Decomposição de amostras de solos assistida por radiação microondas: estratégia para evitar a formação de fluoretos insolúveis

A decomposição de amostras que contêm elevados teores de Si, utilizando mistura com os ácidos nítrico e fluorídrico, pode provocar a precipitação de fluoretos de Al(III), Ca(II), Fe(III) e Mg(II) durante a digestão, impedindo a recuperação total desses elementos. Visando minimizar esse problema, foram testados quatro procedimentos aplicados a diferentes amostras de solos. O procedimento mais eficiente envolveu a decomposição assistida por radiação microondas, seguida pela separação do material sólido residual com elevado teor de Si do sobrenadante ácido. A fração residual foi solubilizada usando HF concentrado e, após complexação com ácido bórico, essa solução foi misturada à fase líquida do digerido. Os teores de metais nos digeridos foram determinados por espectrometria de emissão óptica em plasma induzido (ICP OES) e a exatidão foi demonstrada aplicando o procedimento proposto em amostras provenientes do programa colaborativo International Soil-Analytical Exchange (ISE, Wegening Agricultural University, Holanda) e em amostra certificada de solo proveniente do National Institute of Standard and Technology (NIST, SRM 2709), que apresentou respectivamente, os seguintes percentuais de recuperação: 100,0 ± 2,6; 99,5 ± 1,9; 108,5 ± 2,7; 94,6 ± 8,9; 92,2 ± 9,7; 95,7 ± 1,8; 96,7 ± 2,7; 95,2 ± 0,6; 103,6 ± 2,6 e 96,1 ± 1,6 para, Al, Ba, Ca, Cr, Cu, Fe, Mg, Mn, V e Zn.

Directrius per a l'accessibilitat al contingut web, versió 1.0

Aquestes directrius expliquen com fer que el contingut web sigui accessible a persones amb discapacitats i s'adrecen a creadors de contingut (autors de pàgines web o dissenyadors de llocs web) i a creadors d'eines d'autor. L'objectiu principal d'aquestes directrius és promoure l'accessibilitat. Tanmateix, l'aplicació de les directrius facilitarà l'accés al contingut a tot tipus d'usuari, sigui quin sigui l'agent d'usuari usat (navegador web, navegador de veu, telèfon mòbil, ordinador de cotxe, etc.) o les condicions de l'entorn de consulta (entorns sorollosos, espais mal il·luminats, entorns en què no es poden usar les mans, etc.). L'aplicació d'aquestes directrius també ajudarà els usuaris a trobar la informació d'una manera més ràpida dins el web. Les directrius no pretenen desincentivar l'ús d'imatges, vídeo, etc., sinó que expliquen com fer que el contingut multimèdia sigui més accessible a una àmplia audiència.Aquest és un document de referència per a uns principis d'accessibilitat i idees de disseny. Algunes de les estratègies comentades tracten d'aspectes relatius a la internacionalització del web i a l'accés des de terminals mòbils. Tanmateix, el document se centra en l'accessibilitat i no tracta exhaustivament dels aspectes relacionats amb altres activitats del W3C. Si voleu més informació sobre aquests temes podeu consultar les pàgines inicials W3C Mobile Access Activity (per a l'accés des de terminals mòbils) i W3C Internationalization Activity (per als aspectes d'internacionalització).Aquest document està pensat per a ser estable en el temps i, per tant, no dóna informació específica sobre si els navegadors funcionen o no amb una determinada tecnologia, ja que aquesta informació varia molt ràpidament. Aquesta informació es pot trobar al web de la Web Accessibility Initiative ,WAI, (Iniciativa d'Accessibilitat Web) [WAI-UA-SUPPORT].Aquest document inclou un annex que organitza tots els punts de verificació ordenats per tema i per prioritat. Els punts de l'annex estan enllaçats a les respectives definicions en el document. Els temes recollits en l'annex inclouen les imatges, el contingut multimèdia, les taules, els marcs, els formularis i els scripts. L'annex es presenta en forma de taula o com a simple llista.Un document a part, amb el títol Techniques for Web Content Accessibility Guidelines 1.0 (Tècniques per a les directrius per a l'accessibilitat al contingut web, versió 1.0) ([TECHNIQUES]) explica com posar a la pràctica els punts citats fins aquí. El document de tècniques explica cada punt amb més detalls i dóna exemples usant el llenguatge d'etiquetatge d'hipertext (HTML), fulls d'estil en cascada (CSS), el llenguatge d'integració multimèdia sincronitzada (SMIL) o el llenguatge d'etiquetatge matemàtic (MathML). Aquest document també inclou tècniques per a provar o validar una pàgina web i un índex dels elements i atributs HTML amb les tècniques que els usen. El document de tècniques està pensat per a seguir de prop els canvis tecnològics i es preveu que s'actualitzi més sovint que les directrius.Nota: Algunes de les característiques descrites en les directrius no estan encara implementades en tots els navegadors o eines multimèdia; en concret pot ser que no es puguin utilitzar funcions noves d'HTML 4.0, de CSS1 o CSS2.Les Directrius per a l'accessibilitat al contingut web, versió 1.0 són part d'una col·lecció de directrius sobre accessibilitat publicades per la Web Accessibility Initiative, WAI (Iniciativa d'Accessibilitat Web). La col·lecció comprèn User Agent Accessibility Guidelines (Directrius d'accessibilitat per a agents d'usuari) [WAI-USERAGENT] i Authoring Tool Accessibility Guidelines (Directrius d'accessibilitat per a eines d'autor [WAI-AUTOOLS].

