A. Riveiro

Rapid production of ultralong amorphous ceramic nanofibers by laser spinning

Amorphous nanofibers of several centimeters in length can be grown in microseconds during laser cutting of ceramics if a supersonic gas jet is used to clear the cut edge. Such rapid growth is not achievable with vapor phase or solution based methods. Instead, the growth mechanism involves elongation of very small volumes of molten ceramic by frictional drag action of the gas jet. Analysis of the elongation process shows that rapid elongation and cooling are essential to prevent surface tension causing spheroid formation. Using a modified geometry, the process can be adapted to produce near-continuous nanofibers with tailored compositions.

Study of melt flow dynamics and influence on quality for CO2 laser fusion cutting

The understanding of melt flow dynamics during fusion laser cutting is still a topic of great importance because this determines the quality characteristics of the processed workpiece. Despite the complexity of the experimental study of the physical processes involved in this technique, fusion laser cutting can be visualized during the processing of glass because this material absorbs the laser radiation provided by a CO2 laser but shows transparency to visible radiation. Then, we present in this work the results of the study of the melt flow dynamics during laser cutting of glass. Under different experimental conditions, the dynamics of the cutting front and its complete geometry (front wall inclination), and the evolution of the melt along the cut edge were analysed using a high-speed video camera to study the process. A phenomenon concerning the plasma plume formed during the process was observed, which has not been previously reported in the literature. This can displace the normal shock wave (MSD) commonly formed in the inlet kerf and can affect the assist gas flow into the kerf.On the other hand, the analysis of the recorded images allowed the determination of not only the amount of molten material along the cut edge but also the direction and velocity of the melt. Relevant processing parameters affecting the flow of molten material were assessed. These results were used as a basis to explain the different processes involved in the generation of dross, a typical imperfection appearing in laser cutting.

Three-dimensional bioactive glass implants fabricated by rapid prototyping based on CO2 laser cladding

Three-dimensional bioactive glass implants were produced by rapid prototyping based on laser cladding without using moulds. CO(2) laser radiation was employed to melt 45S5 and S520 bioactive glass particles and to deposit the material layer by layer following a desired geometry. Controlled thermal input and cooling rate by fine tuning of the processing parameters allowed the production of crack-free fully dense implants. Microstructural characterization revealed chemical composition stability, but crystallization during processing was extensive when 45S5 bioactive glass was used. Improved results were obtained using the S520 bioactive glass, which showed limited surface crystallization due to an expanded sintering window (the difference between the glass transition temperature and crystallization onset temperature). Ion release from the S520 implants in Tris buffer was similar to that of amorphous 45S5 bioactive glass prepared by casting in graphite moulds. Laser processed S520 scaffolds were not cytotoxic in vitro when osteoblast-like MC3T3-E1 cells were cultured with the dissolution products of the glasses; and the MC3T3-E1 cells attached and spread well when cultured on the surface of the materials.

Laser cutting of 2024-T3 aeronautic aluminum alloy

Since the beginning of the aeronautic industry, aluminum alloys have played a crucial role in its development. Nowadays, different aluminum alloy families are the base material of many pieces of aerospace vehicles. In this work, a novel approach to process aluminum alloys is explored. The authors efforts are aimed to cut 2024-T3 plates by a CO2 laser. They used a novel laser cutting head assisted by a gas jet working in supersonic regime in order to accomplish this objective. This supersonic nozzle and the intrinsic geometry of the cutting head allow carrying out the processing of these alloys more efficiently than conventional cutting heads. The microstructural characterization, grain morphology, kerf dimensions, and surface finish of the cuts have been analyzed. The cut edges are free of dross and cracks and the heat affected zone is negligible. These successful results confirm laser cutting processing assisted by a supersonic assisting gas jet as a promising technique in the aerospace field.

On the conditions to produce micro- and nanofibres by laser spinning

Laser spinning is a new technique which has recently been demonstrated to produce ultralong amorphous ceramic nanofibres with controllable chemical compositions. A laser is employed to melt a small volume of the precursor material at high temperatures, while a supersonic gas jet is then used to rapidly elongate the molten material. The melt forms glass fibres as a result of its viscous elongation and rapid cooling by the convective heat transfer produced by the gas jet.This work analyses the relation between the operating conditions of laser spinning and the physical process that leads to the formation of nanofibres, with the purpose of controlling and designing the optimum conditions for the process. Two decoupled mathematical models were developed to study, on the one hand, the influence of the initial temperature and volume of the molten material on its elongation process, whereas the second model is solved to analyse the physics of the fusion front as a function of the process parameters and relate it to the process of producing the fibres. These models were verified and complemented by experimental tests studied by analyses of the products and direct observation of the fusion front using a high speed camera.

