Synthesis and Sintering of (K,Na)NbO 3 Based Ceramics
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Synthesis and Sintering of (K,Na)NbO3 Based Ceramics Barbara Malic, Darja Jenko, Janez Bernard, Jena Cilensek and Marija Kosec Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
ABSTRACT
Stoichiometric K0.5Na0.5NbO3 solid solution(KNN) was prepared by solid-state synthesis from alkaline carbonates and niobium oxide. Thermal decomposition of the powder mixture occurs upon heating to 700 oC followed by crystallization of the perovskite phase. After sintering at 1100 oC, 2h the stoichiometric KNN reaches 94 % of theoretical density. Dielectric constant of stoichiometric KNN is 620, and the losses 0.03.
INTRODUCTION
Ceramics based on Pb(Zr,Ti)O3 solid solution (PZT) have been widely used due to their piezoelectric, pyroelectric and ferroelectric properties. One of the major drawbacks of these materials is a high lead content, the ceramics contain about 60 weight % lead, and therefore a possible ecological hasard. An alternative group of lead-free ferroelectric materials are those based on sodium potassium niobate (K,Na)NbO3 solid solution with the composition with K/Na 50/50 (further denoted as KNN) close to morphotropic phase boundary exhibiting optimum piezoelectric response 1. One of the major problems of KNN is sintering: according to early reports the stoichiometric material is extremely difficult to consolidate1. Various strategies have been used to overcome this problem. Hot pressing or hot isostatic pressing has been used to achieve high densities as early as in 1960`s 2,3. In 1975, Kosec and Kolar prepared KNN ceramics sintered at 1125 oC with 97.1 % theoretical density (TD = 4.51 gcm-3) 3, by introducing Mg-ion as A-site dopant as compared to 94.5 % for the stoichiometric composition. Similarly, an improvement in density was achieved by introducing Nb-excess. By both ways they introduced A-site vacancies into the perovskite lattice and therefore promoted sintering 4. Promoted sintering of KNN with Nb-excess has been also confirmed in our recent work 5. The aim of this work was to address the solid-state synthesis and sintering of stoichiometric K0.5Na0.5NbO3 solid solution (KNN). For comparison KNbO3 and NaNbO3 as boundary compositions of the solid solution were also studied.
EXPERIMENTAL
K0.5Na0.5NbO3 (KNN), KNbO3 (KN) and NaNbO3 (NN) were prepared by the solid state synthesis from alkaline carbonates and niobium oxide. The materials were prepared from K2CO3
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(99+ %, Aldrich), Na2CO3 (99.95 – 100.05 %, Alfa) that were thoroughly dried prior use, and Nb2O5 (99.5 %, Aldrich). The powder mixtures were homogenized, calcined at 800 oC, 4h and milled to yield submicron sized powders as confirmed by laser granulometry. Powder compacts of KNN were uniaxially pressed with 100 M Pa and sintered at 1100 oC for 2 hours in air. Thermogravimetry and differential thermal analysis (TG/DTA) were used to follow the decomposition of the powder mixtures using platinum crucibles with a heating rate of 10 oC/min. in flowing air atmosphere (Netzsch STA 409). Phase composition was determined by X-ra
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