With nearly 1,100 species, the fish family Characidae represents more than half of the species of Characiformes, and is a key component of Neotropical freshwater ecosystems. The composition, ...phylogeny, and classification of Characidae is currently uncertain, despite significant efforts based on analysis of morphological and molecular data. No consensus about the monophyly of this group or its position within the order Characiformes has been reached, challenged by the fact that many key studies to date have non-overlapping taxonomic representation and focus only on subsets of this diversity.
In the present study we propose a new definition of the family Characidae and a hypothesis of relationships for the Characiformes based on phylogenetic analysis of DNA sequences of two mitochondrial and three nuclear genes (4,680 base pairs). The sequences were obtained from 211 samples representing 166 genera distributed among all 18 recognized families in the order Characiformes, all 14 recognized subfamilies in the Characidae, plus 56 of the genera so far considered incertae sedis in the Characidae. The phylogeny obtained is robust, with most lineages significantly supported by posterior probabilities in Bayesian analysis, and high bootstrap values from maximum likelihood and parsimony analyses.
A monophyletic assemblage strongly supported in all our phylogenetic analysis is herein defined as the Characidae and includes the characiform species lacking a supraorbital bone and with a derived position of the emergence of the hyoid artery from the anterior ceratohyal. To recognize this and several other monophyletic groups within characiforms we propose changes in the limits of several families to facilitate future studies in the Characiformes and particularly the Characidae. This work presents a new phylogenetic framework for a speciose and morphologically diverse group of freshwater fishes of significant ecological and evolutionary importance across the Neotropics and portions of Africa.
Sintering and nanostability (defined as the stability against sintering) are critical phenomena present in the processing and application of nanoparticles. With important implications in obtaining ...high‐quality dense ceramics with fine grains or in enabling high surface areas in nanoparticles for catalytic applications, the control of these interrelated phenomena has been the focuses of several studies. From a thermodynamic perspective, it is recognized that surface energy is a fundamental parameter in both cases, since it is the main driving force for sintering and also the reason that nanoparticles are thermodynamically unstable and have the tendency to coarsen at elevated temperatures. The role of grain‐boundary energies is less recognized as relevant, but is also connected to densification, grain growth, and nanoparticle stability. In this paper, we review the critical aspects of the role of interfacial energies in the microstructure evolution, in particular addressing them as parameters to allow better control in addition to more conventional kinetic parameters. The concept is based on the nonsingularity of interfacial energies in a given system, which varies with temperature, atmosphere, and most importantly, chemical composition—this last offering a method to induce particular microstructural evolutions. While the model assumes isotropic grain boundaries but consequences to anisotropy are also discussed. The paper presents examples of the role of dopants on interfacial energies, how this is quantitatively related to their segregation at the interfaces, and the impact in sintering and nanostability. Given the importance of interface energetics to these phenomena, we also present a short review on the current methods used to obtain reliable interface thermodynamic data.
Fully dense transparent zinc aluminate ceramics with nanoscaled grain sizes were fabricated by Deformable Punch Spark Plasma Sintering (DP‐SPS). Optical transmission spectra showed high transparency, ...with up to 70% transmitted light in the visible spectrum. Vickers hardness was measured and grain boundary strengthening observed, showing hardness increase from 18.2 GPa up to 22.5 GPa as the grain sizes decreased from 60.3 to 10.1 nm. The trend followed the Hall‐Petch relationship, with hardness linearly proportional to the inverse of square root of grain size. A low grain size limit reported in previous literature below which hardness decreases, known as inverse Hall‐Petch relationship, was not observed within the studied grain size range. Cross‐sections of the hardness tests' indentations were prepared by focused ion beam and observed by electron microscopy and showed radically different crack patterns underneath the indentation imprint when contrasting samples with dissimilar grain sizes, shedding light on the mechanisms behind the observed grain boundary hardening mechanisms.
