Although largely used in mobiles and electric vehicles applications, insertion batteries based on Li-ion technology are high-cost devices. These applications require high-performance energy storage systems with high-energy and high-power densities, which are better attained by Li-ion technology. However, stationary applications such as power plants and uninterruptible power supplies (UPS) mainly require low-cost energy storage devices. In this scenario Na-ion insertion batteries feature as a cheaper option for such applications. Among the several compounds for use as cathode in Na-batteries, a largely studied class of compounds are the sodium-carbonate-phosphates, Na 3 MCO 3 PO 4 (M = Mn, Fe, Co or Ni), with structure analogue to the mineral sidorenkite. In this work, it was developed a new microwave-assisted hydrothermal method as an ultra-fast way to prepare sodium-carbonate-phosphate compounds. This methodology results in high-quality materials with general formula Na 3 MCO 3 PO 4 (M = Mn, Fe, Co or Ni) upon only 5 min processing time. Characterization techniques indicate highly ordered materials with sidorenkite-like phase and different morphologies or crystal habits, including plates, rods, and a 'starfruit' shape. As a proof-of-application the Mn-based material was evaluated from electrochemical tests for Na + insertion reactions. The obtained results evidence initial discharge capacity of 107 mA h g-1 with good reversible capacity (65 mA h g-1) after several charge/discharge cycles.
Este artigo tem por objetivo discutir o contexto de atuação dos tutores nos cursos superiores em Educação a Distância (EaD), desde a implementação da Lei de Diretrizes e Bases da Educação Nacional (LDBEN), que regulamentou a EaD no cenário brasileiro. Trata-se de uma pesquisa bibliográfica com análise documental, em que são apresentadas considerações acerca do exercício da tutoria em instituições públicas e privadas. Os estudos decorrentes de uma revisão da literatura revelaram que, embora se trate de contextos de atuação diferentes, a tutoria tem sido uma forma de precarização do trabalho docente. Nota-se que a ausência de legislação que regulamente o trabalho do tutor como professor favorece a exploração desses tutores, profissionais que exercem um papel de relevância na EaD.
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