Preparation and characterisation of isotopically enriched Ta2_2O5_5 targets for nuclear astrophysics studies

The direct measurement of reaction cross sections at astrophysical energies often requires the use of solid targets of known thickness, isotopic composition, and stoichiometry that are able to withstand high beam currents for extended periods of time. Here, we report on the production and characterisation of isotopically enriched Ta$_2$O$_5$ targets for the study of proton-induced reactions at the Laboratory for Underground Nuclear Astrophysics facility of the Laboratori Nazionali del Gran Sasso. The targets were prepared by anodisation of tantalum backings in enriched water (up to 66% in $^{17}$O and up to 96% in $^{18}$O). Special care was devoted to minimising the presence of any contaminants that could induce unwanted background reactions with the beam in the energy region of astrophysical interest. Results from target characterisation measurements are reported, and the conclusions for proton capture measurements with these targets are drawn.Comment: accepted to EPJ

Capacitance Stability of Polymer Tantalum Capacitors Under Different Environmental Conditions

Polymer Hermetic Sealed (PHS) Tantalum capacitors with pre-polymerized PEDOT cathodes and different dielectric thicknesses were used to study the stability of Polymer Tantalum capacitors under different environmental conditions. In particular, capacitance dependence on temperature, frequency, and dc bias voltage were studied in humid and dry capacitors with varying dielectric thicknesses. Capacitance and ESR measurements were performed to characterize the capacitors.Humid capacitors were observed to have higher capacitance than dry capacitors for all dielectric thicknesses. The capacitance for all dielectric thicknesses was observed to increase with temperature in both humid and dry capacitors. Humid capacitors showed a stronger temperature dependence at lower temperatures while dry capacitors showed a stronger temperature dependence at higher temperatures. These temperature effects were more pronounced in thinner dielectric capacitors, and the results were explained by the integrity of the dielectric-polymer interface. The capacitance for all dielectric thicknesses was also observed to decrease with an increase in frequency, both in humid and dry capacitors. The frequency effect was more pronounced in humid capacitors with thinner dielectrics. These results were explained by the RC ladder effect, secondary transitions of the polar segments of the polymer cathode, and lower reactance and lower self-resonance frequency of the thinner dielectric capacitors

Leakage Currents and Gas Generation in Advanced Wet Tantalum Capacitors

Currently, military grade, established reliability wet tantalum capacitors are among the most reliable parts used for space applications. This has been achieved over the years by extensive testing and improvements in design and materials. However, a rapid insertion of new types of advanced, high volumetric efficiency capacitors in space systems without proper testing and analysis of degradation mechanisms might increase risks of failures. The specifics of leakage currents in wet electrolytic capacitors is that the conduction process is associated with electrolysis of electrolyte and gas generation resulting in building up of internal gas pressure in the parts. The risk associated with excessive leakage currents and increased pressure is greater for high value advanced wet tantalum capacitors, but it has not been properly evaluated yet. In this work, in Part I, leakages currents in various types of tantalum capacitors have been analyzed in a wide range of voltages, temperatures, and time under bias. Gas generation and the level of internal pressure have been calculated in Part II for different case sizes and different hermeticity leak rates to assess maximal allowable leakage currents. Effects related to electrolyte penetration to the glass seal area have been studied and the possibility of failures analyzed in Part III. Recommendations for screening and qualification to reduce risks of failures have been suggested

The effect of gel structure on ionic conductance and capacitor impedance

The aim of this work was the determination of the effects of gelling the electrolytes used in electrolytic capacitors of the type Ta/Ta2O5/gel electrolyte/counter electrode. The investigation was divided between two main areas: firstly, the ionic conductivity of gel electrolytes was examined using Pt/Pt black electrodes; secondly, the influence of various electrolytes on the properties of tantalum and Ta/Ta2O5 electrodes, and on the electrolytic capacitors was examined. The conductivity of a series of gels prepared from fumed silica and 40% sulphuric acid showed a non-linear dependence on silica content, but was sensitive to the method of preparation. A standard mixing technique was developed and the reproducible conductivity of gels prepared by this method showed a linear decrease with increasing silica content. [Continues.

