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Ferrites are ceramic materials that have magnetic properties. They may be defined as magnetic materials composed of oxides containing ferric ions as the main constituent (the word ferrite comes from the Latin “ferrum,” for iron).1 Ferrites are generally categorized as hard or soft, depending on their magnetic properties—i.e., ferrites with permanent magnetization versus nonpermanent magnetization, respectively.
The majority of soft ferrites are based on the material groups MnZn and NiZn. These spinel-type structures combine two important features, namely a high magnetic permeability and a high electrical resistivity. Magnetic permeability is a measure of how well a magnetic field flows in a specific material, compared to air. Electrical resistivity defines the amount of current flowing through a specific material. Major applications for ferrites include transformers and inductors used in telecommunications, power conversion and interference suppression. Much of the ferrite-related research took place after the 1950s, thanks to a technology expansion in the fields of radio, television, telephony and computers.
Ferrite ManufacturingThe ferrite manufacturing process takes place in several stages, similar to those used in the manufacturing of other ceramics. These main stages are summarized in Figure 1. Since the properties of ferrites are structure-sensitive, it is important to obtain optimum manufacturing conditions during the entire process.
The ferrite manufacturing process begins with the weighing and mixing of the various starting materials. These materials are usually oxides such as Fe2O3 and ZnO, but carbonates may also be used. The mixing step is carried out to achieve a homogeneous mixture. The mixed components are then pre-fired, or calcined, at a temperature range of 1000°C. Calcining decomposes the carbonates, thus reducing the evolution of gases during the final sintering stage. Calcining also contributes to a further homogenization of the mixture and controls shrinkage during the final sintering stage. At this stage, the oxides form a spinel structure through a solid-state reaction.
After being calcined, the powder is usually wet-milled. The particle size distribution is reduced and becomes more uniform. The suitable particle size distribution improves grain growth during sintering and is chosen, among others, according to the end application. During the milling process, additives are added to the slurry. Additives function as binders, lubricants or, as in the case of niobium, to enhance certain magnetic and electrical properties. Prior to forming, the mixture is granulated by spray drying, improving the powder's flow properties. The forming process is usually carried out by dry pressing or extrusion. Once the powder has been compacted into the required shape, it is sintered in a controlled atmosphere at a temperature range of 1100-1300°C.
The Role of AdditivesThe practice of incorporating additives with ferrites was not always commonly exercised. In fact, before 1940, it was thought that all magnetic substances should be made of materials with the highest possible purity grade, i.e., without any additives. Around 1950, Takei and Sugimoto discovered that small amounts of dopants could significantly improve the magnetic properties of ferrites by affecting the grain growth and grain density.2. Since then, numerous studies have shown that properties such as power loss, electric resistance, grain growth and permeability can be improved by the addition of dopants, such as CaO, SiO2, Ta2O5, Nb2O5 and TiO2.
Advantages and Properties of Niobium OxalateAdvantages
One of the additives used in ferrites is niobium oxide. The advantages of its use in ferrite devices, as well as in other ceramic components, have been reported in numerous publications. Accordingly, small amounts of Nb2O5 (0.01-0.08 wt%) in transformer Mn-Zn based ferrites reduce power loss in the high frequency range. The reasons are not fully understood; the addition of Nb oxide perhaps impacts the magnetic properties by altering the grain boundaries;3 however, only high purity Nb-compounds can be used to achieve such an effect.
In another example, Nb-oxide-containing ferrites, which are used as cores for line filters, have shown an improved permeability and a superior frequency dependency at frequencies of 100 to 500 kHz.4
According to TDK Corp., 5 power loss and high frequency performance have been improved in Nb-containing ferrites of the Mn-Zn system. As a result, the size of power supply transformers can be reduced, and they can be used over a wider temperature range.
Niobium doping has been reported to alter electrical properties in non-ferrite applications as well, such as in the case of facilitating domain switching in PZT films, 6 or by affecting the permittivity of Nb-doped bismuth titanate. 7 The role of oxygen mobility in connection with niobium seems to play an important role.
