PREPARATION AND ANALYSIS OF BaPr(1.33)Ti(3)O(9) & BaPr(2)Ti(4)O(12) MICROWAVE DIELECTRIC CERAMICS

In the field of microwave dielectric ceramics, BaO-R₂O₃-TiO₂  (R=La, Nd, Sm, Pr) is the most valuable material system, which is used for microwave components such as multi-layer ceramic chip capacitor (MLCC), single-layer ceramic chip capacitor (SLC), dielectric resonator & filter, voltage control oscillator (VCO) and antenna, etc. Also microwave dielectric ceramics has received much attention due to rapid growth of the wireless communication industry. In this project we are concerned with the development of microwave resonator materials. The three vital properties required for these materials are:

1)      A high dielectric constant for miniaturization.

2)      A high quality factor (which is inverse of loss tangent, tanδ) for better frequency stability.

3)      A good temperature stability or near zero temperature coefficient of resonant frequency.

 Dielectric ceramics materials in ternary system BaO-R₂O₃-TiO₂ (R=Rare earth, La, Pr, Nd, Sm, Gd) has gained importance due to their high dielectric constants, high quality factor and near zero temperature coefficient of resonant frequency. The ceramics system with general formulas BaPr_1.33Ti₃O₉ & BaPr₂Ti₄O₁₂ has been synthesized by mixed oxide method using reagent grade powders BaCo_3, Pr₆O₁₁ and TiO_2.  The samples have been calcinated at 1100°C for 2 hrs in silica crucibles in a linear programmable furnace in order to bring about a thermal decomposition, phase transition and removal of a volatile fraction.

       Microwave dielectric properties can be adjusted widely by ionic or structural modification. The effect of such substitution on microstructures and microwave dielectric properties at room temperature has been investigated.  Various analytical tools are used for the study of structural and dielectric properties. The most common analytical tool for structural studies is crystal diffraction using x-rays that undergo wave interface with the regular arrangements of atoms in a crystalline lattice. This technique is x-ray powder diffraction (XRD) and can be used to give both qualitative and quantitative data for the crystal structure. It is expected that substitution of Pr in ceramics would result in promising material with high dielectric constant and low loss.

INTRODUCTION

1.1  Microwave dielectric ceramics:

Temperature-stable, medium-permittivity dielectric ceramics have been used as resonators in filters for microwave (MW) communications for several years. The growth of the mobile phone market in the 1990s led to extensive research and development in this area. The main driving forces were the greater utilization of available bandwidth, that necessitates extremely low dielectric loss (high-quality factor), an increase in permittivity so that smaller components could be fabricated, and, as ever in the commercial world, cost reduction. Over the last decade, a clear picture has emerged of the principal factors that influence MW properties. Proper selection of ceramics materials is necessary in order to achieve suitable dielectric properties. Dielectric ceramics can be selected from a wide range of materials. These materials include titanates, nitrates, tantalates, niobates, silicates, etc. such as – Alumina, Aluminium nitride, Barium Titanates (BaTiO₃), Barium Strontium Titanates, and Lithium Niobates.

The Barium Titanates (BaTiO₃) is used extensively in ceramics because of its high      dielectric constant compared with other ceramics. It is rarely used without modification, because difficulty in controlling grain size during sintering. BaTiO₃ is slightly doped with rare earth oxides (up to a concentration of around 0.3-0.5%) and is used for the preparation of switching and regulating devices. Ceramics in the vicinity of BaO: R₂O₃:4TiO₂ are used in microwave resonators. It will be beneficial to explain ceramics at this point.

1.2 CERAMICS:

1.2.1 Origin of ceramics

The word ceramic is derived from Greek, and strictly refers to clay in all its forms. However, modern usage of the term broadens the meaning to mean inorganic non-metallic materials. Until 1950s or so, the most important of these was the traditional clays, made into pottery, bricks, tiles and similar, along with cements and glass. The traditional crafts are described in the article on pottery. The classic ceramic materials are hard, porous and brittle. The study of ceramics consists to a large extent of methods to mitigate the problems, and accentuate the strengths of the materials, as well as to offer up unusual uses for these materials.

