Red Ferrite Technology-Dream or Reality A. Živič, Z. Živič Keywords: soft-ferrites, production, cost reduction, environment, pollution minimization, red ferrites, technology, high-permeabilta territes, MN-ZN ferrites, input raw materials, homogenization, powders, raw matenal preparation, electromagnetic properties, physical properties, compressibility, shrinkage, patent applications Abstract: Search tor the answer to the question, if good initial homogenization of row materials can substitute all classic ways of homogenization such as dry mixing, pelletizing, calcination and milling brought us to the revival of the so called 'red' ferrite technology. It has been proved that it can be successfully applied in high permeability ferrite production. Rdeča feritna tehnologija - sanje ali resničnost Kjučne besede: feriti mehki, proizvodnja, zmanjšanje stroškov, okolje, minimizacija onesnaženja, feriti rdeči, tehnologije, feriti visoko-permeabilnl, MN-ZN feriti, surovine vhodne, homogenizacija, prahovi, priprava surovin, lastnosti elektromagnetne, lastnosti fizikalne, stisljivost, skrčenje, prijave patentne Povzetek: Iskanje odgovora na vprašanje, če dobra začetna homogenizacija vhodnih surovin lahko zamenja vse klasične načine homogenizacije, kot so suho mešanje, peletiranje, kalcinacija in mletje, je naju pripeljalo do oživitve t i. rdeče' feritne tehnologije. Pokazano je, da jo je možno uspešno uporabiti v proizvodnji visoko-permeabilnih feritnih izdelkov. Introduction In order to be competitive on the nowadays soft ferrite market one has to fulfill certain general demands: electromagnetic properties must be in specified limits being more stringent and demanding from day to day; exactness and repeatability of core dimensions is a must; yield over all parameters must be high ensuring high quality products; products must have price as low as possible; delivery time must be exact and as short as possible. Powder prepration is a part of a ferrite core production which can crucially affect the fulfillment of most of the above mentioned demands. It has pronounced influence on all of these production process properties. There are two main branches of soft ferrite powder preparation technologies - conventional ceramic process (1-4) and wet chemical process (5-8). In the first one, dry oxide or carbonate powders are mixed, calcined, milled and dried after appropriate binder/lubricant addition. In the second one, salts such as chlorides or nitrates are soluted, coprecipitated, usually calcined aferwards, milled and dried after appropriate binder/lubricant addition. However, in both cases the general scheme is homogenization-calcination-milling-drying. Each of these technologies can be judged not only on the basis of above mentioned general requirements but on the basis of the requirements tied to some special aspects of powder preparation such as: process ability to lower or eliminate the influence of row materials; number of operations is to be minimum in order to have powerfüll line production control loop; ease of material rework and waste material recycling is desirable; pollution by eithertoxic agents ordust released in the proces is to be minimum; finally in the area of granulate properties - superior physical and chemical homogeneity is a must and powder compressibility should be minimum. New 'red' ferrite technoiogy objectives and set up All these demands are not easily accomplished. Compromises are usualy made between high magnetic properties and quality on the one side and costs, simplicity and delivery time on the other. The question is: Is it possible to set up a ferrrite powder preparation technology that produces high quality products that are cheap, produces high quality products in a short time, produces high quality products in a simple and elegant way? There is a technology which is simple enough and cheap enough, which could be the starting point for answering that question. It is so called 'red' technology. To our knowledge almost everything about that technology was denoted as poor (2): 1. poor electromagnetic properties, 2. poor dimension exactness, 3. high shrinkage. 