•Introduce concepts of interfacial energy modification in oxide materials.•Establishes rational behind impact of interfacial energies in nanostructural control.•Presents collections of experimental ...thermodynamic data on interfacial energies of oxides for benchmark.
This paper presents a brief description of the role of interfacial energies in the understanding and control of nanocrystalline complex oxides in both particulate and bulk forms. Interfacial energies are fundamental parameters in microstructural evolution processes such as phase transformation, grain growth, and sintering. Although generally considered constant driving forces, experimental evidences confirm the possibility of intentional modification of both surface and grain boundary energies in oxide systems via ionic doping. This opened the perspective for a systematic understanding of their roles as refining parameters in microstructural control during processing and in operation. In this work, the theoretical framework in the context of Gibbs adsorption isotherm and the formation of dopant excess (i.e. interfacial solute segregation) is introduced in a similar manner as formalized for liquid systems. A collection of data demonstrating interfacial energy control in oxides is presented and discussed in terms of microstructural relationships with specific examples. The data advocates for a paradigm shift on nanocrystalline processing control from a traditionally kinetically oriented perspective to a more balanced viewpoint in which thermodynamics can play a governing role, especially at moderate temperatures. The work is not an extensive review, but rather has the goal of introducing the reader to this growing research topic.
This article focuses on the control of nonlinear systems with redundant actuation, where the number of actuators is higher than the number of controllable degrees of freedom. To effectively handle ...actuation redundancy, we propose a control allocation (CA) approach that combines numerical optimization methods with control Lyapunov and control barrier functions. The inclusion of control Lyapunov functions enhances the CA's ability to minimize performance loss in the presence of unattainable virtual inputs. The integration of barrier functions allows the CA to exploit actuation redundancy to enforce safety constraints. The proposed approach is applied to the control of an overactuated system, demonstrating superior transient and safety properties when compared to classical CA algorithms.
The effect of dopants (or additives) on sintering is typically addressed from a mechanistic and diffusivity perspective by focusing on how dopants affect those parameters. However, a comprehensive ...description of sintering needs to address the role of dopants in the thermodynamics of the system, which affects local chemical potentials driving forces and is, therefore, ultimately linked to the mass transport mechanisms themselves by the thermodynamic extremal principle. In this work, Lanthanum doped Yttria-Stabilized-Zirconia (YSZ) sintering was studied from both kinetics and thermodynamic perspectives to demonstrate the need for those complementary analyses to allow proper processing control. La caused inhibition of both grain growth and densification, which was a result of a change of interfacial energies linked to La segregation as well as of the codependence of coarsening and densification on the grain boundary energy. While La caused a modest decrease in the activation energy for densification, surface and grain boundary energies decreased from 0.95 to 0.70 J/m2, respectively, for YSZ, to 0.80 and 0.41 J/m2 for 2 mol% La-YSZ, indicating an increase in sintering stress. The apparent contradiction between the thermodynamic data and the observed densification trend is attributed to the reduced grain growth that trapped the system in a metastable configuration.
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Nanomaterials have intrigued the scientific community for many years due to their unique and unexpected properties. Though the number of technological applications has impressively increased in the ...past years, a fundamental understanding of the behavior of nanomaterials is still incomplete. This is particularly true from a thermodynamic viewpoint, which defines the nanostructure overall thermal stability, polymorphism, and is ultimately connected to an optimal exploitation and reliability of the nano-features. This short review presents recent advancements on the understanding of the energetics of nanocrystalline ceramics and how they relate to nanoscale stability phenomena by focusing on the latest experimental evidences. The role of interface energetics as a key instrument to enable stability prediction and control is discussed based on a quantitative description of nanoscale phase diagrams and coarsening processes. Techniques to assess interface energies of ceramic nanostructures are also briefly discussed, in particular addressing the highly sensitive calorimetric methods recently used to acquire unprecedented interface energy data for oxides. A short collection of experimental evidences and trends on the thermodynamic understanding of nanocrystalline oxides, focusing on aluminum oxide, titanium dioxide, zirconium oxide, and tin dioxide, concludes this review.
