323,990 research outputs found
Generalized mixed-mode S-parameters
No full textThis paper presents an innovative approach to extend the S-parameter definition to multiport networks having conventional single-ended and differential ports, as is the case for operational amplifiers, transformers and baluns. To give maximum generality to this technique, for example, allowing for complex -parameter reference impedances, the mathematical derivation will be carried out with the most general definition of the -parameters. The presented approach gives the same results already published for circuits with differential ports only when the required simplifications are applied
Physics-based large-signal sensitivity analysis of microwave circuits using technological parametric sensitivity from multidimensional semiconductor device models
No full textThe authors present an efficient approach to evaluate the large-signal (LS) parametric sensitivity of active semiconductor devices under quasi-periodic operation through accurate, multidimensional physics-based models. The proposed technique exploits efficient intermediate mathematical models to perform the link between physics-based analysis and circuit-oriented simulations, and only requires the evaluation of dc and ac small-signal (dc charge) sensitivities under general quasi-static conditions. To illustrate the technique, the authors discuss examples of sensitivity evaluation, statistical analysis, and doping profile optimization of an implanted MESFET to minimize intermodulation which makes use of LS parametric sensitivities under two-tone excitatio
Microwave Measurements Part I: Linear Measurements
No full textAn Overview of the most relevant issues concerning RF and microwave linear measurements is presented. Vector Network Analyzer foremost used instrumentation for this kind of measures is describe
Behavioral modeling of GaN-based power amplifiers: impact of electrothermal feedback on the model accuracy and identification
No full textIn this article, we discuss the accuracy of behavioral models in simulating the intermodulation distortion (IMD) of microwave GaN-based high-power amplifiers in the presence of strong electrothermal (ET) feedback. Exploiting an accurate self-consistent ET model derived from measurements and thermal finite-element method simulations, we show that behavioral models are able to yield accurate results, provided that the model identification is carried out with signals with wide bandwidth and large dynamics
A Measurement System for On-line Estimation of Weed Coverage
No full textThis paper describes two different solutions for the estimation of weed coverage. Both measuring systems discriminate the weed from the ground by means of the color difference between the weed and ground and can be used to on-line control tractor sprayers in order to reduce weedkiller use. The solutions differ with respect to the sensor type: one solution is based on a digital camera and a computer that analyzes the images and determines the weed amount, while the other simpler solution makes use of two photo detectors and an analog processing system. The camera-based solution provides an uncertainty of a few percentage, while the photo detector-based one, though extremely cheap, has an uncertainty of about 5% and suffers from changes in light conditions, which can alter the estimation
Elettrodi a diffusione di gas per idrodealogenazione elettrocatalitica su argento di composti organici volatili policlorurati
No full textElettrodi a diffusione di gas per idrodealogenazione elettrocatalitica su argento di composti organici volatili policlorurati.
Alberto Vertova,* Gabriele Aricci, Cristina Locatelli, Carlo Pirola, Sandra Rondinini
Dipartimento di Chimica Fisica ed Elettrochimica, via Golgi 19 – 20133 Milano
*e-mail: [email protected]
Gli alogenuri organici volatili rappresentano una delle categorie di inquinanti organici più difficilmente trattabili per la loro elevata tossicità. Queste sostanze essendo dotate di forte resistenza a trattamenti fisici, chimici e biologici si accumulano facilmente nell’ambiente e possono entrare a far parte della catena alimentare.[1] Grande importanza assume quindi lo studio di metodi validi, efficaci ed economicamente sostenibili per la degradazione di tali substrati. Tra i diversi tipi di trattamenti, i processi riduttivi sono particolarmente interessanti perché i prodotti finali ottenibili sono idrocarburi a basso peso molecolare, ad esempio il metano. In particolare, i processi di degradazione riduttiva elettrolitica su elettrodi a base di argento dei composti citati costituisce un processo innovativo molto promettente.[2]
In questa presentazione vengono discussi i risultati di un processo elettroriduttivo con Gas Diffusion Electrode (GDE) a base di argento su molecole organico modello quali il triclorometano e l’1,1,1-tricloroetano, alimentate alla cella tramite un gas inerte.