Atributos químicos de solos influenciados pela substituição do carbonato por silicato de cálcio

A aplicação de silicatos pode promover incrementos na capacidade de troca de cátions, favorecer a disponibilização de ânions, especialmente o H2PO4- (fosfato diácido), elevar o pH, amenizar a toxidez de Al e, de maneira geral, incrementar a disponibilidade de nutrientes para as plantas, apresentando, porém, uma eficiência agronômica inferior em comparação aos carbonatos. Com o objetivo de avaliar o efeito da substituição do carbonato de Ca por silicato de Ca sobre as propriedades químicas dos solos, especialmente em relação à disponibilidade de P, foram realizados quatro experimentos em casa de vegetação, num delineamento inteiramente casualizado, com quatro repetições. Os tratamentos consistiram de cinco níveis de substituição (0, 25, 50, 75 e 100 %) do carbonato de Ca por silicato de Ca, mantendo uma relação estequiométrica Ca:Mg de 4:1 e o mesmo conteúdo de CaO, suficientes para elevar V = 60 %. Os tratamentos foram aplicados em amostras de 4 dm³ de Neossolo Quartzarênico órtico, Latossolo Vermelho-Amarelo textura média, Latossolo Vermelho-Amarelo textura argilosa e Latossolo Vermelho textura muito argilosa, sendo cada solo um experimento. Foram determinados os valores dos atributos químicos dos solos: pH em H2O, P, P remanescente (P-rem), K, Ca, Mg, Si, Al, H + Al, matéria orgânica (MO), Cu, Mn, Zn e B, soma de bases (S), a CTC efetiva (t), a CTC em pH 7,0 (T), a saturação por bases (V) e a saturação por Al (m), os quais foram submetidos à analise de variância e ao ajuste de modelos de regressão simples, considerando os níveis de substituição de CaCO3 por CaSiO3. Verificou-se que a substituição de carbonato por silicato promoveu aumentos significativos nos valores de Si, Al, H + Al e m e redução nos valores de P-rem, pH, S, t e V; já os valores de P Mehlich-1, K, Mg, MO, T, Mn, Cu e B não foram influenciados significativamente. Houve declínio na disponibilidade de Zn somente no solo RQo. A eficácia do silicato de Ca foi inferior à de carbonato de Ca na melhoria das condições químicas do solo.

Influence of the (111) twinning on the formation of diamond cubic/diamond hexagonal heterostructures in Cu-catalyzed Si nanowires

The occurrence of heterostructures of cubic silicon/hexagonal silicon as disks defined along the nanowire (111) growth direction is reviewed in detail for Si nanowires obtained using Cu as catalyst. Detailed measurements on the structural properties of both semiconductor phases and their interface are presented. We observe that during growth, lamellar twinning on the cubic phase along the (111) direction is generated. Consecutive presence of twins along the (111) growth direction was found to be correlated with the origin of the local formation of the hexagonal Si segments along the nanowires, which define quantum wells of hexagonal Si diamond. Finally, we evaluate and comment on the consequences of the twins and wurtzite in the final electronic properties of the wires with the help of the predicted energy band diagram.

High-pressure ultrasonic study of vibrational anharmonicity in bcc Cu-Al-Be alloys

To quantify the vibrational anharmonicity of the long-wavelength acoustic modes of bcc Cu74.1Al23.1Be2.8 near its martensitic transition temperature Ms (261 K), the hydrostatic pressure derivatives (¿CIJ/¿P)P=0 of the elastic stiffness moduli have been measured. The Grüneisen parameters at 268 K (just above Ms), especially of longitudinal modes, which become smaller than those of the shear modes, are quite different from those at 295 K: the anharmonicity changes markedly in the vicinity of the transition. Similar trends are noted for Cu66.5Al12.7Zn20.8. Experimental data near Ms are used to estimate cubic invariants in the strain order parameters in a Landau formalism.

Elastic constants of bcc Cu-Al-Ni alloys

We have measured the adiabatic elastic constants of two Cu-Al-Ni martensitic alloys using ultrasonic methods and we have compared the results to recent neutron-scattering experiments. It is shown that the elastic behavior of Cu-Al-Ni alloys follows the same trends exhibited by other Cu-based alloys; in particular, the TA2 long-wavelength acoustic modes are softer than all other modes.

