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With the development of electronic technology, composite materials have been widely used for electronic devices where higher densities, limited space and multifunction are required. Recently the ferroelectric-ferromagnetic composite materials were intensively researched for two uses: the magnetic-electric sensors in radio-electronics, optoelectronics, microwave electronics and transducers and the compact electrical filters for suppressing electromagnetic interference (EMI). As for the magnetic-electric sensors, high ferroelectric content was necessary for the composite materials with sufficient resistivity to generate magnetoelectric effect.
Magneto electric coupling describes the influence of a magnetic field (or an electric field) on the polarization (or magnetization) of a material.
In the past few years, extensive research has been conducted on magneto electric effect in single phase and composite materials. Direct polarization of a material under a magnetic field or an in induced magnetization under an electric field requires the simultaneous presence of long range ordering of magnetic moments and electric dipoles.
Magneto Electric materials are of two types:
In a magnetoelectric (ME) composite the magnetostrictive strain in the magnetic phase creates an electric polarization in the adjacent piezoelectric phase and hence is capable of converting magnetic field into electric field and vice versa. Such product property can be utilized in smart materials used in sensors, processors and feedback systems.
The first magneto electric effect was predicted in Cr2O3, but magneto electric materials with a single phase show a weak magneto electric effect , hence the need of composites.
Magneto electric composites on other hand have large magneto electric coefficients of magnitude of magneto electric voltage coefficients. The composites are made exploiting the product property of materials.
Composite materials are engineered materials made from two or more constituent materials with significantly different physical or chemical properties and which remain separate and distinct on a macrospace level within the finished structure.
There are number of physical methods for preparing nano crystalline materials viz inert gas condensation, physical vapour deposition , laser ablatiion, chemical vapour deposition, sputtering, molecular beam epitoxy etc. Among the available solution- chemistryroutes, combustion technique is capable of producing nano crystallline powders of oxide ceramics, at a lower calcination temperature in a surprisingly short time. The solution combustion is a two step process:
Formation of a precursor
The formation of precursor (viscous liquid or gel), is a primary condition for an intimate blending of the starting constituents and preventing the random redox reaction between a fuel and an oxidizer. The very high exothermicity generated during combustion manifests in the form of either a flame or a fire and hence the process is termed as auto ignition process. The nature of the fuel and its amount are some of important process parameters for getting the transparent viscous gel without any phase seperation and precipitation. Thus the basic characterstics of a fuel are that it should be able to maintain the compositional homogenity among the constituents also undergo combustion with an oxidizer at low ignition temperature. Commonly used fuels are glycine, urea, citric acid etc.
Sintered composite materials are much easier as well as cheaper to prepare than unidirectional solidified in situ composites. As regard to the ME effect it was found that ME composites made by unidirectional solidification always gave a higher value than those prepared by solid state sintering of the presintered component phases for a given composition.
Nanocrystalline NiFe2O4 samples can be synthesized by following methods; coprecipitation, combustion, citrate gel and conventional ceramic method. AR grade metal nitrates, Ni(NO3)2·6H2O and Fe(NO3)3·9H2O were used for all three syntheses.
Nanocrystalline BaTiO3 samples can be synthesized by autocombustion, solid oxide route and conventional ceramic method.
Recently auto combustion synthesis method attracted considerable attention in fabricating homogeneous and unagglomerated fine ceramic powder. Availability of comparatively inexpensive precursors, simple calculations, eases in optimizations of process parameters proved to be advantageous in auto combustion synthesis.
Other Magnetoelectric Applications:
Historically BaTiO3- NiFe2O4 composites were first obtained in 1972 by Van Suchtelen.
Type of materials that undergo ME multiferroic: Single material/ Composite
Theoretically the magneto electric effect came into picture in 1894 when curie discussed correlation of magnetic and electric properties in low symmetry crystals. Another strong footing on ME effect theoretically is by L.D. Landau in 1957. According to him, " The magneto electric effect is odd with respect to time reversal and vanishes in materials without magnetic structure. First experimental observation of the ME effect was in 1960 by Astrov who found the electric field induced magneto electric effect in Cr2O3. Conventionally, oxide ceramic powders are made by solid-state reaction method which requires heating at elevated temperatures for long periods of time. In addition to high energy consumption, the production rate is slow. Recently, several wet chemical methods, such as the hydrothermal method, co-precipitation process, and sol-gel technique, have been developed for the synthesis of oxide ceramic powders to improve their properties. These methods may have several drawbacks : high pH sensitivity, stringent drying conditions, complex equipment and expensive precursors, and others. Generally the low temperature environment is mostly preferred for the synthesis of nanoparticles,. Some of the physical and chemical methods widely used in the synthesis of nanoferrites are ball-milling, sol-gel, co-precipitation, spray pyrolysis and hydrothermalmethods . Though the sol-gel route yields more promising results in the synthesis of nanoferrites , several preparation conditions such as dilution, fuel/oxidant ratio, pH and temperature can have an impact on the formation of the ferrites and their properties . As some of the advantageous perspectives are, this method exploits the advantages of cheap precursors, simple preparation and a resulting ultra fine and homogeneous powder .