Lo studio ha previsto due tipi di prove: misure potenziodinamiche, per caratterizzare i GDE in diversi ambienti di reazione; elettrolisi preparative, per verificare il meccanismo di reazione tramite l’analisi dei prodotti gassosi in uscita dalla cella. Nel secondo caso si è proceduto sia con l’impiego di una cella divisa che con cella indivisa, ponendo particolare attenzione all’elettrolita utilizzato.
La degradazione elettroriduttiva del CHCl3 ha portato prevalentemente a metano e in misura minore a prodotti di parziale dealogenazione; nel caso del’1,1,1-tricloroetano il prodotto principale è etano, con tracce di etene e di prodotti di parziale dealogenazione.
[1] C. Costentin, M. Robert, J.M. Seveant, J. Am. Chem. Soc., 125 10729 (2003)
[2] G. Fiori, S. Rondinini, G. Sello, A. Vertova, M. Cirja, L. Conti, J. Appl. Electrochem. , 35 363 (2005
Catalytic conversion of non-food oilseeds into methyl esters : traditional and ultrasound assisted techniques
No full textIntroduction
The most recent challenge concerning the biodiesel production deals with the processing of
non-food, raw oils to make them suitable to be used as biofuels. For these oils several
standardization processes are often required. The search for high efficiency transformation
methods is therefore a key issue in this context. In this work, different kinds of non-edible
oilseeds from crops such as Brassica juncea, Nicotiana tabacum and Cartamus tinctorius have
been selected to be processed processed using traditional and ultrasound (US)-assisted methods.
Experimental
The oilseeds were deacified by free fatty acids (FFA) esterification and then wholly converted
into methyl esters (ME) by transesterification. Sulphonic ion exchange resins Amberlyst®46
(A46) and Purolite®D5081 were used as esterification catalysts. Homemade catalysts of the
kind SO4
=/TiO2-SnO2 (TiO2 loadings from 5 to 20%) and prepared by impregnation were also
tested in the esterification. Both the reactions were carried out in slurry modality using the
conditions reported in the Table 1. US experiments were performed using a tip-type sonicator at
20 kHz. ME yields were monitored through acid base titrations and GC analyses [1,2]. Software
PROII (Simsci Esscor-Invensys) was used to perform process simulations.
Results/Discussion
FFA conversions close to 90% were achieved for all the oils deacidified on A46 and D5081, as
already observed by the authors for other kinds of feedstock [3]. These catalysts were recycled
for several times showing practically no deactivation This stable behaviour is due their
peculiarity of being sulphonated only the external surface. This confers absence to mass
transfer limitations and minimization of side products formation [4]. For these catalysts, a
kinetic pseudohomogenous model [5] was used as for comparison with the experimental data: a
very good correlation for different kinds of oils characterized by different initial acidities was
achieved. This represent an important results for the esterification process scalability.
The catalysts of composition SO4
=/TiO2-SnO2 resulted in less satisfactory performances than
the ones obtained with the ion exchange resins. Moreover, they deactivated after few uses,
probably due to the leaching of the active sulphate groups [6].
In Table 1 the results of both the esterification and transesterification reactions are displayed for
traditional and US-assisted methods.
Table 1. Conversion of an acid rapeseed oil: operative conditions and achieved results.
Esterification catalyst: D5081, transesterification catalyst: NaOCH3.
Reaction Method T (K) MeOH:Oil
(%wt)
Cat:Oil
(%wt)
Time
(min)
Conv. to
ME (%)
Est. Traditional 313a
338b
16:100 10:100 360 52.8a
71.2b
US 313a
338b
16:100 10:100 360 77.3a
76.1b
Trans. Traditional
(2 steps)
333step1
333step2
20:100step1
5:100step2
1:100step1
0.5:100step2
90 step1
60step2
96.9
US (1 step) 293 20:100 1:100 30 86.6
The use of US allows to achieve higher ME conversions in the esterification at lower
temperatures and in the transesterification. In the latter case shorter times and lower amount of
reagents are required.
