality of powder with reuse. This thesis focuses on the analysis of powder degradation mechanisms and their effect on processed components in the case of both, LB-PBF and EB-PBF processes. In the LB-PBF process, powder degradation for AlSi10Mg and Alloy 718 powders has been investigated. The examined AlSi10Mg powder was used for over 30 months, and the fabricated parts exhibited an increase in porosity and decrease in tensile strength with increased reuse of powder. The analysis of reused powder samples showed that spatter accumulation is a dominant mechanism in powder degradation. Spatters are an inevitable by-product of the process, and the number of generated spatters depends upon the material, process parameters, atmosphere, and geometry of the part. The role of part geometry in spatter generation and powder degradation was further revealed by fabricating specially designed capsules from Alloy 718. Obtained results showed that surface-to-volume ratio and overhang structures tend to increase the number of generated spatters. The analysis of produced Alloy 718 spatters put in evidence the severe surface oxidation with thick Al- and Cr-based oxide patches and particulates formation. By employing an external atmosphere purity system connected to the LB-PBF machine, it was revealed that even if spatters are oxidized particles that can\ub4t be fully avoided, their oxidation can be significantly limited by reducing the oxygen partial pressure in the process chamber. The obtained results showed that spatters generated at <20 ppm residual oxygen content showed only a 30 % increase compared to the spatters generated at 1000 ppm, which showed a 300 % increase in oxygen content. This is a very promising approach to slow down the rate of powder degradation and increase powder reusability for the LB-PBF process. In the EB-PBF process, the effect of powder bed oxidation and sublimation of volatile elements from Alloy 718 due to the long-term powder exposure to high temperature and high vacuum level on powder degradation was investigated. It was found that in the case of Alloy 718 powder, Cr was dominantly sublimated during the process, which can be detrimental to the superior oxidation and corrosion resistance properties of Alloy 718 components typically preferred for their high-temperature performances. Hence, it is important to monitor Cr and Al content both in powder and built parts while processing of Alloy 718 in the EB-PBF process

Sarcoidosis Presenting as Acute Respiratory Distress Syndrome.

Sarcoidosis is a multisystem granulomatous disease of unknown origin. It typically involves the lungs and mediastinal lymph nodes in a chronic fashion. However, acute syndrome has been reported possibly in response to systemic release of proinflammatory cytokines. Acute pulmonary manifestations, especially acute respiratory failure or acute respiratory distress syndrome, remain extremely uncommon in individuals without a prior diagnosis. We present the case of a 41-year-old African American female, who presented with ARDS. An extensive workup into the cause of her illness remained negative, and she subsequently succumbed to her illness. A diagnosis of sarcoidosis was made upon autopsy, after exclusion of other granulomatous illness. The case highlights the need to consider this uncommon diagnosis in patients with unexplained ARDS to guide therapy