1.2.2 Examples of Ceramic Materials

• Silicon nitride (Si₃N₄), which is used as an abrasive powder.
• Boron carbide (B₄C), which is used in some helicopter and tank armor.
• Silicon carbide (SiC), which is used as a succeptor in microwave furnaces, a commonly used abrasive, and as a refractory material.
• Magnesium diboride (MgB₂), which is an Unconventional superconductor.
• Zinc oxide (ZnO), which is a semiconductor, and used in the construction of transistors.
• Ferrite (Fe₃O₄), which is ferromagnetic and is used in the core of electrical transformers and magnetic core memory.
• Steatite is used as an electrical insulator.
• Bricks (mostly aluminum silicates), used for construction.
• Uranium oxide (UO₂), used as fuel in nuclear reactors.
• Yttrium barium copper oxide (YBa₂Cu₃O_7-x),a high temperature superconductor

                                                              

1.3 Properties of Ceramics        0

1.3.1Mechanical  properties :

Ceramic materials are usually ionic or glassy materials. Both of these almost always fracture before any plastic deformation takes place, which results in poor toughness in these materials. Additionally, because these materials tend to be pours, the pores and other microscopic imperfections act as stress concentrators, decreasing the toughness yet further, and reducing the tensile strength. These combine to give catastrophic failures, as opposed to the normally much more gentle failure modes of metals.

 It is not true to say that these materials do not show plastic deformation. However, due to the rigid structure of the crystalline materials, there are very few available slip systems for dislocations to move, and so they deform very slowly. With the non-crystalline (glassy) materials, viscous flow is the dominant source of plastic deformation, and is also very slow. It is therefore neglected in many applications of ceramic materials. These materials have great strength under compression, and are capable of operating at elevated temperatures. Their high hardness makes them widely used as abrasives, and as cutting tips in tools.

1.3.2 Refractory behavior:

Some ceramic materials can withstand extremely high temperatures without losing their strength. These are called refractory materials. They generally have low thermal conductivities, and thus are used as thermal insulators. For example, the belly of the Space Shuttles are made of ceramic tiles which protect the spacecraft from the high temperatures caused during reentry. The most salient properties required for a good refractory material is that it not soften or melt, and that it remains unreactive at the desired temperature. This latter point covers both self decomposition, and reacting with other compounds in its vicinity - either of which would be detrimental.

Porosity takes on additional relevance with refractories. As the porosity is reduced, the strength, load bearing ability and environmental resistance decreases as the material gets denser. However as the density increases the resistance to thermal shock (cracking as a result of rapid temperature change) and insulation characteristics are reduced. Many materials are used in a very porous state, and it is not uncommon to find two materials used - a porous layer, with very good insulating properties, with a thin coat of a more dense material to provide strength. It is perhaps surprising to find that these materials can be used at temperatures where part of them is a liquid. For example, silica firebricks used to line steel making furnaces is used at temperatures up to 1650°C (3000°F), where some of the brick will be liquid. Designing for such a situation unsurprisingly requires a substantial degree of control over all aspects of construction and use.

1.3.3 Electrical behavior:

One of the largest areas of progress with ceramics was their application to electrical situations, where they can display a bewildering array of different properties.

1.3.3.1 Electrical insulation and dielectric behavior:

The majority of ceramic materials has no mobile charge carriers, and thus do not conduct electricity. When combined with strength, this leads to uses in power generation and transmission. Power lines are often supported from the pylons by porcelain discs, which are sufficiently insulating to cope with lightning strikes, and have the mechanical strength to hold the cables. A sub category of their insulating behavior is that of the dielectrics. A good dielectric will maintain the electric field across it, without inducting power loss. This is very important in the construction of capacitors. Ceramic dielectrics are used in two main areas. The first is the low-loss high frequency dielectrics, used in applications like microwave and radio transmitters. The other is the materials with high dielectric constants (the ferroelectrics). Whilst the ceramic dielectrics tend not to outmatch other options for most purpose, they fill these two niches very well.

 1.3.3.2 Ferroelectric, piezoelectric and pyroelectric:

A ferroelectric material is one the can spontaneously generate a polarization, in the absence of an electric field. These materials exhibit a permanent electric field, and this is the source of their extremely high dielectric constants.

