2,871 research outputs found
Direct Ink Writing of AISI 316L Dense Parts and Porous Lattices
No full textThe focus of the present work is the development of a metallic ink that possesses controlled rheological properties: by keeping printing parameters constant both AISI 316L dense parts and porous scaffolds have been produced. Shrinkage, porosity, and mechanical properties have been studied to evaluate the link between the ink rheological properties and the final part. Depending on binder composition, linear shrinkage ranging from 9% to 22% and porosity from 34 to 7 vol% are measured. Tensile strength for specimens sintered at 1240 °C reach the value of 444 MPa and elongation at a break of 12.3%. These values are still far from additively manufactured AISI 316L parts with powder bed fusion technologies, but represent an improvement compared to previously reported data in the literature for AISI 316L parts 3D printed by DIW. Porous scaffolds with a spanning distance of 1.2 mm are printed and sintered. Porosity of 74 vol% and compression strength of 74 MPa are measured for this set of samples showing how the produced ink represents a valuable alternative to pastes already present in the literature
Enhanced mechanical properties of 3D printed alumina ceramics by using sintering aids
No full textStereolithography based 3D printing provides an efficient pathway to fabricate alumina ceramics, and the exploration on the mechanical properties of 3D printed alumina ceramics is crucial to the development of 3D printing ceramic technology. However, alumina ceramics are difficult to sinter due to their high melting point. In this work, alumina ceramics were prepared via stereolithography based 3D printing technology, and the improvement in the mechanical properties was investigated based on the content, the type and the particle size of sintering aids (TiO2, CaCO3, and MgO). The flexural strength of the sintered ceramics increased greatly (from 139.2 MPa to 216.7 MPa) with the increase in TiO2 content (from 0.5 wt% to 1.5 wt%), while significant anisotropy in mechanical properties (216.7 MPa in X-Z plane and 121.0 MPa in X–Y plane) was observed for the ceramics with the addition of 1.5 wt TiO2. The shrinkage and flexural strength of the ceramics decreased with the increase in CaCO3 content due to the formation of elongated grains, which led to the formation of large-sized residual pores in the ceramics. The addition of MgO help decrease the anisotropic differences in shrinkage and flexural strength of the sintered ceramics due to the formation of regularly shaped grains. This work provides guidance on the adjustment in flexural strength, shrinkage, and anisotropic behavior of 3D printed alumina ceramics, and provides new methods for the fabrication of 3D printed alumina ceramics with superior mechanical properties
Financial stress dynamics in the MENA region: evidence from the ArabSpring
No full textIn this paperwe analyse the impact of instability caused by the Arab Spring on the co-movements and volatility spillovers of aggregated Financial Stress Indices for eight MENA countries. Using a dynamic frequency connectedness framework,we conclude that stress transmission between markets is higher at low frequencies than at high frequencies, which implies that MENA markets are slow in adjusting to the information they receive. The Global Financial Crisis generated stronger spillover effects between MENA markets than the political turmoil of the Arab Spring. These results are useful for investors with different investment horizons, and have policy implications for the maintenance of financial stability in this region
Baldratia karamae Elsayed and Skuhrava, new species
