Dipika Saha

Effect of sintering atmosphere on the dielectric properties of barium titanate based capacitors

It has been found that BaTiO3 based capacitor compositions containing lithium tetraborate (flux) and bismuth oxide (flux and Curie peak suppressor) can be fired in an inert (argon) atmosphere without sacrificing densification. However, the inert atmosphere firing can lead to a dramatic increase of both the dielectric constant and dissipation factor of the samples containing bismuth oxide which can be suppressed by adding a small amount of manganese dioxide in the compositions. The temperature coefficient of capacitance of such samples shows the desirable flattened response when fired in argon atmosphere. The observations have been explained by considering the formation of defects at elevated temperatures in bismuth containing samples when fired in an inert atmosphere.

Solid-state synthesis of precursor MgNb2O6 for the preparation of Pb(Mg1/9Nb2/9)O9

Lead magnesium niobate Pb(Mgl/gNb2/9)O 9 (designated as PMN) with perovskite structure has recently been considered as important in the area of electronic ceramics because of its high dielectric constant with flat temperature dependence. To obtain the desired perovskite PMN, it was concluded [1] that the intermediate pyrochlore phase(s) formation must be eliminated. To achieve this, the Columbite precursor method [1,2] is generally employed where MgO and Nb205 are pre-reacted to form the Columbite MgNb206 (designated as MN) followed by its reaction with PbO to form the perovskite PMN. However, Shrout et al. [1, 2] observed that the Columbite precursor method can yield 95% perovskite phase, and to obtain 100% perovskite phase (as detected by X-ray), excess MgO has to be added. On the other hand, Lejeune and Boilet [3] have been able to synthesize 100% perovskite phase by adding excess PbO. Horowitz [4] also reported that up to 94% perovskite phase can be obtained by the Columbite precursor method without using any additive. It is to be noted that [5] excess PbO and MgO are responsible for lowering and ageing the dielectric properties of PMN. Incidentally, Shrout et al. [1, 2] and Horowitz [4] reported the formation of MN by reacting MgO and Nb205 at around 1000 °C, whereas, McCarthy [6] mentioned 1200 °C as the calcination temperature to obtain phase-pure MN. Here we report the formation behaviour of MN for two different solid-state reaction routes and show that if phase-pure precursor MN can initially be formed by a proper calcination schedule, 100% perovskite PMN phase can be obtained without the use of any additive. The raw materials used for synthesis were magnesium hydroxycarbonate (E. Merck, Germany), Nb205 (E. Merck, Germany), magnesium metal powder (S. D. Fine, India) and niobium metal powder (E. Merck, Germany). In the first route (designated as batch A), magnesium hydroxycarbonate and niobium pentoxide were used as raw materials, and in the second route (designated as batch B), magnesium metal powder and niobium metal powders were used. The raw materials were mixed in an agate mortar and pestle under acetone. Initial calcinations were done at 900 °C for 20 h for both A and B batches. These calcined materials were recalcined at different temperatures and the schedules are depicted in Table I. X-ray diffraction studies were carried out with a Philips PW 1730 diffractometer using CuKo~ radiation. Fig. 1 depicts some of the representative X-ray

Preparation of bixbyite phase (MnxFe1-x)2O3 for NTC thermistor applications

Abstract Spinel compounds like doped Mn 3 O 4 and ferrites are conventionally used as negative temperature coefficient (NTC) thermistors. Incidentally, Mn 2 O 3 has got two orders of magnitude lower room temperature resistivity than that of Mn 3 O 4 and it is expected that doped Mn 2 O 3 should perform better than doped Mn 3 O 4 as NTC thermistor. However, Mn 2 O 3 can not be fabricated in the ceramic form as it decomposes to Mn 3 O 4 above 900 °C. We have found that iron, as solid solution, can stabilize Mn 2 O 3 as bixbyite (Mn x Fe 1− x ) 2 O 3 phase even at high temperatures We have successfully prepared phase-pure nano-size powders of (Mn x Fe 1− x ) 2 O 3 (where x =0.98, 0.66 and 0.37) by citrate–nitrate gel method. After sintering the precursor powders at high temperature (1500 °C), their electrical behaviour was studied. It has been found that the NTC sensitivity index ( β ) of the bixbyite phase (Mn x Fe 1− x ) 2 O 3 (where x =0.37 and 0.66) can be as high as 6000 K (in the temperature range of 50–150 °C) which is appreciably higher than that of conventional NTC materials.