4. impossibility to produce complicated shapes, 5. high granulate compressibility, 6. sintering equipment corrosion. On the basis of all these statements it would never occur to anyone to even try to use this technology in high permeability ferrite production. However, this is exactly what was done and presented in this paper. Why? In the sixties, when conventional ceramic process was developed, based on the metal oxide and carbonate production development, the quality of the iron oxide was rather poor. The impurity content of both anion and cation impurities was rather high. Specific surface area (SSA) was not reproducible, as shown in Table 1. Organic additives were not such as to provide low granulate compressibility for the powders of the average particle size < 0.5 |.im. Calcination was the the only logical answer to these problems. Table 1. History of raw material properties - calcination conditions relation Properties of Time period Fe203 1960-1970 1970-1980 1980-1990 1990- trend PSSAT' ±2 ±2 ± 1 ±0.5 ±0.5 Purity (%) 97-98 99.0-99.5 99.1 -99.4 99.4-99.7 99.96-99.995 Anion content (%) 1 -2 0.1 -0.6 0.1-0.6 0.10-0.40 0.005-0.09 Calcination Temperature Cc) 900-1150 900-1150 900-1100 1000-1100 750 Duration 2-4h 2-4h 30 min < 1 min 15s Kiln type Milling time (h) tunnel tunnel rotational spray spray [ M 1 12-24 12-24 3-12 »1 'PSSAT - particle specific surface area tolerance What happens during calcination? Release of all volatile products of red-ox reactions such as CO2, CI, SO3. The calcinate is free from corrosive gases and all other volatile compounds that could cause cracks during sintering. Through the reaction of sintering and the reaction of spinel crystal structure formation all physical and chemical parameters are homogenized, assuring uniform microstructure and composition during sintering. Due to hihger calcinate particle dimensions, granulate compressibilities approach user-friendly values of < 200 MPs. Is there something unfavorable about calcination? Yes, it is the fact that it must be followed by milling, in order to recover calcinate reactivity by decreasing its particle size and increasing its specific surface area. What is wrong about milling? Milling introduces composition change through the iron pick-up, due to the steel ball abrasion. The quantity of the iron pick-up changes in time as the intensity of steel ball abrasion increases. The iron pick-up can be as high as 1.3 wt % (10). Another effect of abrasion is the change of steel ball dimensions and eventually shapes, which causes the powder physical parameters not to be repeatable in time. It is very well known how deteriorating the change of composition and powder physical properties affects ferrite electromagnetic properties. So, shortly speaking, the shortcomings of conventional ceramic ferrite powder preparation include limitations on compositional control (3,10), incomplete chemical homogenization, introduction of impurities from milling, relatively coarse particles leading to pressed bodies with large and inhomogenous porosity. In order to surpass these problems and answer to constanly higher demands regarding electromagnetic properties of ferrite materials novel powder preparation techniques (3-8) were developed as well as clean raw materials. The tendency of lowering calcination temperatures and decreasing calcination time is evident in Table 1. The direct consequence of this are shorter milling times, partially eliminating its negative effects. The challenge we wanted to face is to produce high permeability ferrite material with the nowadays commercial row materials by means of the simplest, shortest and cheapest process ever known. This process avoids the problems present in all technologies which incorporate calcination and subsequent milling. The main objectives of this process are to achieve sufficient composition homogeneity during the first wet mixing step, leaving spinel crystal lattice formation, grain growth and densi-fication to sintering step and transferring everything else to the raw material and binder system level. Flow chart of the new 'red' ferrite technology, we used to produce high permeability ferrite cores is presented in Fig. 1. As evident, it follows the old 'red' ferrite processing, with the special attention paid to wet mixing optimization and the choice choice and content of organic additions (9). Sintering is another step that should be especially adjusted, due to its additional function. In this paper sintering was not adjusted to these requirements. Standard sintering Fig. 1: Flow chart of the new 'red' ferrite process procedure normally used In our production facilities has been used,leaving free space for future improvements. Experimental procedure The starting materials of commercial purity were spray roasted iron, manganese oxide and zinc oxide. Their properties are given in the Table 2.. Ferrite composition to be realized was MnOo.49ZnOo.43Fe2.o7304. The dry premixed starting materials are suspended in water and underwent technically superior wet-mixing step. At this stage the appropriate binder/plasticizer combination (9) was added. The slurry was then spray dried. Powder compressibility was measured as a function of a green density. Table 