Electrolytic preparation of high dielectric thin films

Electrolytic formation techniques for high dielectric materials in thin films - barium salts - titanate, zirconate, and niobat

Memristive Anodic Oxides: Production, Properties and Applications in Neuromorphic Computing

Memristive devices generally consist of metal oxide elements with specific structure and chemical composition, which are crucial to obtain the required variability in resistance. This makes the control of oxide properties vital. While CMOS compatible production technologies for metal oxides deposition generally involve physical or chemical deposition pathways, we here describe the possibility of using an electrochemical technique, anodic oxidation, as an alternative route to produce memristive oxides. In fact, anodization allows to form a very large range of oxides on the surface of valve metals, such as titanium, hafnium, niobium and tantalum, whose thickness, structure and functional properties depend on process parameters imposed. These oxides may be of interest to build neural networks based on memristive elements produced by anodic oxidation

Reliability studies on solid tantalum electrolytic capacitors

The primary objective of this thesis is to determine and analyze the failure mechanisms of solid tantalum capacitors, pinpoint the causes, and suggest the precautionary measures that prevent the occurrence of these failures. To achieve this goal, the study went in two directions, theoretical and experimental. The theoretical part-is a comprehensive review of the work -done on tantalum capacitors since 1960 up to date. The experimental part of the study is a life test (1000 hours) of solid tantalum capacitors under humidity and high temperature. To avoid the misconception that could happen by investigating a particular manufacturer\u27s product, units from five international companies are tested. For each unit, three parameters are measured before and after the life test; namely capacitance, equivalent series resistance (ESR), and leakage current. Both humidity and temperature were found to have profound effects on capacitor behaviour. The study was also able to rank the five major capacitor producers according to their units behaviour before and after the life test

Development of stacked porous tantalum oxide layers by anodization

Interest in nanoporous tantalum oxide (Ta2O5) has been increasing due to its high variety of applications, from protective coatings, photocatalysts to biomedical devices. Anodization is a surface modification technique, which is inexpensive, versatile, easily scalable and widely used to produce these nanostructures. In this study, Ta2O5 nanoporous surfaces were produced by anodization in HF-free electrolyte composed of ethylene glycol, water and ammonium fluoride (NH4F) with different anodization parameters (electrolyte concentration, applied potential and time). The surface morphology of each sample was investigated by scanning electron microscopy (SEM) and the sample with the more uniform porous nanostructure was characterized in terms of cross-section morphology, chemical composition and crystalline structure. The concentration of NH4F and applied potential demonstrated a significant impact on current-density-time curve, and thereafter in surface morphology. Multiple thin porous nanolayers were formed under strong electrochemical conditions (very high current density and electrolyte temperature). Through chemical analysis, it was possible to detect the presence of fluoride, which is consistent with an amorphous Ta2O5 layer with fluoride ions incorporation. Thereby, managing the electrochemical conditions is crucial to control the morphology of an anodic Ta2O5 layer.This research was supported by Norte2020, through European Social Fund (FSE), under the National Doctoral Program in “Surfaces Engineering and Protection”, NORTE-08-5369-FSE-000047. The authors also thank the financial support by Portuguese Foundation for Science and Technology (FCT) in the framework of the HEALTHYDENT (co-financed via FEDER (PT2020) POCI-01-0145-FEDER-030708 and FCT (PIDDAC)), On-Surf (co-financed via FEDER (PT2020) POCI-01-0247-FEDER-024521) and Strategic Funding (co-financed via UID/FIS/04650/2019 and FCT) projects