Fast firing of lead magnesium niobate at low temperature

A fast firing technique for the sintering of lead magnesium niobate relaxor ceramics at relatively low temperature has been described. In this process, the samples containing excess PbO (up to 5 wt. {percent}) are directly introduced into a furnace maintained at a temperature of 950{degree}C and kept there for 15{endash}80 min, followed by a postsintering annealing treatment at 800{degree}C for 10 h. The importance of fast heating as well as annealing treatment has been justified. The sintered samples are near-phase- pure perovskite materials showing high bulk densities ({approx_gt}94{percent}), uniform and dense microstructure, and satisfactory dielectric properties ({ital K}{sub max}{approx_gt}13,000). The technique is simple and economic, does not require any controlled atmosphere, and minimizes hazards from lead volatilization. {copyright} {ital 1996 Materials Research Society.}

Low-frequency dispersion extended to higher frequencies: A new look at relaxor behaviour

Abstract A new theoretical framework and its experimental verification are presented in order to understand and generalize the dielectric response for relaxor ferroelectrics. By considering the fluctuating nanodomains of relaxors as fluctuating ‘giant dipoles’ we argue that the relaxor effect is indeed a low-frequency dispersion (LFD) phenomenon extended to the r.f. range. Because of the presence of disorder in relaxor ferroelectrics, configurational tunnelling plays an important role; we show that all the conditions necessary for LFD exist in the relaxors and at the same time we provide sufficient reasons to establish why LFD in relaxors is extended as well as prominent in the r.f. range. Our model explains the importance of random electric fields for exhibiting relaxor behaviour. Finally we substantiate our model by showing that dielectric constant and dissipation values as well as dispersion in lead magnesium niobate relaxor ferroelectrics are enhanced in a humid atmosphere in agreement with many well ...

The role of dilatancy on expansion during early stage liquid phase sintering of lead magnesium niobate

Abstract During an attempt to sinter (900°C/30 min) lead magnesium niobate [Pb(Mg1/3Nb2/3)O3] of average particle (aggregate) size 5.8 μm at by adding 3 wt.% excess PbO as a sintering aid, it has been observed that around 3% volume shrinkage takes place. However, an addition of excess Nb2O5 (6 wt.%) along with the same excess PbO gives rise to an appreciable volume expansion (upto 14%) instead of shrinkage under identical firing conditions. We have concluded after systematic studies that such expansion can be understood primarily by considering the generation of insufficient (for consolidation of powders) capillary pressure owing to the formation of a liquid phase (PbO melt with some dissolved Nb2O5) at 900°C and the subsequent role of dilatancy which considers the expansion of well-packed powders during shear.

Effect of humidity on dielectric properties of lead magnesium niobate containing excess MgO

The enhancement of dielectric constant (K) in humid atmosphere (Kmax = 13360 in dry atmosphere to 17450 in 100% relative humidity) for lead magnesium niobate [Pb(Mg1/3Nb2/3)O3] relaxor materials sintered at 950°C for 30 min (using 3 wt% excess Pbo as a sintering aid) can be explained by the extended low frequency dispersion (LFD) model proposed earlier. However, it has been observed that for samples containing excess MgO, such enhancement of Kmax in humid atmosphere is not so remarkable. It has been shown that in case of samples containing excess MgO, owing to the presence of already formed OH− ions at the surface arising out of the reaction of adsorbed moisture with MgO, the physical absorption of polar H2O molecules in humid atmosphere will be much less and consequently the enhancement of dielectric response in humid atmosphere should be low.

Lead Oxide as a Sintering Aid for Liquid Phase Sintering of Lead Magnesium Niobate

It is known that sintering of pure lead magnesium niobate (designated PMN) powder requires relatively high temperature (>1100°C) and the latter necessitates the use of costly high palladium based electrode for multilayer capacitors. Here, we have shown that PMN can be rapidly sintered at 900°C (30 min soaking) by using 2–3 wt% excess PbO. However, appreciable densification by the aforesaid method is only possible when the particle (aggregate) size of the PMN powder is quite large (≥ 15 μm). Possible reasons for such behaviour have been delineated.