Alexandra I. Costa

Analysis of Beam Material Interaction in Welding of Titanium with Fiber Lasers

Ti-6Al-4V is one of the most widely used titanium alloy in industrial applications because of its lightweight and corrosion resistance. The new generation of high power fiber lasers presents several benefits, namely, high power, low beam divergence, and compact size. These lasers can be used in a diversity of materials as the low wavelength that characterizes them allows absorption by almost all metals and alloys. This article presents a research about the weldability of the Ti-6Al-4V alloy using a fiber laser. Weld beads produced with different processing parameters were morphologically characterized under optical microscopy and the microstructures obtained were investigated.

Color and Luminescence stability of selected dental materials in vitro

PURPOSE To study luminescence, reflectance, and color stability of dental composites and ceramics. MATERIALS AND METHODS IPS e.max, IPS Classic, Gradia, and Sinfony materials were tested, both unpolished (as-cast) and polished specimens. Coffee, tea, red wine, and distilled water (control) were used as staining drinks. Disk-shaped specimens were soaked in the staining drinks for up to 5 days. Color was measured by a colorimeter. Fluorescence was recorded using a spectrofluorometer, in the front-face geometry. Time-resolved fluorescence spectra were recorded using a laser nanosecond spectrofluorometer. RESULTS The exposure of the examined dental materials to staining drinks caused changes in color of the composites and ceramics, with the polished specimens exhibiting significantly lower color changes as compared to unpolished specimens. Composites exhibited lower color stability as compared to ceramic materials. Water also caused perceptible color changes in most materials. The materials tested demonstrated significantly different initial luminescence intensities. Upon exposure to staining drinks, luminescence became weaker by up to 40%, dependent on the drink and the material. Time-resolved luminescence spectra exhibited some red shift of the emission band at longer times, with the lifetimes in the range of tens of nanoseconds. CONCLUSIONS Unpolished specimens with a more developed surface have lower color stability. Specimens stored in water develop some changes in their visual appearance. The presently proposed methods are effective in evaluating the luminescence of dental materials. Luminescence needs to be tested in addition to color, as the two characteristics are uncorrelated. It is important to further improve the color and luminescence stability of dental materials.

Materials Behavior in Laser Welding of Hardmetals to Steel

Tool manufacturing industry faces the problem of permanently joining hardmetals to steel holders with high shear strength. The mostly used welding process still is brazing. However, brazed joints have poor lifetimes, mostly when high temperatures are achieved and often break in operation. In a previous study about the ability of CO2 and Nd:YAG lasers to weld hardmetals to steels, it was found that Nd:YAG lasers, working in continuous wave mode, could be used especially for welding hardmetals with Co content around 12%. This article discusses the materials behavior under laser radiation and analyzes the microstructural features observed.

Studies Towards the Living Polymerisation of Phenylethynyl-calix[4]arene Compounds with Rh-based Ternary Catalytic Systems

The living polymerisation of mono and difunctional phenylethynylcalix[4]arene compounds 1 and 2 by Rh(I) ternary catalytic systems (TCS) was examined. Two TCS were tentatively prepared in situ, adapting known methodologies: (1) Rh(C = CPh)(norbornadiene)(PPh3) and (2) Rh(C(Ph)C = CPh2)(norbornadiene)(PPh3). Using the first TCS, the conjugated polymers poly 1 and poly 2 could be obtained in very good yields (77–86%), in short reaction times and freed from low-molecular-weight products, only when NEt3 was used as a co-catalyst. With the second TCS, excellent results were obtained. Indeed, this catalytic system proved to be quite efficient in the polymerisation of calix[4]arenes 1 and 2, affording the correspondent poly 1 and poly 2 essentially in almost quantitative yields (by GPC analysis), under appropriate conditions. The living nature of the polymerisation has been proved. For instance, in the case of calix[4]arene 1, the M n of the polymer obtained at high monomer conversion increased proportionally with the [1]:[Rh] molar ratio in the feed, keeping [1]o constant, thus showing that irreversible chain transfer or termination reactions did not occur to a major extent. Under the most favourable conditions {[1]:[Rh] = 50 and [2]:[Rh] = 50}, the polydispersities of poly 1 and poly 2 were kept in a narrow range (1.16–1.30).

Microstructural aspects of laser dissimilar welds of hard metals to steels

Laser beam welding has been widely used in industry in the past decade. Recently, applications in dissimilar welds involving hard metals have been investigated. Hard metals are commonly used in tool manufacturing and this application requires joining to steel holders. The physical, structural, and mechanical properties of these materials pose several welding problems thus stimulating the application of laser welding. Research efforts have been mainly focused on developing procedures in order to achieve narrow fusion and heat affected zones with minimal distortions. Investigations on the weld microstructures and defect formation were also carried out. This paper presents an analysis of the influence of CO2 and Nd:yttrium–aluminum–garnet laser welding parameters on the microstructural aspects of the welds, discusses solidification microstructures and solid state transformations in dissimilar welds of carbon steels to hard metals, aiming at understanding the weldability of these materials.

Chiroptical and emissive properties of a calix[4]arene-containing chiral poly (P-phenylene ethynylene) with enantioselective recognition ability

Supramolecular chirality was achieved in solutions and thin films of a calixarene-containing chiral aryleneethynylene copolymer. The observed chiroptical activity, which is primarily allied with the formation of aggregates of high molecular weight polymer chains, is the result of a combination of intrachain and interchain effects. The former arises by the adoption of an induced helix-sense by the polymer main-chain while the latter comes from the exciton coupling of aromatic backbone transitions. The co-existence of bulky bis-calix[4]arene units and chiral side-chains on the polymer skeleton prevents efficient π-stacking of neighbouring chains, keeping the chiral assembly highly emissive. In contrast, for a model polymer lacking calixarene moieties, the chiroptical activity is dominated by strong interchain exciton couplings as a result of more favourable packing of polymer chains, leading to a marked decrease of photoluminescence in the aggregate state. The enantiomeric recognition abilities of both polymers towards (R)- and (S)-α-methylbenzylamine were examined. It was found that a significant enantiodiscrimination is exhibited by the calixarene-based polymer in the aggregate state.