For the different oils it was in general observed that the positive effect of US is more
pronounced at lower temperatures, whereas at higher temperatures it does not seems to bring
any advantage with respect to the traditional method. This suggests that at lower temperatures
the acoustic cavitation effects are enhanced: it has in fact already been reported the existence of
an optimum temperature for the occurrence of the acoustic cavitation in different reactive
systems, oilseeds included [7]. In the US-promoted FFA esterification, these effects may be
described in terms of the mechanical events enabled by the US waves inside the liquid reaction
medium and in particular in the proximity of the catalyst’s surface. In the case of the
homogeneously catalyzed transesterification, the high reactivity observed with the use of US
may be ascribable to the effects caused by the acoustic cavitation in an homogenous medium,
which generates very high local temperatures and pressures [8].
References
1. C. L. Bianchi, D. C. Boffito, C. Pirola, S. Vitali, G. Carvoli., D. Barnabè and A. Rispoli,
Biodiesel/Book 1. ISBN 978-953-307-713-0, 2011.
2. C. Pirola, D. C. Boffito, G. Carvoli, A. Di Fronzo, V. Ragaini and C. L. Bianchi,
Soybean/Book 2, ISBN 978-953-307-533-4, 2011.
3. C. L. Bianchi, C. Pirola, D. C. Boffito and V. Ragaini, Catal. Lett.., 134, 179 (2010)
4. C. Pirola, C. L. Bianchi, D. C. Boffito, G. Carvoli and V. Ragaini, Ind. Eng. Chem. Res., 49,
4601 (2010)
5. T. Pöpken, T. Götze and J. Gmehling, Ind. Eng. Chem, Res., 39, 2601 (2000)
6. D. C. Boffito, C. Pirola, and C. L. Bianchi, Chem. Today., 30, 42 (2012)
7. H. Lu, Y. Liu, H. Zhou, Y. Yang, M. Chen, B. Liang, Comput. Chem. Eng., 33, 1091 (2009)
8. K. S. Suslick, Science, 247, 1439 (1990
Massey Products and Fujita decompositions on fibrations of curves
No full textLet f: S→ B be a fibration of curves and let f∗ωS/B= U⊕ A be the second Fujita decomposition of f. In this paper we study a kind of Massey products, which are defined as infinitesimal invariants by the cohomology of a curve, in relation to the monodromy of certain subbundles of U. The main result states that their vanishing on a general fibre of f implies that the monodromy group acts faithfully on a finite set of morphisms and is therefore finite. In the last part we apply our result in terms of the normal function induced by the Ceresa cycle. On the one hand, we prove that the monodromy group of the whole U of hyperelliptic fibrations is finite (giving another proof of a result due to Luo and Zuo). On the other hand, we show that the normal function is non torsion if the monodromy is infinite (this happens e.g. in the examples shown by Catanese and Dettweiler)
Design and Cahracterization of a 10-Gb/s Dual Drive Z-Cut Ti:LiNbO3 Electrooptical modulator
No full textAn investigation of the relationships between supply risk awareness, assessment, management, and supply disruption occurrence
No full textThe today supply chain trends along with the current financial crisis have increased the awareness among professionals that risk assessment and mitigation play a crucial role in successfully managing supply chains. This increasing emphasis on risk management and the even more predominant trend to focus on core activities that creates greater dependencies on upstream supply, emphasizes the importance of the supply risk management. This study focuses on the supply side of the risk, looking at the relationships between top management awareness, supply risk assessment, supply risk management, and disruption occurrence. In particular, through a structural equation model, this research will demonstrate that increasing the top management awareness of supply risk raises the employment of risk assessment tools. Risk assessment allows a company to better understand risk sources increasing the implementation of risk management techniques in order to improve enterprise resiliency and decrease disr..
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