► The thermodynamics of nanoceramics is addressed, focusing on the quantitative description of the effects of interface energies on phenomena such as phase stability and coarsening.► The usage of dopants prone to interface segregation is formally described as a tool to enable interface energy control and, consequently, improve nanostability. ► The techniques required to assess interface energetics of nanoceramics are also addressed.
One of the most classic size-effects in materials is the increase in the strength and hardness as the grain size decreases. However, a practical low size limit for this so-called grain boundary ...strengthening has been extensively reported for both metals and ceramics. Here, it is demonstrated that this limit is not observed in fully dense nanocrystalline magnesium aluminate, where hardness increases from 17.2 to 28.4GPa (surpassing sapphire hardness) when grain sizes are refined from 188nm to 7.1nm, respectively. The increasing trend is proportional to the square root of the grain size, following the Hall-Petch relationship, reassuring that common weakening mechanisms described in nanocrystalline metals might not be present in ceramics. To achieve such small grain sizes in fully dense ceramics, a new processing technique is introduced, Deformable Punch Spark Plasma Sintering, DP-SPS, in which nanoparticles are sheared under high pressures (~2GPa) during densification at moderate temperatures (720–870°C). This inhibits grain growth due to the low processing temperatures and destabilizes/eliminate isolated residual pores, known to detrimentally affect mechanical behavior of ceramics. Noticeably, the sintered material showed high transparency in the visible spectrum, being reported as one of the hardest transparent oxide material to date.
•Fully dense nanocrystalline transparent magnesium aluminate was produced.•Unprecedented hardness of 28.4GPa was observed for 7.1nm grain size.•Hall-Petch relation held true from micro to nano-sizes.•Mechanism of densification relates to punch deformation during sintering.
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•Anatase-rutile phase transition diagram was built for nano Nb2O5-doped-TiO2.•Nb2O5-doping postpones the anatase-to-rutile transition.•The stability crossover for TiO2 was 17.3nm, for ...2mol% Nb2O5-doped-TiO2 ∼30nm.•The surface energy for Nb2O5-doped-TiO2 decreases systematically with Nb concentration.
Titanium dioxide nanoparticles are widely used for photocatalysis, and the relative fraction of titanium dioxide polymorph, i.e. anatase, rutile, or brookite, significantly affects the final performance. Even though conventional phase diagrams indicate a higher stability for the rutile polymorph, it is well established that nanosizes benefit the anatase phase due to its smaller surface energy. However, doping elements are expected to change this behavior, once changes in both surface and bulk energies may occur. Nb2O5 is commonly added to TiO2 to allow property control. However, the effect of niobium on the relative stability of anatase and rutile phases is not well understood from the thermodynamic point of view. The objective of this work was to build a new predictive nanoscale phase diagram for Nb2O5-doped TiO2. Water adsorption microcalorimetry and high temperature oxide melt solution were used to obtain the surface and bulk enthalpies. The phase diagram obtained shows the stable titania polymorph as a function of the composition and size.
Nanocrystalline bulk materials (also called nanograined materials) are intrinsically unstable due to the excess grain boundary (GB) free energies. Dopants designed to segregate to boundaries have ...been proposed to lower excess GB energies, increasing stability against coarsening and enabling nanostructure features to survive high temperature processing and operational environments. It has been theoretically proposed that the GB energy of a material can eventually become zero as a function of dopant concentration, signifying negligible driving force for growth—an infinitely stable nanomaterial. In this work we use ultrasensitive microcalorimetry to experimentally measure the absolute GB energy of gadolinium-doped nanocrystalline zirconia as a function of grain size and show that the energy can indeed reach a quasi-zero energy state (∼0.05 J/m2) when a critical GB dopant enrichment is achieved. This thermodynamic condition leads to unprecedented coarsening resistance, but is a temperature dependent function; since increasing temperatures deplete the GB as the dopant dissolves back in the crystalline bulk.
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