A piezoelectric material is one where an electric field can be changed or generated by applying a stress to the material. These find a range of uses, principally as transducers - to convert a motion into an electric signal, or vice versa. These appear in devices such as microphones, ultrasound generators, strain gauges and many more.

A pyroelectric material develops an electrical field when heated. Some ceramic  pyroelectrics are so sensitive they can detect the temperature change caused by a person entering a room (approximately 40 micro Kelvin). Unfortunately, such devices lack accuracy, so they tend to be used in matched pairs - one covered, the other not, and only the difference between the two used.

1.3.4 Semi conductivity:

There are a number of ceramics that are semiconductors. Most of these are transition metal oxides that are II-VI semiconductors, such as zinc oxide. Whilst there is talk of making blue LEDs from zinc oxide, ceramicists are most interested in the electrical properties that show grain boundary effects. One of the most widely used of these is the transistor. These are devices that exhibit the unusual property of negative resistance. Once the voltage across the device reaches a certain threshold, there is a breakdown of the electrical structure in the vicinity of the grain boundaries, which results in its electrical resistance dropping from several mega-ohms down to a few hundred. The major advantage of these is that they can dissipate a lot of energy, and they self reset - after the voltage across the device drops below the threshold, its resistance returns to being high.

This makes them ideal for surge protection applications. As there is control over the threshold voltage and energy tolerance, they find themselves in all sorts of applications. The best demonstration of their ability can be found in electricity sub stations, where they are employed to protect the infrastructure from lightning strikes. They have rapid response, are low maintenance, and do not appreciable degrade from use, making them virtually ideal devices for this application. The semiconducting ceramics can also be found employed as gas sensors. When various gases are passed over a polycrystalline ceramic, its electrical resistance changes. With tuning to the possible gas mixtures, very inexpensive devices can be produced.

1.3.5 Superconductivity:

Under some conditions, such as extremely low temperature, some ceramics exhibit superconductivity. The exact reason for this is not known, but there are two major families of superconducting ceramics. The complex copper oxides are exemplified by Yttrium barium copper oxide, often abbreviated to YBCO, or 123 (after the ratio of metals in its stiochiometric formula YBa₂Cu₃O_7-x). It is particularly well known because it is quite easy to make, does not require any particularly dangerous materials to do so, and has a superconducting transition temperature of 90K (which is above the temperature of liquid nitrogen (77K)). The x in the formula refers to the fact that fully stiochiometric YBCO is not a superconductor, so it must be slightly oxygen deficient, with x typically around 0.3.

The other major family of superconducting ceramics is magnesium diboride. It is currently in a family all of its own, and its behavior is not particularly remarkable, other than its very different from all other superconductors (not being either a complex copper oxide, nor a metal), and thus it is hoped that studying it will reveal more information about why superconductivity exists, which will feed into high temperature superconductivity research.

1.4 Processing of Ceramic Materials :

Non-crystalline ceramics, being glasses, tend to be formed from melts. The glass is shaped when either fully molten, by casting, or when in a toffee like viscosity, by methods such as blowing to a mould.

Crystalline ceramic materials are not amenable to a great range of processing. Methods for dealing with them tend to fall into one of two categories - either make the ceramic in the desired shape, by reaction in situ, or by forming powders into the desired shape, and then sintering to form a solid body. A few methods use a hybrid between the two approaches.

1.4.1 In situ manufacturing:

The most common use of this type method is in the production of cement and concrete. Here, the dehydrated powders are mixed with water. This starts hydration reactions, which result in long, interlocking crystals forming around the aggregates. Over time, these result in a solid ceramic.

The biggest problem with this method is that most reactions are so fast that good mixing is not possible, which tends to prevent large scale construction. However, small scale systems can be made by deposition techniques, where the various materials are introduced above a substrate, and react and form the ceramic on the substrate. This borrows techniques from the semiconductor industry, such as chemical vapor deposition, and it very useful for coatings. These tend to produce very dense ceramics, but do so slowly.

1.4.2 Sintering based methods:

The principles of sintering based methods are simple. Once a roughly held together object is made (called a "green body"), it is baked in a kiln, where diffusion processes cause the green body to shrink, and close up the pores in it, resulting in a denser, stronger product. The firing is done at a temperature below the melting point of the ceramic. There is virtually always some porosity left, but the real win of this method is that the green body can be produced in any way imaginable, and still be sintered. This makes it a very versatile route.

