In this paper artificial aggregates based on recycled plastic materials, mostly polyolefin and polyethylene terephthalate waste, were used as partial replacement of natural aggregates for ...manufacturing hydraulic mortars. In particular, different amounts (10-50% by weight) of siliceous sand were substituted by the same weight of the above plastic waste, to obtain six mortars with different composition. The influence of plastic addition on physical and mechanical properties (density, porosity, compressive and flexural behavior and water vapour permeability) was studied. Moreover the thermal conductivity of the obtained mortars was evaluated. Recycled plastic substitution enhances the open porosity, causing a decrease in flexural and compressive strength, with an increase in water vapour permeability. Nevertheless, the presence of plastic aggregate leads to a significant reduction in thermal conductivity, which improves the thermal insulation performances of the mortar. For this reason the addition of recycled plastic aggregate in the manufacturing of hydraulic mortars can be considered a way to reduce the growing environmental impact of polymers and, at the same time, it allows the development of increasingly eco-sustainable building materials.
•We prepared Sr-, Zn- and Cd-exchanged zeolitic materials as water vapor adsorbents.•Water vapor adsorption isotherms at different temperatures were collected.•Water vapor adsorption isotherms were ...modeled by means of the Dubinin equation.•The isosteric heat of water vapor adsorption was successfully estimated.•The specific heat storage density of the adsorbents was successfully calculated.
This paper reports the characterization of Sr-, Zn- and Cd-exchanged zeolitic materials as water vapor adsorbents, in order to evaluate the influence of the extraframework species on their adsorption properties. Both synthetic and natural substrates are taken into account. Water vapor adsorption isotherms on each ion-exchanged sample have been obtained at 298, 318, 338, and 358K and have then been modeled using the Dubinin-Astakhov equation. Focusing on the possible implementation of such adsorbents in thermodynamic cycles, an estimation of their specific heat storage densities has been expressed. Results revealed that adsorbents of natural origin are not suitable for a valid employment in thermodynamic cycles, while FAU-type zeolite X samples exchanged with Sr2+ or divalent transition metal ions (i.e., Zn2+ or Cd2+) show a significant potential as heat storage media. The same trend of the specific heat storage density with the cationic content of the adsorbent can be identified for both series of synthetic and naturally originating materials (i.e., Zn>Sr>Cd>Na), confirming how ion exchange allows effective tuning of zeolitic substrates when employed in thermodynamic cycles based on the reversible adsorption of water vapor.
•Hydraulic lime is a good alternative to cement in manufacturing repair mortars.•Use of short fibers can reduce plastic shrinkage and improve durability of masonry.•Zeolitic additions further improve ...mechanical performances of hydraulic mortars.
In a previous paper the authors demonstrated that adding glass fibers to hydraulic lime-based mortars leads to clear improvement in the post-cracking behavior. In order to enhance physical and mechanical performances of the above fiber reinforced mortars, the use of zeolitic addition was proposed. A phillipsite-rich tuff and a zeolite A LTA were selected and added in the mixtures. A significant decrease in open porosity and increase in mechanical behavior was achieved with each addition. In particular 20% of LTA produces an improvement of flexural and compressive strength up to 150% compared with fiber reinforced mortars without addition.
Natural fibres have been widely studied as environment-friendly alternative to synthetic ones in composite manufacturing; they can be considered as reinforcement of geopolymeric foams for use as ...insulating materials. In this regard, the paper focuses on lightweight geopolymers reinforced with hemp fibre grids. These novel composite materials, produced in different ways, are characterized by means of wide experimental tests and their properties are compared with the ones of the plain geopolymer. Morphological analysis shows good bonding between the matrix and the hemp reinforcement; in addition, the main physical properties of the foam are not negatively affected by the presence of hemp fibres. From a mechanical point of view, the composites when subjected to quasi-static load conditions do not collapse in a brittle manner, and show improved flexural strength under dynamic load conditions. Finally, from thermal analysis, they guarantee a good thermal stability.
Unexpected threshold voltage (<inline-formula> <tex-math notation="LaTeX">{V}_{\text {TH}} </tex-math></inline-formula>) behaviors are experimentally observed in gallium nitride (GaN)-on-Si ...metal-oxide-semiconductor-channel high electron mobility transistors (MOSc-HEMTs) with fully recessed gate and back-barrier (BB): 1) <inline-formula> <tex-math notation="LaTeX">{V}_{\text {TH}} </tex-math></inline-formula> increases with decreasing gate length (<inline-formula> <tex-math notation="LaTeX">{L}_{G} </tex-math></inline-formula>) (<inline-formula> <tex-math notation="LaTeX">{V}_{\text {TH}} </tex-math></inline-formula> roll-up); 2) discrepancies between <inline-formula> <tex-math notation="LaTeX">{V}_{\text {TH}} </tex-math></inline-formula> values extracted from drain current characteristics <inline-formula> <tex-math notation="LaTeX">{I}_{\text {D}} </tex-math></inline-formula>(<inline-formula> <tex-math notation="LaTeX">{V}_{\text {G}} </tex-math></inline-formula>) (<inline-formula> <tex-math notation="LaTeX">{V}_{\text {TH}\_\text{IV}} </tex-math></inline-formula>) and from gate-to-channel capacitance characteristics <inline-formula> <tex-math notation="LaTeX">{C}_{\text {GC}}{(}{V}_{\text {G}} </tex-math></inline-formula>) (<inline-formula> <tex-math notation="LaTeX">{V}_{\text {TH}\_\text{CV}} </tex-math></inline-formula>); and 3) significant dependence of <inline-formula> <tex-math notation="LaTeX">{V}_{\text {TH}\_\text{CV}} </tex-math></inline-formula> with frequency. Using TCAD simulations and experimental measurements, it is demonstrated that conduction band confinement, especially at gate corners, is responsible for these peculiar <inline-formula> <tex-math notation="LaTeX">{V}_{\text {TH}} </tex-math></inline-formula> behaviors. This band confinement is strengthened by the fully recessed gate configuration coupled with the proximity of a BB. It depends on recessed gate topology and BB efficiency.