Third order elastic constants of bcc Cu-Al-Ni

We have measured the changes in the ultrasonic wave velocity, induced by the application of uniaxial stresses in a Cu-Al-Ni single crystal. From these measurements, the complete set of third-order elastic constants has been obtained. The comparison of results for Cu-Al-Ni with available data for other Cu-based alloys has shown that all these alloys exhibit similar anharmonic behavior. By using the measured elastic constants in a Landau expansion for elastic phase transitions, we have been able to give an estimation of the value of a fourth-order elastic constants combination. The experiments have also shown that the application of a stress in the [001] direction, reduces the material resistance to a (110)[110] shear and thus favors the martensitic transition.

On the stability of the bcc phase in Cu-Al-Mn shape memory alloys

Measurements of the entropy change at the martensitic transition of two composition-related sets of Cu-Al-Mn shape-memory alloys are reported. It is found that most of the entropy change has a vibrational origin, and depends only on the particular close-packed structure of the low-temperature phase. Using data from the literature for other Cu-based alloys, this result is shown to be general. In addition, it is shown that the martensitic structure changes from 18R to 2H when the ratio of conduction electrons per atom reaches the same value as the eutectoid point in the equilibrium phase diagram. This finding indicates that the structure of the metastable low-temperature phase is reminiscent of the equilibrium structure.

Disorder-induced critical phenomena in magnetically glassy Cu-Al-Mn alloys

Measurements of magnetic hysteresis loops in Cu-Al-Mn alloys of different Mn content at low temperatures are presented. The loops are smooth and continuous above a certain temperature, but exhibit a magnetization discontinuity below that temperature. Scaling analysis suggest that this system displays a disorder-induced phase transition line. Measurements allow one to determine the critical exponents ß=0.03±0.01 and ß¿=0.4±0.1, which coincide with those reported recently in a different system, thus supporting the existence of universality for disorder-induced critical points.

Kinetics of martensitic transitions in Cu-Al-Mn under thermal cycling: Analysis at multiple length scales

In this paper we study the evolution of the kinetic features of the martensitic transition in a Cu-Al-Mn single crystal under thermal cycling. The use of several experimental techniques including optical microscopy, calorimetry, and acoustic emission, has enabled us to perform an analysis at multiple scales. In particular, we have focused on the analysis of avalanche events (associated with the nucleation and growth of martensitic domains), which occur during the transition. There are significant differences between the kinetics at large and small length scales. On the one hand, at small length scales, small avalanche events tend to sum to give new larger events in subsequent loops. On the other hand, at large length scales the large domains tend to split into smaller ones on thermal cycling. We suggest that such different behavior is the necessary ingredient that leads the system to the final critical state corresponding to a power-law distribution of avalanches.

Hysteresis in a system driven by either generalized force or displacement variables: martensitic phase transition in single-crystalline Cu-Zn-Al

We report on experiments aimed at comparing the hysteretic response of a Cu-Zn-Al single crystal undergoing a martensitic transition under strain-driven and stress-driven conditions. Strain-driven experiments were performed using a conventional tensile machine while a special device was designed to perform stress-driven experiments. Significant differences in the hysteresis loops were found. The strain-driven curves show reentrant behavior yield point which is not observed in the stress-driven case. The dissipated energy in the stress-driven curves is larger than in the strain-driven ones. Results from recently proposed models qualitatively agree with experiments.

Temperature and magnetic-field dependence of the elastic constants of Ni-Mn-Al magnetic Heusler alloy

We report on measurements of the adiabatic second-order elastic constants of the off-stoichiometric Ni54Mn23Al23 single-crystalline Heusler alloy. The variation in the temperature dependence of the elastic constants has been investigated across the magnetic transition and over a broad temperature range. Anomalies in the temperature behavior of the elastic constants have been found in the vicinity of the magnetic phase transition. Measurements under applied magnetic field, both isothermal and variable temperature, show that the value of the elastic constants depends on magnetic order, thus giving evidence for magnetoelastic coupling in this alloy system.

Heat capacity measurements on the equiatomic compounds in Ni-X (X = Al, In, Si, Ge and Bi) and M-Sb (with M = Ni, Co and Fe) systems

Molar heat capacities of the binary compounds NiAl, NiIn, NiSi, NiGe, NiBi, NiSb, CoSb and FeSb were determined every 10 K by differential scanning calorimetry in the temperature range 310-1080 K. The experimental results have been fitted versus temperature according to C-p = a + b . T + c . T-2 + d . T-2. Results are given, discussed and compared to estimations found in the literature. Two compounds, NiBi and FeSb, are subject to transformations between 460 and 500 K. (C) 1999 Elsevier Science Ltd. All rights reserved.