There are thousands of possible refinements that can be added to this process. Some of the most common are pressing the green body, to give the densification a head start, and reduce the sintering time needed. Sometimes organic binders are added, to hold the green body together, which burn out during the firing. When pressing, something organic lubricants are added, to get maximum density from pressing. It's not uncommon to combine these, and add binders and lubricants to a powder, then press. Rather than a powder, a slurry can be used, and then cast into a desired shape, dried and then sintered. Indeed, the traditional pottery is done with this type of method, using a plastic mixture worked with the hands. If a mixture of different materials is used together in a ceramic, it sometimes is that the sintering temperature is above the melting point of one minor component - a liquid phase sintering. This results in faster sintering times over solid state sintering.

1.5   Applications of ceramics :

• Ceramics are used in the manufacture of knives. The blade of a ceramic knife will stay sharp for much longer than that of a steel knife, although it is more brittle and can be snapped by dropping it on a hard surface.

• Ceramics such as alumina and boron carbide have been used in ballistic armored vests to repel large-caliber rifle fire. Such plates are known commonly as small-arms protective inserts (SAPI). Similar material is used to protect cockpits of some military airplanes, because of the low weight of the material.

• Ceramic balls can be used to replace steel in ball bearings. Their higher hardness means that they are much less susceptible to wear and can offer more than triple lifetimes. They also deform less under load meaning they have less contact with the bearing retainer walls and can roll faster. In very high speed applications, heat from friction during rolling can cause problems for metal bearings; problems which are reduced by the use of ceramics. Ceramics are also more chemically resistant and can be used in wet environments where steel bearings would rust. The major drawback to using ceramics is a significantly higher cost. In many cases their electrically insulating properties may also be valuable in bearings.

• In the early 1980s, Toyota researched production of an adiabatic ceramic engine which can run at a temperature of over 6000°F (3300°C). Ceramic engines do not require a cooling system and hence allow a major weight reduction and therefore greater fuel efficiency. Fuel efficiency of the engine is also higher at high temperature, as shown by Carnot's theorem. In a conventional metallic engine, much of the energy released from the fuel must be dissipated as waste heat in order to prevent a meltdown of the metallic parts. Despite all of these desirable properties, such engines are not in production because the manufacturing of ceramic parts in the requisite precision and durability is difficult. Imperfection in the ceramic leads to cracks, which can lead to potentially dangerous equipment failure. Such engines are possible in laboratory settings, but mass-production is not feasible with current technology.

• Work is being done in developing ceramic parts for gas turbine engines. Currently, even blades made of advanced metal alloys used in the engines' hot section require cooling and careful limiting of operating temperatures. Turbine engines made with ceramics could operate more efficiently, giving aircraft greater range and payload for a set amount of fuel.

• Recently, there have been advances in ceramics which include bio-ceramics, such as dental implants and synthetic bones. Hydroxyapatite, the natural mineral component of bone, has been made synthetically from a number of biological and chemical sources and can be formed into ceramic materials. Orthopedic implants made from these materials bond readily to bone and other tissues in the body without rejection or inflammatory reactions. Because of this, they are of great interest for gene delivery and tissue engineering scaffolds. Most hydroxyapatite ceramics are very porous and lack mechanical strength and are used to coat metal orthopedic devices to aid in forming a bond to bone or as bone fillers. They are also used as fillers for orthopedic plastic screws to aid in reducing the inflammation and increase absorption of these plastic materials. Work is being done to make strong, fully dense nano crystalline hydroxyapatite ceramic materials for orthopedic weight bearing devices, replacing foreign metal and plastic orthopedic materials with a synthetic, but naturally occurring, bone mineral. Ultimately these ceramic materials may be used as bone replacements or with the incorporation of protein collagens, synthetic bones.

• High-tech ceramic is used in watch making for producing watch cases. The material is       valued by watchmakers for its light weight, scratch-resistance, durability and smooth touch. IWC is one of the brands that initiated the use of ceramic in watch making. The case of the IWC 2007 Top Gun edition of the Pilot's Watch Double chronograph is crafted in high-tech black ceramic. 

Next lecture will contain the discussion of dielectric properties. till then good bye..