In this article, threshold-voltage V TH instabilities under positive gate voltage stress V GStress in GaN-on-Si devices are thoroughly investigated. Measurement-stress-measurement pBTI technique ...using ultrafast V G ramp was applied in this study. PBTI transients performed at different V GStress and several temperatures highlight the influence of two trap populations, one being related to Al 2 O 3 gate oxide defects and the other one to C N acceptors in GaN lattice. Both trap populations are located close to the Al 2 O 3 /GaN interface and lead to V TH instabilities via two different underlying mechanisms simulated by TCAD. PBTI transients obtained under several dc and ac stress conditions have also been modeled using capture emission time (CET) maps and allowed the identification of the two trap populations. Analysis of the temperature-dependent CET maps gives an activation energy of 0.8-0.9 eV related to C N traps and an energy range between 0.7 and 1.5 eV ascribed to Al 2 O 3 defects above the GaN conduction band energy. This study provides a better understanding of the underlying physical mechanisms, leading to BTI degradation in GaN-HEMT technologies.
•The use of zeolite-rich tuffs for manufacturing eco-sustainable cements was evaluated.•The parameters affecting short- and long term pozzolanic activity were assessed.•The presence of more open ...zeolites within the tuffs leads to faster reaction kinetics.
The feasibility of some widespread zeolite-rich tuffs to act as pozzolanic material for manufacturing blended cements was evaluated by chemical and mechanical characterization. Two different methods were used: (a) Fratini’s test, that allowed to evaluate the ability of the pozzolanic material to combine with Ca(OH)2 in a blended cement; (b) Saturated Lime Test, where the pozzolanic behaviour was directly evaluated in a lime saturated solution.
Mechanical characterization was carried out by measuring compressive strength of blend mortars, after 28-day curing. The good pozzolanic behaviour proved by all the tuffs, coupled with their low cost, makes very promising the use of zeolitic tuffs for the production of eco-sustainable blended cements.
Wide band gap (WGB) materials are the most promising semiconductors for future electronic devices, and are candidates to replace the conventional materials (Si, GaAs, …) that are approaching their ...physical limits. Among WBG materials, silicon carbide (SiC) and gallium nitride (GaN) have achieved the largest advancements with respect to their material quality and device processing. Clearly, the devices performances depend on several surface and interface properties, which in turn are often crucially determined by the quality of the available material, as well as by the device processing maturity. In this paper, some surface and interface issues related to SiC and GaN devices processing are reviewed. First, the control of metal/SiC barrier uniformity and surface preparation will be discussed with respect to the performance of Schottky-based devices. Moreover, the impact of high-temperature annealing required for high-voltage Schottky diodes and MOSFETs fabrication, on the surface morphology and device performances will also be briefly presented. In the second part, it will be shown that for GaN the material quality is still the main concern, since dislocations have a severe influence on the current transport and barrier homogeneity of metal/GaN interfaces. Other practical implications of thermal annealing and surface passivation during GaN-based devices fabrication will also be addressed.
•The specific surface area affects zeolite reactivity in the initial stages.•Si/Al ratio controls the pozzolanic activity at longer hydration times.•Among the exchanged forms, K+-clinoptilolite has ...the highest lime fixation capacity.•Ability of lime fixation does not always lead to a better mechanical behavior.•XRD is a more sensitive technique to detect reaction products than FTIR.
The pozzolanic action played by five natural zeolite-rich materials (three clinoptilolite- and one each mordenite- and analcime-bearing rocks) coming from Turkey, has been examined, evaluating also the influence of various chemical–physical parameters, such as grain size of the zeolitic materials and nature of the cation present as extra-framework component of the structure. Pozzolan activity has been estimated by the official test of the European Standards and by thermogravimetry, finding a good accordance between the two procedures. Clinoptilolite-rich rocks gave the best results, but performance turned out to depend on the specific surface area and pre-enrichment in a potassium form. Experimental data analysis demonstrated that the pozzolanic reaction is kinetically controlled by the diffusion of reactants through a layer of dense reaction products. Experimental blended cements with the five zeolite-rich rocks were also prepared and the compressive strengths measured as a function of the curing time. The effectiveness of the pozzolanic action was monitored by XRD and FTIR analyses. An accurate microstructural study of the hardened pastes was also carried out and point analysis performed, pointing out that the zeolite-lime interaction is characterized by two stages: initially zeolite subtracts calcium from the environment by cation exchange, then it acts as a true pozzolan as soon as its structure breaks down.
The forward current–voltage (I–V) characteristics of Ni/Au Schottky contacts on AlGaN/GaN heterostructures have been studied in this work. The electrical characteristics exhibited a strongly ...non‐ideal behavior that could not be described by the thermionic emission theory. Hence, we used a “two diodes model,” considering both the presence of the Ni/AlGaN barrier and of a second barrier height at the AlGaN/GaN heterojunction. Capacitance–voltage (C–V) measurements enabled us to experimentally determine the properties of the two dimensional electron gas (2DEG) and, hence, of the second barrier at the AlGaN/GaN interface. Following this approach, the anomalous I–V curves could be explained. Moreover, the value of the barrier height at zero‐electric field (flat‐band barrier height) was introduced and determined with this procedure, and resulted in a good agreement with literature data based on photoemission measurements. This approach provides a valid procedure for an accurate determination of the barrier height in AlGaN/GaN heterostructures, and the results can have useful implications for the fabrication of AlGaN/GaN HEMTs devices.
