Thermal contact resistance between mold steel and additively manufactured insert for designing conformal channels: an experimental study

The focus of this research is on thermal contact resistance between a mold and its insert, specifically inserts made by additive manufacturing (AM). Using a thermal steady-state system and varying contact pressures (0, 50, 75, and 100 bars), we present experimental results of the thermal contact resistance at the contact interface between steel A (1.2344), obtained from an extruded rod, and steel B (1.2709), produced by laser powder bed fusion. Thermal tests were performed for unbonded and bonded configurations. Results showed that increasing the contact pressure allows the system equilibrium to be reached at lower temperatures. Furthermore, thermal tests showed that in the transition zone of the bonded configuration, a well-defined resistance is not formed between the two steel samples as it occurs in the unbonded configuration. For the unbonded configuration, the thermal contact resistance values decrease with increasing applied contact pressure, improving the system’s heat transfer.This work was financially supported by project PRIDOP (POCI-01-0247-FEDER-040271), co-financed by the European Community Fund FEDER through POCI—Programa Operacional Competitividade e Internacionalização. This work was also supported by FCT—Fundação para a Ciência e Tecnologia within the R&D Units Project Scope: UIDB/04077/2020 (METRICS Center). Furthermore, this work was supported by Portuguese FCT under the reference project UIDB/04436/2020

Characterization of TiAlSiN/TiAlSiON/SiO2 optical stack designed by modelling calculations for solar selective applications

Preparation and characterization of TiAlSiN/TiAlSiON/SiO2 solar selective absorber is reported in this contribution. All layers were deposited in a continuous mode using a industrial equipment, the nitride and oxynitride were prepared by reactive magnetron sputtering and the SiO2 layer by Plasma Enhanced Chemical Vapour Deposition. The optical constants of individual layers were calculated by modelling of spectral transmittance and reflectance of the individual layers. The three layer stack absorber was then designed using those optical properties. The thickness of the individual layers was optimized until a solar absorptance of 96% was obtained resulting in a total thickness of about 200 nm, deposited in copper and extruded aluminium absorbers. An emissivity of 5 % for an absorber temperature of 100 ºC was obtained by analyzing the measuring data from a FTIR spectrometer with integrating sphere. After test duration of 600 h, the samples subjected to a thermal annealing at 278 ºC in air showed a performance criterion (PC) below 4% for, while the samples in the humidity tests showed a PC below 2 %.Savo Sola

Optical characterization of TiAlN/TiAlON/SiO2 absorber for solar selective applications

Characterization of a TiAlN/TiAlON/SiO2 tandem absorber is reported in this contribution. The first two layers were deposited by magnetron sputtering and the third layer was prepared by plasma enhanced chemical vapour deposition (PECVD). The optimization was performed by determining the optical constants of individual layers by first measuring spectral transmittance and reflectance of the individual layers. Subsequently the measuring spectra were fitted using the SCOUT software and dielectric function of each layer was determined. The three layer stack absorber on copper was then designed using those optical properties. The thickness of the individual layers was optimized until a solar absorptance of 95.5% was obtained resulting in a total thickness of about 215 nm (65 nm/51 nm/100 nm for the individual layers, respectively). A thermal emittance of 5% for an absorber temperature of 100 °C was obtained by analyzing the measuring data from a FTIR spectrometer with integrating sphere. During continuous thermal annealing at 278 °C for 600 h the absorptance decreased by 0.4%Savo Sola

Solar selective absorbers based on Al2O3:W cermets and AlSiN/AlSiON layers

Solar selective coatings based on double Al2O3:W cermet layers and AlSiN/AlSiON bilayer structures were prepared by magnetron sputtering. Both were deposited on stainless steel substrates using a metallic tungsten (W) layer as back reflector. The coating stacks were completed by an antireflection (AR) layer composed of Al2O3, SiO2, or AlSiOx. Spectrophotometer measurements, X-Ray diffraction, Scanning electron microscopy, Energy Dispersive X-Ray Spectroscopy and Rutherford Backscattering Spectrometry were used to characterize the optical properties, crystalline structure, morphology and composition of these coatings. The spectral optical constants of the single layers were calculated from the reflectance and transmittance measurements and used to design the optical stack. The coatings exhibit a solar absorptance of 93%-95% and an emissivity of 7%-10% (at 400 ºC). The coatings also exhibit excellent thermal stability, with small changes in the optical properties of the coating during heat-treatments at 400 ºC in air for 2500 h and at 580 ºC in vacuum for 850 h. The coating based on the AlSiN/AlSiON bilayer structure was obtained with an Al:Si ratio of 2.5:1. These coatings revealed similar performance as the one obtained with coatings based on Al2O3:W cermet layers.The authors acknowledge the funding from the Finnish Funding Agency for Technology and Innovation, Tekes, and from FEDER funds through the “Programa Operacional Factores de Competitividade – COMPETE” and from national funds by FCT- “Fundação para a Ciência e a Tecnologia”, under project no. PEst-C/FIS/UI0607/2011.info:eu-repo/semantics/publishedVersio