No full textBaldratia karamae Elsayed and Skuhravá, new species Adult description. Color (freshly emerged individuals): head black, antennae brown, thorax dark brown, wings smoky grey, legs light brown, upper and lower parts of abdomen dark brown, lateral parts red. Body length. 1.8 mm (n= 5) in female when the ovipositor not extended and 1.6 mm (n= 5) in male. Head (Fig. 11): Compound eyes with rounded facets, gap between eyes on vertex about 0-1 times as wide as facet. Palpi one-segmented; labella globular, setose, widely separated. Antenna 2 + 10 –segmented (n= 23), scape conical, pedicel rounded, flagellomeres 1–9 subequal, slightly longer than wide, each with two connected rings of circumfila in both sexes; male terminal flagellomere with circumfila arranged in a network pattern (Fig. 12); female terminal flagellomere (Fig. 13) consisting of the fusion of the three distal flagellomeres. Thorax: Wing (Fig. 14) length about 1.3 mm (n= 5) in male and 1.4 mm (n= 5) in female. Vein R 5 joining C approximately at mid-length; C broken behind the junction point with R 5; Sc and M present; CuA simple. Tarsal claws (Fig. 15) toothed and curved. Empodia shorter than claws. Hind legs of males much longer and thicker than fore- and midlegs of the female. Abdomen, Male: Tergites 1–7 rectangular with posterior row of setae; tergites 3–7 with anterior pair of trichoid sensilla. Tergite 8 about 0.3 times as wide as tergite 7. Sternites rectangular; sternites 1 and 3–5 with posterior row of setae; sternites 2, 6 and 7 with two posterior rows of setae. Genitalia (Fig. 16): Gonostylus about 0.6 times as long as gonocoxite, arched, setulose and setose, apically with blunt tooth. Gonocoxite wide, massive with scattered long setae. Mediobasal lobes small. Cerci fused, notched, setose and setulose, with rounded tips. Hypoproct entire, rounded apically. Aedeagus slender, and rounded at apex, surrounded with wide setulose parameres. Female (Fig. 17): Tergites 2–7 rectangular, with anterior pair of trichoid sensilla and posterior row of setae; tergite 8 about half tergite 7. Sternites rectangular; sternites 3–6 with posterior row of setae; sternites 6 and 7 with 1–2 posterior rows. Ovipositor (Fig. 18): segment 9 anteriorly with dorsal and ventral dark sclerotized patches, posteriorly with some hyaline setae; the two sclerotized rods widened basally. Lateral plate bears ~ 21 straight, hyaline, split setae. Aculeus concave ventrally, with three rows of strong, squamiform, apically hooked setae on the dorsal site. Sclerotized thin spine extends dorsally along the lateral plate. Apical lamella ovoid, densely covered with short setae. Holotype. Female, Egypt, El-Amria district (30 ° 59 '54.00"N, 29 ° 49 '7.00"E), 27.I. 2013, A. K. Elsayed, reared from pustule galls on leaves of Suaeda pruinosa. Paratypes. All from Egypt, Alexandria, and reared by A. K. Elsayed from leaf galls on Suaeda pruinosa. El- Amria district: 2 females, 1 male, 29.I. 2013; 2 females, 30.I. 2013; 1 female, 17.III. 2013; Abo-Talat district: 1 male, 7.III. 2013; 1 female, 27.IV. 2013; 1 female, 30.IV. 2013; Sidi Kreer district: 2 females, 1 male, 4.V. 2013; 1 female, 1 males, 5.V. 2013; 1 female, 1 male, 7.V. 2013; 4 females, 8.V. 2013; 1 female, 2 males, 15.V. 2013. Distribution. Egypt (Sidi Kreer, Abo-Talat, and El-Amria district). Etymology. This species is named in honor of Mrs. Hedaya H. Karam, professor of Economic Entomology at Alexandria University, Egypt. Biology. Larvae of B. karamae develop inside leaves of S. pruinosa (Chenopodiaceae). Attacked leaves do not show any external signs of infestation except for a dark reddish spot, but can be recognized once adults have emerged, leaving behind emergence holes and the protruding pupal exuviae. Each gall consists of a single chamber in which pupation takes place. The adults were collected from plants from the end of January to the beginning of March, and from the end of April to the middle of October, 2013. Baldratia karamae may have more than two generations per year. Remarks. According to Fedotova (1991 a) the genus Baldratia is divided into five groups on the basis of morphological characters of adults. By reviewing these characters, it was clear that the new species, B. karamae, belongs to the salicorniae Group, which is characterized by the apical lamella positioned at an obtuse angle relative to segment 9, and the lateral plate embraces the entire base of the apical lobe. The salicorniae Group previously contained three species, viz. B. salicorniae, B. suaedifolia, and B. balchanensis (Fedotova 1991 a, 1992). The thin spine of the female ovipositor is longer and thinner in B. suaedifolia, and B. balchanensis than in B. salicorniae. This new species has a long thin spine