2. Raw material properties Raw material Particle shape Particle size (jim) SSA (m^/g) Purity (%) Anion content (%) Fe203 spherical 0.25 ±0.05 3.5 ±0.5 >99.4 <0.100 1 Mn304 spherical 0.07 + 0.01 15-20 >707 <0.027 ZnO spherical 0.50 + 0.10 4-8 >99.9 <0.001 To study magnetic properties ring cores of different dimensions (FT 36/23 20, FT26/1420, FT 26/14 10, FT 22/14 07, FT 10/06 04 and test toroid FT 26/17 06) and cores RM 4 and RM 8 were dry pressed to a green density of 3.00 g/cm^. The samples were heated to 300°C to remove binder. Sintering was carried out in a tunnel and chamber kiln at 1350°C for about 9 h in air. In a tunnel kiln sintering was followed by cooling maintaining the stochiometry achieved by 90 - 99 % disintegration of excess iron (11). In a chambre kiln sintering was followed by a stabilization at 1300°C for 5 h in a 0.1 % O2 + N2 atmosphere maintaining stochiometry achieved by 90 - 99 % disintegration of excess iron and after that by cooling. Small ring cores sintered in a chambre kiln were buried in raw ferrite powder of the same composition. After sintering, RM cores were grinded and polished to improve the quality of mating surfaces. Ring cores were subjected to initial permeability - ^i, loss factor value - tg5/pi, hysteresis material constant - "B, disaccommodation factor - Df and n - T dependence measurements. Besides magnetic properties, dimension shrinkage dependence of ring cores on their green density was recorded as well as RM core shrinkage and worpage. Results and discussion Fig. 2 shows the dependence of red ferrite powder compressibility on green body density. It is evident that 220 200 100 CL 160 i 140 !5 i 120 Um a 100- E 0 0 80 GO- 40- Fig. 2: Z56 ze6 2.76 2.65 £95 3.a5 3.16 3 25 Green body density (g/cm3) Powder compressibility dependent on green body density green densities in the range of 3.00 ± 0.05 g/cm^ can be achieved with the pressures well below 200 MPa, which is considered to be the user friendly limit for the dry pressing. Without that condition 'red' ferrite powder would not be considered usable. We suppose that one of the main reasons for such a low compressibility is spherical particle shape of the used raw materials, making the fact that their dimensions are well below 1 |.im unimportant. Proper choice and content of binder/plasti-cizer/lubricant combination is the second one. Low 'red' powder compressibility obtained by new 'red' process discards the 5th of the 'poor' statements about red ferrite processing. With different sintering procedures we achieved different high-permeability levels such as 4600 ± 20 %, 6000 ± 20 % and 10000 ± 30 %. Typically achieved values of tg5/n @ 100 kHz are 12E-6 for permeability levels of 4300 and 6000 and < 30E-6 for permeability level of 10000. Hysteresis losses are < 1.25E-3/T for the permeability levels of 4300 and 6000 and < 0.8E-3/T for the permeability level of 10000. Disaccommodation factor is < 3.6E-6 for the permeability levels of 4600 and 6000 and <0.2E-6 for the permeability level of 10000. Fig. 3. shows respective ring core permeability - temperature dependence. Electromagnetic properties of RM cores are given in the Table 3. It is evident that all parameters both of ring and RM cores are in the world- wide accepted limits. So the 1st of the 'poor' statements is discarded. At this point we can answer the question given at the very beginning of this paper, if good initial homoge-nization of row materials can substitute all classic ways of homogenization such as dry mixing, pelletizing, calcination and milling. The answer is positive. We expect even better results after adjusting the sintering to the noncalcined powder requirements. Achieved dimension exactness and its repeatability is illustrated in Fig.4., where the largest dimension q of RM 8 core was measured at the bottom and at the top of the ü rt = TJ i3 C E g t- C. t O. -25 -16 15 25 35 46 55 65 75 B5 05 106 115 Temperalura (°C) Fig. 3: Permeability dependence on temperature for different ring cores RM core magentlc properties Core of pormonblllty level RM 04 4600 10000 4600 100 )8 00 Parameter Catalofluo data New process data Catalogue data New proc««» data Catalogue data New procesa data Catalogue data New process data A! (nH) +Ä) 1700 -20 +20 1900 -20 + 30 3690 -20 +20 3700 -20 4-30 5700 -20 + 20 6320 -20 + 30 12500 -30 +20 10000 -20 MBff 2610 2900 BOOO 5030 3200 3400 5870 5600 fj5/ii@100 kHz(E-6) 14 14 14 30 '^B (E-3/T) 1.16 1.19 1.12 0.96 Table 3: Electromagnetic properties of RM cores for 19G, 22G and 12G quality Top-hystogr. BoOom-hysL Bc«to(n-dl«r. -Requirement Top -distrlbut 0,00 0.0« 0.12 0.1« 0.24 O.W Ü.J6 0.42 0.4« Difference q(bottotn)-q(top) (mm) £=3 dtff.