Studying these examples reveals that the amount of Nb added is typically small. It is also apparent that the full effectiveness of Nb-doping usually takes place within a narrow composition window. Therefore, it is important to distribute the Nb-additive finely and evenly in the ceramic slurry. Otherwise, islands of various niobium concentrations may evolve, thus distorting the magnetic properties of the device.
The significance of a fine and homogeneous distribution grows as the ferrite device becomes smaller in size. One way to improve the Nb-dispersion within the ceramic slurry is by using a soluble form of niobium, which has superior blending characteristics. Nb-oxalate is a soluble form of niobium, and because it is water-soluble, it offers additional advantages:
- It eliminates the need for organic solvents.
- Since it is stable against hydrolysis, inert gas techniques are not required.
- It is easy to handle.
Niobium oxalate is a white, water-soluble compound containing ammonia and crystal water, and it is stable in air (see Figure 2). Compared to other soluble niobium sources, it is easy to handle and store, as it requires no protecting atmosphere (to prevent hydrolysis), and HCl evolution does not have to be taken into consideration (as in the case of NbCl5).The water solubility of Nb-oxalate is temperature-dependant (see Figure 3). At room temperature the solubility is >45 g Nb/l (grams niobium per 1 liter water), and it increases to more than 150 g Nb/l above 70°C. The solubility can be increased by adding oxalic acid.8 Keeping the solution at a temperature of above 80°C may result in hydrolysis and the clouding of the solution.
Niobium oxide complexes in aqueous oxalic acid solutions show equilibrium between two ionic species, containing two or three oxalate groups. This equilibrium can be shifted by changing the pH-value and the oxalic acid concentration.9 When the pH value is increased by adding ammonia, hydrolysis begins. At a pH value of 5, niobium hydroxide precipitates. A quantitative precipitation of niobium hydroxide can be performed at a pH value above 6.Heating niobium oxalate in an oxidizing atmosphere will lead to its decomposition and to the formation of Nb-pentoxide. X-ray diffraction studies show that the oxalate peaks disappear at above 300°C (see Figure 4). At 400°C the powder is amorphous, and at 600°C Nb2O5 peaks appear.
Additional Nb-Oxalate ApplicationsNb-oxalate is not limited to the production of ferrites. It can also be applied in the production of catalysts and dielectrics.
Catalysts. Due to its acidic and oxidizing properties, niobium oxide can enhance various catalytic reactions such as selective oxidation, hydrocarbon conversion and carbon monoxide hydrogenation. Examples include the ammoxidation of propane to acrylonitrile;10 the reaction of oxiranes with carboxylic acids;11 the formation of mesitylene from acetone;12 and reactions such as hydration, alkylation and esterification.13 Similar to the case of ferrites, the use of Nb-oxalate allows the preparation of homogeneous mixtures on an atomic scale.
Dielectrics. By using niobium oxalate in the production of dielectrics, sintering temperature can be lowered significantly.14 Barium titanate ceramics for multilayer ceramic capacitors (MLCCs) can be coated with niobium oxide to reach X7R-temperature characteristics.15
SummaryFerrite materials are used in a wide range of applications. Thanks to their electro-magnetic properties they are used as transformers and inductors in the fields of telecommunications, power conversion and interference suppression. Adding niobium during the manufacturing process has been reported to reduce power loss in ferrites at high frequencies and to increase the permeability over a wide range of frequencies. A fine and homogeneous dispersion of Nb ions within the ferrite body is an important key to improved properties. Thanks to the superior blending characteristics of solutions, soluble Nb-oxalate can contribute to material homogeneity, especially in the production of miniaturized devices. Since Nb-oxalate is water-soluble, it offers additional advantages such as a relatively easy handling and the minimization of hazardous risks. Nb-oxalate can also be used as an additive in the production of catalysts and dielectrics.
For More InformationFor more information about the use of Nb-oxalate in ferrites and other ceramic materials, contact H.C. Starck Inc., P.O. Box 25 40, D-38615 Goslar, Germany; (49) 5321-751145; fax (49) 5321-751193; e-mail Bettina.firstname.lastname@example.org (U.S. contact: Jonathan Margalit, 45 Industrial Place, Newton, MA 02461; 617-630-4879; email@example.com).