that does not exceed the base of the aculeus, in contrast to B. balchanensis which has a longer thin spine. Baldratia suaedifolia has a thin spine covered with split setae, while it is bare in B. karamae. Currently only five gall midge species are known to be associated with the host plant Suaeda (Gagné & Jaschhof 2014). Two of them, B. aelleni Möhn, 1969, and B. suaedae Möhn, 1969, were described on the basis of larvae alone (Möhn 1969), and can therefore not be compared to other adults in the genus. Baldratia karamae larvae that develop in leaves of Suaeda pruinosa, differ from the three other species, viz. B. occulta Dorchin, 2001, associated with S. monoica Forssk; B. suaedifolia Fedotova, 1991, associated with Suaeda acuminata (Meyer); and B. terteriani Mamaev & Mirumian, 1990, associated with Suaeda altissima (L.), on the basis published descriptions of these species (Mamaev & Mirumian 1990, Fedotova 1991 a, Dorchin 2001). An unique feature of the B. karamae is the stable number of antennal flagellomeres (2 + 10) in both sexes, in contrast to other species of Baldratia which have a variable number of flagellomeres between the sexes, viz. B. suaedifolia (2 + 12 in female versus 2 + 10 in male), B. occulta (2 + 13-14 in female versus 2 + 12 in male), and B. terteriani (2 + 14 in female versus 2 + 12 in male). The lateral plate of the ovipositor of B. karamae is broad at its base, narrow in the middle and covered with split setae, but B. occulta has a lateral plate which is narrow at the base and bearing 10-15 straight setae, with split setae only on the basal part. The lateral plate of B. terteriani has a small lateral projection at the base that is not present in B. karamae.Published as part of Elsayed, Ayman Khamis, Skuhravá, Marcela, Karam, Hedaya Hamza, Elminshawy, Abdelaziz & Al-Eryan, Mohamed Awad, 2015, New records and new species of gall midges (Diptera: Cecidomyiidae) developing on Chenopodiaceae in Egypt, pp. 105-115 in Zootaxa 3904 (1) on pages 108-110, DOI: 10.11646/zootaxa.3904.1.6, http://zenodo.org/record/23434
Sustainable manufacturing of new construction material from alkali activation of volcanic tuff
No full textThe current climate emergency leads to reduction of virgin raw material extraction and promotes circular economy. In this framework, alkali activation of unemployed fraction of grey tuff, combined with glass waste, provides a range of sustainable construction materials. For the sake of sustainability, tuff powder was subjected to rapid attack (30 min), operated by a 'weak' alkaline solution (3 M NaOH), and then left to dry at low temperature (75 degrees C) for 72 h. The addition of Triton X-100 surfactant was considered to obtain foams starting from slurries with different liquid-to-solid ratio. A thermal treatment was applied to selected samples, at low temperature (700 degrees C). Despite the 'mild' activation conditions, all products survived after immersion in boiling water or acid solution, already in the unfired state, according to the formation of a multiphasic gel. The strength-to density ratio, especially for foams, in both unfired and fired form, compares well with that of already existing construction materials (e.g. it could exceed 5 MPa cm3/g)
Multi-Material Additive Manufacturing: Creating IN718-AISI 316L Bimetallic Parts by 3D Printing, Debinding, and Sintering
No full textAllowing for complex shape and low energy consumption, 3D printing, debinding, and sintering (PDS) is a promising and cost-effective additive manufacturing (AM) technology. Moreover, PDS is particularly suitable for producing bimetallic parts using two metal/polymer composite filaments in the same nozzle, known as co-extrusion, or in different nozzles, in a setup called bi-extrusion. The paper describes a first attempt to produce bimetallic parts using Inconel 718 and AISI 316L stainless steel via PDS. The primary goal is to assess the metallurgical characteristics, part shrinkage, relative density, and the interdiffusion phenomenon occurring at the interface of the two alloys. A first set of experiments was conducted to investigate the effect of deposition patterns on the above-mentioned features while keeping the same binding and sintering heat