Controlling particle size in the Stöber process and incorporation of calcium

The Stӧber process is commonly used for synthesising spherical silica particles. This article reports the first comprehensive study of how the process variables can be used to obtain monodispersed particles of specific size. The modal particle size could be selected within in the range 20-500 nm. There is great therapeutic potential for bioactive glass nanoparticles, as they can be internalised within cells and perform sustained delivery of active ions. Biodegradable bioactive glass nanoparticles are also used in nanocomposites. Modification of the Stӧber process so that the particles can contain cations such as calcium, whilst maintaining monodispersity, is desirable. Here, whilst calcium incorporation is achieved, with a homogenous distribution, careful characterisation shows that much of the calcium is not incorporated. A maximum of 10 mol% CaO can be achieved and previous reports are likely to have overestimated the amount of calcium incorporated.

Toward smart implant synthesis: bonding bioceramics of different resorbability to match bone growth rates.

Craniofacial reconstructive surgery requires a bioactive bone implant capable to provide a gradual resorbability and to adjust to the kinetics of new bone formation during healing. Biomaterials made of calcium phosphate or bioactive glasses are currently available, mainly as bone defect fillers, but it is still required a versatile processing technique to fabricate composition-gradient bioceramics for application as controlled resorption implants. Here it is reported the application of rapid prototyping based on laser cladding to produce three-dimensional bioceramic implants comprising of a calcium phosphate inner core, with moderate in vitro degradation at physiological pH, surrounded by a bioactive glass outer layer of higher degradability. Each component of the implant is validated in terms of chemical and physical properties, and absence of toxicity. Pre–osteoblastic cell adhesion and proliferation assays reveal the adherence and growth of new bone cells on the material. This technique affords implants with gradual-resorbability for restoration of low-load-bearing bone.

Production of nanoparticles from natural hydroxylapatite by laser ablation

Laser ablation of solids in liquids technique has been used to obtain colloidal nanoparticles from biological hydroxylapatite using pulsed as well as a continuous wave (CW) laser. Transmission electron microscopy (TEM) measurements revealed the formation of spherical particles with size distribution ranging from few nanometers to hundred nanometers and irregular submicronic particles. High resolution TEM showed that particles obtained by the use of pulsed laser were crystalline, while those obtained by the use of CW laser were amorphous. The shape and size of particles are consistent with the explosive ejection as formation mechanism.

Single-Pass and Multi-Pass Laser Cutting of Si–SiC: Assessment of the Cut Quality and Microstructure in the Heat Affected Zone

Experimental investigations on the laser cutting of thick silicon infiltrated silicon carbide (Si–SiC) elements are presented. Si–SiC is a fully dense ceramic composite with high hardness and chemical and thermal stability, which makes it a valuable material in severe conditions. However, machining is still a challenging task. A pulsed Nd:YAG laser was employed to explore the quality of different techniques for the laser cutting of Si–SiC elements with a thickness of 6 mm. In this work, the feasibility of a single-pass technique in comparison with a multi-pass laser cutting was explored for different processing parameters and using both inert and reactive assist gas. A systematic evaluation of the cut quality was assessed by examination of the cross section of samples by optical and electron microscopy, compositional microanalysis, and Raman spectroscopy.Experimental investigations on the laser cutting of thick silicon infiltrated silicon carbide (Si–SiC) elements are presented. Si–SiC is a fully dense ceramic composite with high hardness and chemical and thermal stability, which makes it a valuable material in severe conditions. However, machining is still a challenging task. A pulsed Nd:YAG laser was employed to explore the quality of different techniques for the laser cutting of Si–SiC elements with a thickness of 6 mm. In this work, the feasibility of a single-pass technique in comparison with a multi-pass laser cutting was explored for different processing parameters and using both inert and reactive assist gas. A systematic evaluation of the cut quality was assessed by examination of the cross section of samples by optical and electron microscopy, compositional microanalysis, and Raman spectroscopy.

Laser Surface Texturing of Polymers for Biomedical Applications

Polymers are materials widely used in biomedical science because of their biocompatibility, and good mechanical properties (which, in some cases, are similar to those of human tissues); however, these materials are, in general, chemically and biologically inert. Surface characteristics, such as topography (at the macro-, micro, and nanoscale), surface chemistry, surface energy, charge or wettability are interrelated properties, and they cooperatively influence the biological performance of materials when used for biomedical applications. They regulate the biological response at the implant/tissue interface (e.g., influencing the cell adhesion, cell orientation, cell motility, etc.). Several surface processing techniques have been explored to modulate these properties for biomedical applications. Despite their potentials, these methods have limitations that prevent their applicability. In this regard, laser-based methods, in particular laser surface texturing (LST), can be an interesting alternative. Different works have showed the potentiality of this technique to control the surface properties of biomedical polymers and enhance their biological performance; however, more research is needed to obtain the desired biological response. This work provides a general overview of the basics and applications of LST for the surface modification of polymers currently used in the clinical practice (e.g. PEEK, UHMWPE, PP, etc.). The modification of roughness, wettability, and their impact on the biological response is addressed to offer new insights on the surface modification of biomedical polymers.