Wear behavior of a new composite formulation, with TEOS addition, for abrasive vitrified grinding wheels

Vitrified grinding wheels are used in many manufacturing industries to shape and finish metals and other materials in an efficient way. This work addresses a new approach with the use of tetraethyl orthosilicate solution (TEOS solution or silica sol), as an additive, in vitrified abrasive composites for grinding wheels. Different types of composite were produced using always the same materials and processing methods, but with different TEOS contents. Compared with a reference abrasive grinding wheel, the addition of TEOS promoted an improvement in the abrasive tool performance. The new formulation for vitrified abrasive composites showed better dimensional stability during processing (drying and sintering), and could be applied to the production of grinding tools with high dimensional accuracy. With the addition of 3.75 wt% of TEOS there was an increase in the strength of the matrix bonds. The porosity of the composite increased by 7% (from 43.6 to 46.9%), also improving its wear properties, with the quadrupling of the grinding ratio of the abrasive composite. In addition, the surface roughness of the composite decreased (from Ra = 22.9 to 8.4 μm) allowing to improve the surface finish of the workpieces.This work was supported by Programa Operacional Competitividade e Internacionalização , with the code POCI-01-0247-FEDER-038492. This work was supported by FCT national funds, under the national support to R&D units grant, through the reference project UIDB/04436/ 2020 and UIDP/04436/2020

Influence of the microstructure on the creep behaviour of Tin-Silver-Copper solder

A common failure mode of electronic printed circuit boards (PCB’s) is the appearance of cold solder joints between the component and PCB, during product life. This phenomenon is related to solder joint fatigue and is attributed mainly to the mismatch of the coefficients of thermal expansion (CTE) of component-solder-PCB assembly. With today’s solder joint thickness decreasing and increasing working temperatures, among others, the stresses and strains due to temperature changes are growing, leading to limited fatigue life of the products. As fatigue life decreases with increasing plastic strain, creep occurrence should have significant impact, especially during thermal cycles and, thus, should be studied. Through the cooling phase, on the production of PCB assembly’s by the reflow technology, the hoven atmosphere temperature is adjusted in order to control the cooling rate. Narrow criteria is used so as to control the inter-metallic compounds (IMC) thickness, PCB assembly distortion and defects due to thermal shock. The cooling rate also affects solder microstructure, which has direct impact on creep behaviour and, thus, on the soldered joint reliability. In this paper, a dynamic mechanical analyser (DMA) is used to study the influence of the solder cooling rate on its creep behaviour. SAC405 samples with two distinct cooling rates were produced: inside a hoven cooling and by water quenching. Creep tests were made on three-point-bending clamp configuration, isothermally at 25 °C, 50 °C and 75 °C and under three separate levels of stress, 3, 5 and 9 MPa. The results show that creep behaviour has a noticeable cooling rate dependence. It was also noticed that creep propensity is exacerbated by the temperature at which stresses are applied, especially for the slower cooling rates. Creep mechanisms were related to the solder microstructural constituents, namely by the amount of phases ant their morphology.The authors would like to express his acknowledgments for the support given by the Portugal Incentive System for Research and Technological Development. Project in co-promotion This research is sponsored by the Portugal Incentive System for Research and Technological Development. This work is supported by: European Structural and Investment Funds in the FEDER component, through the Operational Competitiveness and Internationalization Programme (COMPETE 2020) [Project nº 002814; Funding Reference: POCI-01-0247-FEDER-002814]. This work was financed by FCT, under the Strategic Project UID/SEM/04077/2013; PEst2015-2020 with the reference UID/CEC/00319/2013 and UID/FIS/04650/2013