treatment. Different sintering temperatures (1260 °C, 1300 °C, and 1350 °C) and holding times (4 h and 8 h) were then investigated to improve the density of the printed parts. Co-extruded parts showed a better dimensional stability against the variations induced by the binding and sintering heat treatment, compared to bi-extruded samples. In co-extruded parts, shrinkage depends on scanning strategy; moreover, the higher the temperature and holding time of the sintering heat treatment, the higher the density reached. The work expands the knowledge of PDS for metallic multi-materials, opening new possibilities for designing and utilizing functionally graded materials in optimized components. With the ability to create intricate geometries and lightweight structures, PDS enables energy savings across industries, such as the aerospace and automotive industries, by reducing component weight and enhancing fuel efficiency. Furthermore, PDS offers substantial advantages in terms of resource efficiency, waste reduction, and energy consumption compared to other metal AM technologies, thereby reducing environmental impact
Volatility and return connectedness of cryptocurrency, gold, and uncertainty: Evidence from the cryptocurrency uncertainty indices
No full textThis paper examines the dynamic connectedness of return- and volatility spillovers among cryptocurrency index (CRIX), Gold, and uncertainty measures. Apart from traditional uncertainty measures, we also consider two novel uncertainty measures: Cryptocurrency Policy Uncertainty and Cryptocurrency Price Uncertainty indices. We observe that cryptocurrency policy uncertainty is the main transmitter of the return spillovers to other variables. In addition, Gold is a net receiver of both the return and the volatility spillovers. These results are valid under bearish, bullish, and normal market conditions. Our findings contribute to the literature considering the spillover effect between cryptocurrencies and other assets and their determinants
Hybrid direct ink writing of bioglass-calcite-carbon composite scaffolds supported by novel silicone-based emulsions
No full text70S30C (70 mol% SiO2, 30 % CaO) bioglass is one of the most promising bioceramics for bone tissue engineering. We discuss the feasibility of 70S30C bioglass/calcite/carbon composites derived from novel silicone-based emulsions, leading to highly porous lattice scaffolds. These are produced by direct ink writing (DIW) 3D printing, followed by ceramic conversion at 700 degrees C in flowing nitrogen. The emulsions consisted of droplets of concentrated calcium nitrate aqueous solution incorporated into blends of H44 commercial polysiloxane and photocurable acrylate resin. This formulation offered unprecedented opportunities in both synthesis and shaping. Specifically, the homogeneous dispersion of the CaO precursor in silicone enabled a uniform SiO2/CaO distribution, favoring the formation of a glass matrix. Additionally, the acrylate component and water content allowed for tuning of the microstructure both immediately after printing and upon firing. Photopolymerization of acrylates consolidated the printed bodies (configuring a 'hybrid DIW') after extrusion, while water evaporation enhanced gas evolution during ceramic conversion, promoting pore interconnectivity
Determinants of spillovers between Islamic and conventional financial markets: exploring the safe haven assets during the COVID-19 pandemic
No full textWe analyse the impact of the COVID-19 pandemic on the spillovers between conventional and Islamic stock and bond markets. We further analyse comparatively whether gold, oil, Bitcoin prices, and the risk measures VIX and EPU indexes affect the relationships between these different markets during the COVID-19 pandemic outbreak. The results show that the Islamic bonds (Sukuk) demonstrate safe haven properties during this pandemic crisis, while the spillovers between conventional and Islamic stock markets become stronger during the pandemic outbreak. COVID-19, Oil and gold are strong predictors of the conventional-Islamic markets spillovers, while Bitcoin is not a significant determinant of these relationships
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