Cu(2) nuclear resonance evidence for an original magnetic phase in aged 60K-superconductors RBa2Cu3O6+x (R=Tm,Y)

It is widely believed that the long-range antiferromagnetic order in the RBa2Cu3O6+x compounds (R=Y and rare earths except of Ce, Pr, Tb) is totally suppressed for the oxygen index x>0.4 (AFM insulator-metal transition). We present the results of the copper NQR/NMR studies of aged RBa2Cu3O6+x (R=Tm,Y) samples showing that a magnetic order can still be present at oxygen contents x up to at least 0.7 and at temperatures as high as 77K.Comment: 7 pages, 6 figures. Submitted to Phys.Rev.

First time determination of the microscopic structure of a stripe phase: Low temperature NMR in La2NiO4.17

The experimental observations of stripes in superconducting cuprates and insulating nickelates clearly show the modulation in charge and spin density. However, these have proven to be rather insensitive to the harmonic structure and (site or bond) ordering. Using 139La NMR in La2NiO4.17, we show that in the 1/3 hole doped nickelate below the freezing temperature the stripes are strongly solitonic and site ordered with Ni3+ ions carrying S=1/2 in the domain walls and Ni2+ ions with S=1 in the domains.Comment: 4 pages including 4 figure

Spin dynamics and ordering of a cuprate stripe-antiferromagnet

In La1.48Nd0.4Sr0.12CuO4 the 139La and 63Cu NQR relaxation rates and signal wipe-out upon lowering temperature are shown to be due to purely magnetic fluctuations. They follow the same renormalized classical behavior as seen in neutron data, when the electronic spins order in stripes, with a small spread in spin stiffness (15% spread in activation energy). The La signal, which reappears at low temperatures, is magnetically broadened and experiences additional wipe-out due to slowing down of the Nd fluctuations.Comment: 4 pages including 3 figures - ref. 16 adde

Correlations Between Charge Ordering and Local Magnetic Fields in Overdoped YBa2_2Cu3_3O6+x_{6+x}

Zero-field muon spin relaxation (ZF-$\mu$SR) measurements were undertaken on under- and overdoped samples of superconducting YBa$_2$Cu$_3$O$_{6+x}$ to determine the origin of the weak static magnetism recently reported in this system. The temperature dependence of the muon spin relaxation rate in overdoped crystals displays an unusual behavior in the superconducting state. A comparison to the results of NQR and lattice structure experiments on highly doped samples provides compelling evidence for strong coupling of charge, spin and structural inhomogeneities.Comment: 4 pages, 4 figures, new data, new figures and modified tex

Resistance to acid attack, abrasion and leaching behavior of alkali-activated mine waste binders

This paper report results of a research project on the development of alkali-activated binders using mine wastes. Abrasion and acid resistance of two ordinary Portland cement (OPC) strength class concrete mixtures (C20/25 and C30/37) and several mine waste (MW) mixtures were compared. This study indicates that MW binders possess higher acid and abrasion resistance than OPC based concrete mixtures.The leaching assessment of the MW binders shows it can be considered an inert material which indicates that it could be used as a building material

Estudo comparativo da análise de ciclo de vida de concretos geopoliméricos e de concretos à base de cimento Portland composto (CP II)

A Análise de Ciclo de Vida (ACV) é um método que inclui a compilação e avaliação das entradas, saídas e dos impactos de um produto ao longo do seu ciclo de vida. Os resultados desta análise são utilizados para escolher alternativas favoráveis para uma aplicação específica. Este artigo utilizou as ferramentas de ACV, apoiada pelo software Umberto, para comparar o processo de obtenção de concretos de cimento Portland com o de concretos geopoliméricos, obtidos da ativação alcalina de aluminossilicatos. O impacto ambiental foi avaliado considerando 1 m³ de cada concreto, sendo (i) as emissões de CO2 (kg CO2/ m³) e (ii) a demanda energética (MJ/m³) as variáveis para determinar o potencial sustentável de ambos materiais. O objetivo principal foi avaliar se os concretos geopoliméricos são mais sustentáveis do que os concretos tradicionais. Os resultados obtidos mostraram que o consumo energético é reduzido em 45,8% na produção do concreto geopolimérico, quando comparado à produção do concreto de cimento Portland de desempenho mecânico equivalente. Com relação às emissões de CO2, o concreto geopolimérico reduz as emissões em 72,4%, em comparação ao concreto tradicional de cimento Portland CPII. Assim, o primeiro se mostra uma alternativa a ser considerada, na produção de materiais de construção de menor impacto ambiental

Examination of alkali-activated material nanostructure during thermal treatment

The key nanostructural changes occurring in a series of alkali-activated materials (AAM) based on blends of slag and fly ash precursors during exposure to temperatures up to 1000 °C are investigated. The main reaction product in each AAM is a crosslinked sodium- and aluminium-substituted calcium silicate hydrate (C-(N)-A-S-H)-type gel. Increased alkali content promotes the formation of an additional sodium aluminosilicate hydrate (N-A-S-(H)) gel reaction product due to the structural limitations on Al substitution within the C-(N)-A-S-H gel. Heating each AAM to 1000 °C results in the crystallisation of the disordered gels and formation of sodalite, nepheline and wollastonite. Increased formation of N-A-S-(H) reduces binder structural water content after thermal treatment and correlates closely with previous observations of improved strength retention and reduced microcracking in these AAM after heating to 1000 °C. This provides new insight into thermally induced changes to gel atomic structure and thermal durability of C-(N)-A-S-H/N-A-S-H gel blends which are fundamental for the development of new fire-resistant construction materials