Sintering mechanism of LiTaO/sub 3/ ceramics by the addition of lithium and magnesium fluorides

Abstract

Highly densified LiNbO3 and LiTa03 ceramics have been obtained by sintering in a moist atmosphere in the presence of LiF and MgF2. The sintering temperatures was decreased by 300oC for LiNbO3 and 500oC for LiTa03. An open air sintering mechanism based on the partial hydrolysis of the starting fluorides and the formation of an intermediate MgqM2Xg (M = Li,Ta; X = O,F,OH) type hydroxofluoride phase, has been proposed and a three stage reactional sketch has been given.1.Introduction LiNb03 and LiTa03 are well known by their interesting non-linear properties, such as piezo-and pyroelectricity, electrooptics, etc. A lot of work has been undertaken on single crystals, however, ceramics of this ferroelectric family had not been developed because of the required high sintering temperature (> 1300oC) leading to the volatilisation of Li20. Systematic research has been undertaken recently by the way of oxide or fluoride addition or by sol-gel preparation process, in order to improve the ceramic qualities (1-4). Satisfying results have been obtained, amongst which the addition of litium and magnesium fluorides has permitted to prepare highly densified LiNbO3 or LiTa03-type ceramics by sintering in free atmosphere at a temperature of 900oC (2, 3). Dielectric, piezo-and pyroelectric measurements on LiTa03 ceramics showed that the ferroelectric Curie temperature is higher than that of pure LiTa03 and that piezo-and pyroelectric coefficients reach respectively 20% and 30% of the values of LiTa03 single crystals. These piezo-and pyroelectric properties are the best performances so far realized on LiTa03-type ceramics (Table 1). In this paper the sintering mechanism of LiTaO and MgF2 will be reported.ceramics at low temperatures in the presence of Li 8 2.Substantial Comparative Sintering Effects Highly densified ceramics were prepared by sintering at 900OC in air atmosphere the mixtures: (1-x)LiM03 + x(LiF+MgF2) (M=Nb; Ta). Figure 1 shows the thermal evolution of the thickness of a cylindrical sample made of compacted powder of starting composition x = 0.05, recorded by means of a 0-7803-0190-0/91$01 .OO OIEEE Netzsch differential dilatometer. As a comparison, the curves of pure LiNb03 and LiTa03 samples are also given. I1 can be seen that the addition of small amounts of the lithium and magnesium fluorides decreased the sintering temperature by 300 degrees for LiNb03 and by 500 degrees for LiTa03. 3.Experimental Studies Revealing the Sintering Mechanism 3.1 Air atmosphere conditionsA quite different behaviour of the (l-x)LiTa03 + x(LiF+MgFZ) mixtures at 900oC was observed according to the atmosphere. A previous study carried out on the same starting compositions but in sealed platinium tubes showed that the obtained samples were poorly densified at 900OC (the shrinkage coefficient Ad/# e 0.01 !) (14). Further experiments showed that the level of moisture in the atmophere greatly influenced the sintering process.Water vapour seems to promote the partial hydrolysis of the additive fluorides, particularly that . A high-temperature X...

Powder Synthesis and Hot‐Pressing of a LiTaO3 Ceramic

“…Dense, pure polycrystalline LiTaO 3 ceramics have never been reported, though limited success has been achieved through the use of various sintering aids such as MgF 2 or MnO x , or through extensive substitution on the A ‐site, B ‐site, or both . In addition to the difficulties associated with the extreme differences in cation activity and mobility common to alkali niobates and tantalates, LiTaO 3 has a trigonal (rhombohedral) crystal structure of space group R 3 cH with significant crystallographic anisotropy in coefficients of thermal expansion (CTE) (α a = 16 × 10 ‐6 /°C, α c = 4 × 10 −6 /°C), which leads to tremendous stresses during cooling of a consolidated body.…”