Educational recommender systems and their application in lifelong learning

The almost unlimited access to educational information plethora came with a drawback: finding meaningful material is not a straightforward task anymore. Recommender algorithms can be used to make smart decisions in complex information systems and help the users decide upon useful materials; therefore, they become a promising area in academia and industry. The current paper presents a survey on educational recommender systems (RS): a set of analysis criteria are exposed and the technological specifications and challenges of each analysed system are provided, in the context of the main trends in the development of RS. Also, an ontology-based educational recommendation mechanism is proposed and its application to lifelong learning is highlighted, proving that RS can successfully support new learning paradigms.     

On a 3D micromechanical damage model

In the present contribution a 3D two-scale damage model is obtained by asymptotic-developments homogenization, assuming a locally periodic micro-crack distribution. At the microscopic level an energy-based propagation criterion is considered and the macroscopic damage equations are completely deduced through the change of scale procedure. We show that the new damage equations naturally capture microscopic lengths. Their presence in the macroscopic equations leads to size effects, which are investigated in the case of brittle and quasi-brittle damage. The differences with respect to previous results, concerning the 2D case, are pointed out.     

An intermediate crack model for flaws in piezoelectric solids

Summary The aim of this paper is to study a crack model for piezoelectric bodies. In recent years it became evident that the electrically impermeable or the perfect electric contact boundary conditions on the crack faces are inadequate for many physical situations, over- or underestimating the electric field influence on the propagation process. The crack model here investigated is intermediate between these two limit cases. The generalized plane problem for an infinite piezoelectric body with a central crack is converted into a system of integro-differential equations, then reduced to an integro-differential equation similar to Prandtl's equation of aerodynamics. For poled ceramics with transversely isotropic symmetry, the integral equation is numerically solved using quadrature formulae for both plane and antiplane states of deformation. The energy release rate calculated with the discrete solution is then compared with that given by the exact solution for an elliptic hole embedded in the infinite piezoelectric body. A range of values of the cavity thickness is found, for which the considered crack model is a good approximation of the exact two-body problem, while the impermeable and the perfect contact models are not appropriate.     

On the electrical conduction in the interpolysilicon dielectriclayers

To reduce the low-field electrical conductivity of interpolysilicon dielectrics used in electrically erasable programmable read-only (EEPROM) memories devices, the roughness of the poly-SiO₂ interface until now has been decreased in two ways: (1) by increasing the temperature of oxidation and doping of polysilicon combined with low-pressure chemical vapor deposition (LPCVD) of silicon (undoped or in-situ doped) in the amorphous phase, or (2) by the use of LPCVD high-temperature oxide (HTO) deposited over polycrystalline silicon. The advantages of both methods are combined, and electrical conduction results for an interpoly structure based on LPCVD smooth surface polysilicon and LPCVD HTO SiO₂ are presented. The data are interpreted in terms of the Fowler-Nordheim mechanism     

The Role of Endothelium in COVID-19

The 2019 novel coronavirus, known as severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2) or coronavirus disease 2019 (COVID-19), is causing a global pandemic. The virus primarily affects the upper and lower respiratory tracts and raises the risk of a variety of non-pulmonary consequences, the most severe and possibly fatal of which are cardiovascular problems. Data show that almost one-third of the patients with a moderate or severe form of COVID-19 had preexisting cardiovascular comorbidities such as diabetes mellitus, obesity, hypertension, heart failure, or coronary artery disease. SARS-CoV2 causes hyper inflammation, hypoxia, apoptosis, and a renin–angiotensin system imbalance in a variety of cell types, primarily endothelial cells. Profound endothelial dysfunction associated with COVID-19 can be the cause of impaired organ perfusion that may generate acute myocardial injury, renal failure, and a procoagulant state resulting in thromboembolic events. We discuss the most recent results on the involvement of endothelial dysfunction in the pathogenesis of COVID-19 in patients with cardiometabolic diseases in this review. We also provide insights on treatments that may reduce the severity of this viral infection.     

Antiplane two-scale model for dynamic failure

A new dynamic damage law is proposed for the anti-plane loading case. The model is deduced from the energy criterion describing the dynamic propagation of microcracks by using the mathematical homogenization method based on asymptotic expansions. A study of the local macroscopic response predicted by the new model is conducted to highlight the influence of parameters like the size of the microstructure and the rate of loading on the evolution of damage. Results of macroscopic simulations of dynamic failure and the associated branching instabilities are presented and compared with those reported by experimental observations and theoretical studies on dynamic fracture in brittle materials.     

Theory of VHF detection and frequency multiplication with space-charge-limited current (SCLC) silicon diodes

A non-linear theory of transit-time effects upon VHF detection and frequency multiplication with SCLC silicon diodes, is put forward. Diffusion is neglected and carrier mobility is assumed to be field-independent. The theory applies to the SCLC resistor (n⁺νn⁺ structure) and the punch-through diode (n⁺πn⁺). An analytical theory for relatively small signal-amplitudes is developed. Then, computer calculations yield the frequency and bias dependence of the detected current and the second-harmonic amplitude.It is shown that by properly biasing the n⁺νn⁺ device (the transition region between the ohmic and square-law regions), the detected current is almost frequency-independent up to extremely high frequencies. On the other hand, the transit-time effects upon the detection characteristics of the punch-through diode are by far more important and limit the device usefulness to frequencies below the transit-time frequency. The amplitude of the second harmonic strongly depends upon frequency for both n⁺νn⁺ and n⁺πn⁺ structures.