Noise and Transport Analysis of the Niobium Oxide Layers

Kondenzátor na bázi oxidu niobu je novým typem pasívní součástky, jehož vývoj byl motivován snahou vyřešit hlavní nedostatky tantalového kondenzátoru – omezený zdroj tantalové suroviny a nebezpečí hoření při průrazu. Chování kondenzátoru na bázi oxidu niobu lze stejně jako u tantalového kondenzátoru popsat prostřednictvím reverzní MIS struktury. Pro studium mechanismu transportu nosičů nábojů v dielektriku Nb2O5 a pro stanovení fyzikálních parametrů, které řídí zbytkový proud, bylo využito měření V-A charakteristik v normálním a reverzním módu při 77 a 300K, dále závislosti kapacity ochuzené vrstvy na napětí a frekvenci, teplotní a časové závislosti zbytkového proudu a spektrální hustoty šumu ve frekvenční a časové doméně při různých napětích. Experimentální data potvrdila platnost navrženého pásového diagramu MIS struktury a poskytla jeho klíčové parametry. Bylo ověřeno, že transport nosičů náboje v NbO kondenzátorech je určen ohmickou, Poole-Frenkelovou a tunelovou složkou v normálním módu, a Schottkyho emisí v reverzním módu. V rozsahu standardních aplikačních napětí dominují v normálním módu Poole-Frenkelova emise a v reverzním módu Schottkyho emise. Při vyšších napětích v normálním módu určuje průrazné napětí kondenzátoru tunelový mechanismus. V reverzním módu rozhoduje o odolnosti vůči tepelnému průrazu kondenzátoru výška bariéry mezi dielektrikem a anodou. Bylo zjištěno, že NbO a Tantalové kondenzátory mají stejný mechanismus vodivosti. Specifika NbO anody se projevují pouze v rozdílných hodnotách parametrů pásového diagramu, nikoliv v principech mechanismu transportu nosičů náboje. To vysvětluje základní rozdíl mezi oběma kondenzátory, který je v kvalitě dielektrické vrstvy na přechodu anody a dielektrika. Nižší potenciálové bariéry a vyšší počet defektů v dielektriku, který je způsoben dalším stabilním oxidem, má za následek vyšší zbytkový proud NbO kondenzátoru. Tento jev však nemá žádný vliv na spolehlivost součástky. Teoretické modely a vybrané testovací metody byly použity k volbě vhodných materiálů anody, ke stanovení vhodných dopantů a k optimalizaci technologie anodické oxidace. Byla nalezena korelace mezi parametry transportu nosičů náboje a spolehlivostí, a na základě experimentálních dat byly navrženy optimalizace výrobního procesu kondenzátorů. Lepší porozumění transportním mechanismům v NbO kondenzátorech umožnilo úplný popis nové součástky na bázi oxidu niobu. Byly zdůrazněny silné a slabé stránky této nové technologie a nalezeny nástroje pro optimalizaci procesů, které umožní vyšší spolehlivost a efektivitu NbO kondenzátorů.Niobium oxide capacitor is a new type of passive components, which was developed to overcome basic disadvantages of Tantalum capacitors – limited ore source and burning failure mode. Similarly to Tantalum one, the niobium oxide capacitor can be described as a reverse MIS structure. V-A characteristics in normal and reverse mode at 77 and 300K, capacitance dependence of depleted layer on voltage and frequency, temperature and time dependence of leakage current and noise spectral density in frequency and time domains for different voltages were used to study charge carrier transport mechanisms in Nb2O5 dielectric and to define physical parameters, controlling the leakage current. Experimental data verified validity of proposed MIS structure band diagram and provided its key parameters. It was proven that the charge carrier transport in NbO capacitors is given by ohmic, Poole-Frenkel and tunnelling component in normal mode, and Schottky emission in reverse mode. Poole-Frenkel mechanism in normal mode and Schottky emission in reverse mode are dominant in the standard application voltage range. At high voltages, the tunnelling mechanism in normal mode determines breakdown voltage of the capacitor. In reverse mode, the barrier height between dielectric and anode determines resistance against thermal breakdown of the capacitor. It was found that NbO and Ta capacitors have identical conductivity mechanisms. Specifics of NbO anode material are reflected only in different values of the band diagram parameters, not in the principles of charge carrier transport mechanisms. This explains basic difference between both capacitors, which is in the quality of the dielectric layer at the anode - dielectric interface. Lower potential barriers and higher number of defects in the dielectric, caused by additional stable oxide, result in higher leakage current of NbO capacitor. This effect has however no effect on reliability of the component. Theoretical models and determined testing methods were used to select proper anode materials, to evaluate suitable dopants for NbO raw material improvement and to optimize the anodic oxidation technology. Charge carrier transport parameters were correlated with the reliability and on the basis of experimental data optimization of the capacitors manufacturing process was proposed. Better understanding of transport mechanisms in the NbO capacitors gave complete overview of the new Niobium oxide based component with highlighting strong and weak points of this new technology, and provided tools for better understanding of driving forces which improve efficiency and reliability of NbO capacitors.