-hyst ..... dlff.-dlstr. upper limit Fig. 4: Dimension exactness and repeatability RM 8 length -q Fig. 5: Warpage of RM 8 outer walls; measurement of RM 8 length -q CRITERIA TECHNOLOGY New 'red' (9) Conventional dry mlxing(1) Conventional wet mixing(2) Spray firing _M Co-spray roasting (5,6) 'oprecipitation (7,8) RAW MATERIAL INFLUENCE Particio phjrBioal propertlps Cutlona m Aniona PROCESSING Number of procoosing atops Ö-7 Lino production control loop O Yield of toxic agorrts Recycling of wasrte materials O Rework poaalbKIty R^uctlon of duat Addaptöllon wilh Gonv. forrlto prod, POWDER Homogeneity physical chemical O O O o o o Workobillty CORE PROPERTIES COST O more, better O o favotsblö less, wor^ o o unfövordbtö Table 4: Comparison of new 'red' ferrite processing with the other ones RM outer walls on arbitrarily chosen 80 out of the series of 3000 pieces. It is obvious that dimension exactness is satisfactory, process capability measure Cp being >1.33 for both of the measuring points. This proof discards the 2nd of the 'poor' statements. Warpage of complicated ferrite cores might be the reason why complicated cores can not be successfully produced as stated in the 4th of the 'poor' statements. One of the ways to estimate the warpage is to measure the difference of the largest Rf\yi dimension q at the bottom and at the top of the RM outer wall. The frequency distribution of this difference is shown in Fig. 5. The average difference is 0.18 mm being sufficiently below the tolerance of 0.45 mm. Such an extent of warpage is usual with standard calcined ferrite RM cores. The 4th of the 'poor' statements is discarded as well. Finally, Fig. 6 presents the dependence of ring core dimension shrinkage on its green density. Shrinkage of the ring core height is 15.0 ± 0.5 %, while shrinkage of its outer and inner diameter is 16 ± 0.5 % for the green density in the interval 3.00 ± 0.05 g/cm^. This shrinkage is exactly the same as the one corresponding to the calcined ferrite powders with approximately equal binder content, being sintered under approximately the same conditions. The 3rd of the 'poor' statements is discarded. The 6th of the 'poor' statements concerns sintering equipment corrosion. This one holds, but let us see to i»«. IS 18 17 t> O) « 16 C £ 15 (O 14 13 -1-1-1-1-r-i-1-1-1-1-1- 2.55 2.66 2.75 2.85 2.BS 3.05 3.15 Green body density (g/cm3) O. D. I.D. Heloht Fig. 6: Shrinkage as a function of a toridal green body density what extent and if that problem can be solved. Iron oxide we use incorporates CI". According to U. Wagner (12) chlorides completely evaporate in the temperature range from 400 - 1000°C. This means that probable corrosion problem can be solved on the fumace hardware level in that range of temperatures, using corrosion resistant and gas tight ceramic insullation, stainless steel exaust piping and finally corrosion resistant heating elements, all available in the market. Using higher purity materials for reliable production of ferrite materials of permeabilities > 10000 the 6th of the 'poor' statements practically disappears. Let us consider other aspects of the new 'red' technology - the economy and pollution related aspects. Even a glance thrown on the new 'red' process flow chart reveals the following. It is the shortest, the simplest, the cheapest and the best controlable ferrite core processing ever used. Dust pollution is minimal. Comparison of the new 'red' ferrite powder preparation technology with the other ones taking into consideration all of the criteria given in the Introduction is given in Table 4. Judgement of other techologies has already been given by U, Wagner (3). to be nonexistent, such as poor electromagnetic properties, high shrinkage, high powder compressibility etc, or easily solvable such as corrosion. New 'red' ferrite processing is the simplest, the shortest and the cheapest ferrite production technology ever known. Moreover, this technology overcomes classical shortcommings of the conventional ceramic ferrite technologies. Are we entering the new era of 'red' ferrite processing? The time will show whether this orientation is the right one ! References {1) M. Paulus, "Effect of Homogeneity on Sintering and the Process to Improve it",Sei, of Sintering, Vol. 12, No. 1, (1980), pp.25-38 (2) M. Limpel, "The Review of Ferrite Powder Preparation Technologies", private communication, Ljubljana, Feb. 1992 (3) U, Wagner, "Spray Firing for Preparation of Presintered Powder for Soft Ferrites", J. Magn. & tvlag. Mat. 19, (1980), pp. 99-104 (4) M. J. Ruthner, "Fast Reaction Sintenng Process for the Production of Ferntes", Colloque CI, Suppi. 4, Tome 38, (1977), pp. C1-311 (5) K. OI