Evaluation of gel production and antiradicalar activity in several Aloe species

Introduction Genus Aloe (Family Liliaceae) is worldwide known genus with 600 species naturally from Africa. Aloes have been long used in traditional medicine, specially the gel produced, and nowadays are used for several purposes – like cosmetics and dietary supplements. Nowadays some species, namely Aloe barbadensis, are cultivated as raw-material for agro-food, cosmetic or even pharmaceutical industries. Nevertheless, as indicated, many Aloe species exists that might be also used for these purposes. In this work our goals were i- evaluate several aloe species concerning gel production and antioxidant activity, and ii- compare eventual differences between species grown in greenhouse or open field conditions. Method A total of 19 different Aloe species were evaluated for gel production capacity, in percentage of fresh epidermal part of leaf (skin, peel) and fresh parenchyma transparent mucilaginous gel (gel), and for their antioxidant capacity (by ABTS method). The species were cultivated in equivalent conditions in the same field, in Santarem (Portugal). Additionally, some species (10) were grown both outside (ext) and in a greenhouse (int) conditions. Samples (leaves) from different plants (3) of each species and condition (int or ext) were collected in February and June of 2015, in a period that does not rain for at least a week. Fresh leafs were weighed, and the gel and peel collected separately and weight. Gel and peel were lyophilized and stored at -20 °C, until the processing. In order to evaluate antiradicalar capacity, aliquots of lyophilized gels were solubilized in DMSO (1 mg/mL). ABTS assay was used for antiradicalar activity evaluation and comparison among species and growth conditions. Results and Discussion The lowest percentage of fresh gel (8.2% ± 0.6) was measured in Aloe zebrina (ext). The highest percentage of fresh gel (69.2% ± 1.4) was obtained in hybrid of Aloe arborescens x Aloe barbadensis (int). The productions of gel were affected by growing plants outside and in greenhouse, but no clear tendency was observed. Aloe chabaudii, and Aloe inyangensis, both at greenhouse conditions (int), showed the highest antiradicalar activities with 57.1% ± 0.4 and 39.9% ± 0.6, respectively. The lowest scavenging activity was assessed in Aloe zebrina (int): 2.3 % ± 1.7. Aloe barbadensis (ext) and (int) showed an activity of 16.5%± 1.3 and 5.9% ± 0.1, respectively. Aloe arborescens x Aloe barbadensis hybrid (int) and (ext) was 33.2% ± 1.3 and 3.9% ± 1.2. So the variability among Aloe species, concerning gel production and antiradicalar activity, is high and Aloe barbadensis might not be the best species in terms of these properties for agriculture practices.FCTinfo:eu-repo/semantics/publishedVersio

Dimensional variations on drying of composite components for abrasive wheels

Abrasive wheels are composed of abrasive grains, vitreous bond precursors and a temporary binder that, normally, includes a liquid part to give consistency and plasticity to the green body [1]. During the drying thermal cycle, there are length variations in the material caused by thermal expansion and water elimination. These can originate the formation of cracks in the composite. In this work, the effect of the amount of water added to the vitreous precursor and the organic additive (dextrin) was analyzed, up to 80 °C, by Thermomechanical Analysis and Dynamic Mechanical Analysis in compression mode. Firstly, it was observed that the vitreous bond precursor does not significantly contribute to the length variation of the composite (< 0.05%) in the drying process. Figure 1a presents the dimensional variation for dextrin with different H2O contents. Initially, the added water is incorporated into the dextrin molecules, as water of hydration (zone Z1). Sample with 7.0% H2O presents an expansion, up to ~47 °C, followed by a contraction. As the water content in the samples increases, free water begins to form and, therefore, the shrinkage generated by the water evaporation occurs (zone Z2) becoming the predominant effect. The trend lines of samples with water content in the range 22-30%, converge to a value of ~ 15 ± 1% of H2O as the transition zone to free water formation. Compression tests (figure 1b) show that the two types of water incorporation in dextrin have a different effect: inducing a transition from a mainly elastic to a plastic deformation behavior. The necessary water content to guarantee the plasticity of the mixture without subjecting the composite to excessive dimensional variations during the drying step, was determined. Excess of water increases the global dimensional variation on drying and can induce formation of cracks.This work is within the scope of the Sharlane Costa Ph.D. degree in pro-gress, financially supported by the Portuguese Foundation for Science and Technology (FCT) through the PhD grant reference 2021.07352.BD. This work is also supported by FCT national funds, under the national support to R&D units grant, through the reference project UIDB/04436/2020 and UIDP/04436/202

Effect of sintering temperature on mechanical and wear behaviour of a ceramic composite

© 2017 Elsevier Ltd A new vitrified bonded abrasive composite for grinding wheels and a reference material used in the industrial production of grinding wheels were compared in terms of mechanical properties and wear behaviour. The new formulation abrasive samples were processed using different sintering temperatures. With the new composite, it was possible to, simultaneously, obtain similar mechanical properties and increase the open porosity, at a lower sintering temperature. The results demonstrated that the grinding performance of vitrified bonded abrasives can be optimized varying the heat treatment maximum temperature. The best combination of results was achieved with a sintering temperature of 1000 °C.info:eu-repo/semantics/publishedVersio

Effect of distance between grooves on the wear of structured grinding wheels

During roughing and surface finishing operations a lot of heat is generated in the grinding wheel/workpiece contact zone. The surface structuring/texturing of abrasive wheels has been a proposal to reduce the operating temperature. In this work, three spiral grooved abrasive discs were produced, with different distances between grooves. The wear behavior of structured grinding wheels was evaluated by lubricated pin-on-disc tests. Results show that the inclusion of grooves, and the reduction of the distance between them, allows a more efficient lubrication of pin/disc contact zone, reducing the temperature in this zone while wear debris are eliminated, thus modifying the wear conditions