Surface photochemistry: alloxazine within nanochannels of Na+ and H + ZSM-5 zeolites.

This work reports the surface photochemistry study of alloxazine adsorbed within nanochannels of MFI zeolites, namely in a series of Na+ and H+ ZSM-5 zeolites where the hydrophobic and hydrophilic character of the host varies systematically. Laser-induced room temperature and 77 K luminescence of air-equilibrated solid powdered samples of alloxazine adsorbed onto the two sets of zeolites, which we will name NaZSM-5 and HZSM-5, revealed the existence of a single emission of alloxazine as a broad band centred at about 450 nm in some cases, while in others an emission with a maximum at about 510 nm was detected. The decay times of the alloxazine emission vary greatly going from solution to entrapment within the nanochannels of the ZSM-5 zeolites. In the latter case a lifetime distribution analysis has shown that the longest lifetime for the alloxazine fluorescence emission exists in the case where an isoalloxazine-type emission was detected, i.e. whenever the hydrophobic character of the host increases. Alloxazine entrapped in the more acidic zeolites exists in the form of emissive monomers. However, alloxazine emits both from monomeric and aggregated emissive forms in the case of the hydrophobic zeolites. These data indicate the formation of planar dimers of alloxazine whenever the number of active sites in the zeolite decreases. These dimers have to be formed at the intersections of the zig-zag and linear nanochannels of the zeolite since there is no space available for their formation inside the zeolite channels. The isoalloxazine tautomers are formed due to the existence of alloxazine dimers which may undergo double proton transfer in the excited state, following laser excitation. Delayed fluorescence of alloxazine was also detected for the HZSM-5 and NaZSM-5 entrapment both at room temperature and at 77 K. The present study is paradigmatic as regards the host influence on the photochemistry of the guest.

Analysis of Different Laser Welding Processes for Joining Hardmetals to Steel

Laser welding has been investigated as a potential technology for joining hardmetals to steels for tool manufacturing industry, due to its particular characteristics such as low heat input and narrow fusion and heat-affected zones, which minimize microstructural modifications, residual stresses and distortions. Hardmetals have good high temperature and wear resistances together with high mechanical strength which make them suitable for applications like tool manufacturing industry. The poor ductility exhibited is of major concern when welding has to be used. This paper describes the work performed aiming to study the weldability of hardmetals with 6 and 12 % Co in dissimilar welds to a tool steel widely used in tool manufacturing industry with different commercially available lasers including a recent high power fibre laser. Laser welding was performed in order to analyse the effect of major processing parameters and the joints evaluated for microstructural analysis under Scanning Electron Microscopy with Energy Dispersion Spectroscopy (EDS) and mechanical resistance through three point bending tests and residual stresses measured by X-ray diffraction. Mathematical modelling of the welding process was done using a market available software regarding the thermal and residual stress fields and validated with experimental data.

Surface Photochemistry: Benzophenone as a Probe for the Study of Modified Cellulose Fibres

This work reports the use of benzophenone, a very well characterized probe, to study new hosts (i.e., modified celluloses grafted with alkyl chains bearing 12 carbon atoms) by surface esterification. Laser-induced room temperature luminescence of air-equilibrated or argon-purged solid powdered samples of benzophenone adsorbed onto the two modified celluloses, which will be named C12-1500 and C12-1700, revealed the existence of a vibrationally structured phosphorescence emission of benzophenone in the case where ethanol was used for sample preparation, while a nonstructured emission of benzophenone exists when water was used instead of ethanol. The decay times of the benzophenone emission vary greatly with the solvent used for sample preparation and do not change with the alkylation degree in the range of 1500–1700 micromoles of alkyl chains per gram of cellulose. When water was used as a solvent for sample preparation, the shortest lifetime for the benzophenone emission was observed; this result is similar to the case of benzophenone adsorbed onto the “normal” microcrystalline cellulose surface, with this latter case previously reported by Vieira Ferreira et al. in 1995. This is due to the more efficient hydrogen abstraction reaction from the glycoside rings of cellulose when compared with hydrogen abstraction from the alkyl chains of the modified celluloses. Triplet-triplet transient absorption of benzophenone was obtained in both cases and is the predominant absorption immediately after laser pulse, while benzophenone ketyl radical formation occurs in a microsecond time scale both for normal and modified celluloses.

Cool Façades - Thermal Performance Assessment Using Infrared Thermography

High reflective paints (cool paints) can reduce solar heat gains, improving thermal comfort and energy efficiency in buildings in summer period. To evaluate the thermal performance of different reflective paints applied in façades (over ETICS like solution) a research study is being developed at LNEC using full-scale test cells built in LNEC’s campus.For this purpose, non-destructive infrared thermography method is being used. This method allows knowing the temperature distribution on façades by measuring infrared radiation emitted from its surfaces. Thermocouples placed on wall outside surfaces support thermographic diagnosis.Optical properties of cool paints (reflectance and emittance) used in this research study were achieved by additional laboratory tests.Comparative analysis between thermal performance of a white cool paint and a white conventional paint is presented in this paper. This research study also shows the potential of infrared thermography in the assessment of thermal performance of cool paints.