As reported above solid state reaction or conventional ceramics method is usually followed to prepare BaTiO3- NiFe2O4 or BaTiO3. Nickel ferrite (Ni2Fe2O4) is widely used as a soft ferrite in electronic devices. It is also used in catalysis. Nickel ferrite belongs to inverse spinel structure, in which the tetrahedral sites (A) are occupied by Fe3+ ions and the octahedral sites (B) by Fe2+ and Ni2+ ions. 3- NiFe2O4 based composites. The advantages of this route are: simple, cheap and free choice of composition of the constituents. Using this method various composites have been made such as NiFe2O4/PZT, Ni0.75Co0.25Fe2O4 + Ba0.8Pb0.2TiO3 etc. Among these different composites BaTiO3-NiFe2O4 composite seems to be most promising for applications. We therefore put the effort to study that system. Multiferroic BaTiO3-NiFe2O4 composite could be regarded as model system illustrating magneto electric effect. BaTiO3 is a typical ferroelectric material which has a large piezoelectricity. NiFe2O4 is ferromagnetic with large magnetization. Wet chemical methods are coming into this field of particulate composite with a lot of advantages. Firstly the sintering temperature likely to be reduced as that is followed in conventional ceramic method. This will save electrical energy in processing. Playing with the properties with varrying compositions is also possible. Composite properties could be improved by proper mixing of constituents. Wet chemical method is very much helpful.
As a summary of some of the literatures which came across is tabulated as follows:
Sung-Soo Ryu , Sang-Kyun Lee , Dang-Hyok Yoon
Reaction Temperature was decreased by doping Calcium
Enhanced Reaction Rates due to increase in Contact area due to Small particles
Maria Teresa Buscaglia1, Vincenzo Buscaglia, Massimo Viviani,
Giovanni Dondero, SergeR¨ohrig, Andreas R¨udigerand
Teoh Wah Tzu, Ahmad Fauzi Mohd Noor, Zainal Arifin Ahmad
Complete formation of Ba0.7Sr0.3TiO3happen at 1150oC and above, several different phases are detected if the calcination is done below 1150oC.
Vittorio Berbenni , Chiara Milanese, Giovanna Bruni, Amedeo Marini
XRPD patterns of samples of both physical and milled mixture annealed at temperatures between 400 â-¦Cand 1100 â-¦C show that NiFe2O4is obtained by 12 h annealing at temperatures as lowas 400 â-¦C while 24 h at 1100 â-¦C are needed to yield NiFe2O4when starting from the physical mixture.
ZHANG Lei,ZH OU Ke-chao, LI Zhi-you,Y ANG Wen-jie
High temperature has a sintering effect on the composite powder, and the microstructure with high density and fine grain inside the particle is gained.
H.G. El-Shobaky, N.R.E. Radwan
Solid interaction between _-Fe2O3and NiO occurred at temperatures starting from 700 â-¦C to produce NiFe2O4. The degree of reaction propagation was increased as a function of temperature.
F. Novel0 and R Valenzuela
This study contributes to the investigation of reaction
kinetics mainly in two aspects: i) the use of a DTA system to perform the reactions in small quantities, thus decreasing the problems of heat transfer, and more important, providing precise reaction conditions (time and temperature); and ii) by using difhaction peak areas instead of peak heights, which leads to a higher regression coefficient.
Synthesis of Barium Titanate:
BaTiO3 is prepared through solid state reaction method. The precursors were BaCO3 and TiO2. BaCO3 used was of Qualigens with 98% purity and TiO2 used was of LOBA chemie with 99% purity.
BaCO3 and TiO2 were taken in 1:1 mole ratio into a small agate mortar. To achieve a homogeneous mixture convenient amount of iso-propanol was used as wet mixing media. After homogeneous mixing it is calcined. During calcinations CO2 gas is evolved. It follows the following reaction :
BaCO3 + TiO2 à BaTiO3 + CO2
Considering the molecular weights of BaTiO3 and NiFe2O4 the amount of BaCO3, TiO2, NiO and Fe2O3 required is calculated.
Synthesis of Nickel ferrite:
NiFe2O4 is prepared through dry route using NiO and Fe2O3. NiO used was of LOBA CHEMIE with comlexometric Ni 70% and Fe2O3 used was of LOBA CHEMIE with 98.5% purity.
It follows the following reaction:
NiO + Fe2O3 à NiFe2O4
NiO and Fe2O3 in1:1 molar ratio is taken and mixed using iso-propanol. After homogeneous mixing the mixture is calcined. XRD analysis is done for phase conformation.
Preparation of the Composite of Barium Titanate and Nickel Ferrite:
In the next step calcined NiFe2O4 and BaTiO3 are mixed using iso-propanol in required proportion. Four batches of BaTiO3-NiFe2O4 with composition ratio 50:50, 60:40, 70:30 and 80:20 are prepared.
After preparation of all the batches 3% PVA (binder) is added to each batch and mixed uniformly.
The PVA mixed mixture was used to make several pellets. Each pellet being formed from around 0.75gm mixture.
The powder was pressed using CARVER PRESS USA and circular die.
For the preparation of pellets 4T force and dwell time of 90 sec was set.
Each pellet prepared is sintered at 1250oC for 2 hrs.
The pellets prepared with 50:50 composition of BaTiO3 and NiFe2O4 are divided into three categories:
BaTiO3 fired at 1000oC and NiFe2O4 fired at 900oC.
BaTiO3 fired at 1000oC and NiFe2O4 fired at 800oC.
BaTiO3 fired at 1100oC and NiFe2O4 fired at 800oC.
From above each sample two pellets were taken and their dry weight was measured. Then suspended weight and soaked weight of the sample was measured. From these measurement density of the pellets was calculated. The calculated density was compared to that of theoretical density of BaTiO3-NiFe2O4 (50:50).
The sintered pellets were polished and XRD analysis was done.
NiO + Fe2O3 à mixed by dry route using iso-propanol
FLOW CHART FOR SYNTHESIS OF BARIUM TITANATE-NICKEL FERITE COMPOSITE
BaCO3 + TiO2àmixed by dry route using iso-propanol
Pressing is done at 4tonne force and 90 sec dwell time make pellets
3% PVA (binder) is added to the mixture
Dry mixing is done to give BaTiO3-NiFe2O4 composite(Four batches are prepared with composition 50:50, 60:40,70:30 and 80:20. )
Calcination at 900OC for 2hrs
Calcination at 700OC for 2 hrs
Sintering of pellets
R. Grossinger, Giap V. Duong and R. Sato-Turtelli, Journal of Magnetism and magnetic materials 320 (2008) 1972-1977.
Jungho Ryu, Shashank Priya, Kenji Uchino and Hyoun-EE Kim, Journal of electroceramics, 8, 107-119, 2002.
W. Eerenstein, N.D. Mathur and J.F. Scott, nature05023.
Sung-Soo Ryu , Sang-Kyun Lee , Dang-Hyok Yoon, J Electroceram (2007) 18:243-250 .
Maria Teresa Buscaglia1, Vincenzo Buscaglia, Massimo Viviani, Giovanni Dondero, SergeR¨ohrig, Andreas R¨udiger and Paolo Nanni, Nanotechnology 19 (2008) 225602 (7pp).
U.Manzoor and D.K.Kim, J. Mater. Sci. Technol., Vol.23 No.5, 2007.
Teoh Wah Tzu, Ahmad Fauzi Mohd Noor and Zainal Arifin Ahmad, formation of barium strontium titanate powder by solid state reaction using different calcinations temperatures.
Vittorio Berbenni , Chiara Milanese, Giovanna Bruni, Amedeo Marini, Thermochimica Acta 469 (2008) 86-90.
ZHANG Lei,ZH OU Ke-chao, LI Zhi-you,Y ANG Wen-jie, Thermochimica Acta 469 (2008) 86-90.
H.G. El-Shobaky, N.R.E. Radwan, Trans. Nonferrous Met. SOC. China 16(2006) 1076-1079.
F. Novel and R Valenzuela, Materials Research Bulletin, Vol. 30, No. 3, pp. 33X340.1995.