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Ивашутенко Александр Сергеевич
Ivashutenko Aleksandr Sergeevich Национальный исследовательский Томский политехнический университет
National Research Tomsk Polytechnic University Заведующий кафедрой «Электромеханические комплексы и материалы» Head of Electromechanical Facilities and Materials Department
к.т.н.
E-Mail: [email protected]
Анненков Юрий Михайлович
Annenkov Yuri Mikhailovich Национальный исследовательский Томский политехнический университет
National Research Tomsk Polytechnic University Профессор кафедры «Электромеханических комплексов и материалов» Professor of Electromechanical Facilities and Materials Department
д.ф.-м.н./профессор
E-Mail: [email protected]
Сивков Александр Анатольевич
Sivkov Aleksandr Anatolievich Национальный исследовательский Томский политехнический университет
National Research Tomsk Polytechnic University Профессор кафедры «Электроснабжение промышленных предприятий» Professor of Industrial Electric Power Supply Department
д.т.н./профессор E-Mail: [email protected]
Double-action magnetic-impulse compaction of oxide nanoceramics
Двустороннее магнитно-импульсное прессование оксидной нанокерамики
The Abstract: The technology of two-sided magnetic pulsed compaction is offered as an effective method of obtaining high-density powder compacts (up to 78% of the theoretical density). The high level of density of compacts is provided by two-sided pulsed application of pressure stimulating the process of dispersion of powder and the generation of excess defects as well as adiabatic heating of the pressed mass.
Keywords: Double-action magnetic-impuls press; Al2O3; Nanomaterial.
Аннотация: Технология двустороннего магнитно-импульсного прессования
предлагается в качестве эффективного метода получения высокой плотности порошковых компактов (до 78 % от теоретической плотности). Высокий уровень плотности у компактов обеспечивается двустороннем приложением импульсного давления, стимулирующего процессы диспергирования порошков и генерации в них избыточных дефектов, а также адиабатическим нагревом при прессовании порошковых масс.
Ключевые слова: Двусторонний магнитно-импульсный пресс; Al2O3; наноматериалы.
Главный редактор - д.э.н., профессор К.А. Кирсанов тел. для справок: +7 (925) 853-04-57 (с 1100 - до 1800) Опубликовать статью в журнале - http://publ.naukovedenie.ru
1. Introduction
According to present research trends in solid state physics nanostructured materials and nanoceramics, in particular, are currently of interest. The development in this direction can only proceed when technological problems in nanostructured material production are solved.
In order to obtain dense nanoceramics three basic conditions have to be complied with:
- to use starting material in nanodisperse state;
- to develop efficient compacting methods providing maximum density of bulk material;
- to implement new methods of compact activated sintering with suppression of the recrystallization process.
The present paper is devoted to realization of the second condition which provides effective compaction and powder activation. Dynamic pressing methods are considered to be the most perspective for nanostructured ceramics production by virtue of the following advantages:
- ability to develop extremely high compacting pressure [1];
- high performance in overcoming adhesive forces between powder particles that considerably decreases pressing energy losses for internal friction and increases pressing quality [1];
- intensive heating of powder due to adiabatic character of pressing that positively affects many stages of pressing [2];
- powder dispersion and activation under impulse pressing can cause compression and considerable temperature reduction of compacts sintering which is required for high-quality nanoceramics production.
All advantages of dynamic compaction given above are typical for magnetic-impulse (MI) pressing. The members of the Institute of Electrophysics, Russian Academy of Science, the Urals Division, have made a great contribution to the development of the presented method [2, 3]. This article presents current results of work on further progress in nanopowder MI-compaction. In particular, a double-action MI-press has been developed and applied [4]. The main advantage of the double-action MI-press in comparison with a single-action press lies in doubling of the compaction pressure and the achievement of high density compacts. One of the aims of the present article is to prove the advantageous character of powder masses counterflow in the mould.
2. Experimental Procedure
2.1. Construction of a double-action magnetic-impulse press (MI-press)
The structure and the mode of operation of a double-action MI-press is shown on Fig. 1. The inductors (6) are flat spirals made of copper strips which strands are isolated from each other and securely fixed. The inductors are connected across a circuit of a storage capacitor C with a capacity of 6 mF. The concentrators (4) are made of instrument steel and serve for transfer of the mechanical force to the punches (3). Inbetween the inductors and concentrators there is a set copper disks, i.e. satellites (5), which provide a high level of eddy currents and consequently intensive compaction forces.
Главный редактор - д.э.н., профессор К.А. Кирсанов тел. для справок: +7 (925) 853-04-57 (с 1100 - до 1800) Опубликовать статью в журнале - http://publ.naukovedenie.ru
Fig. 1. Schematic diagram of double-action MI-press: 1-powder, 2-press-tool, 3-punches, 4-
concentrators, 5-satellites, 6-inductors
When the contacts of the switch K are closed, the capacitor bank C discharges into the inductors causing a pulsed current flow. This current generates an intense magnetic field which induces currents in the satellites opposite to those in the inductors (directions of currents are shown on the figure by means of dots and crosses). The interaction of the pulsed magnetic fields of the inductor and the satellite generates pulsed forces F which put pressure onto the concentrators. Due to the mechanical connection of the concentrators with the punches the compaction pressures produced in the press-tool and can reach several GPa.
2.2. Determination of compaction pressures in the MI-press
The registration of pressure is of highest importance both during modes completion and during execution of basic technological compaction operations. That is why the used technique of compaction pressure determination must be strongly reliable.
Measuring of high dynamic pressures by standard methods such as tensometry and piezometry in particularly in case of small dimensions, usually cannot be realized due to some technical difficulties. In spite of all their virtues resistance strain gages based on metal alloys which are widely spread in measuring technology have one common disadvantage, i.e. a relatively low strain-sensitivity rate that limits the practical application of these devices [5]. Strain gages on piezocrystals used for measurement of static and impulse pressures in spite of their high sensitivity have one serious disadvantage. Upon impulse pressures applied to one of the piezocrystals faces an acoustic wave is going through the crystal and is back reflected from the opposite face. This leads to a strong distortion of the signal due to overlapping of incident and reflected waves.
In view of the above statements for secure measurement of pressure at least two independent methods are used. In the present paper in addition to the standard method of resistive tensometry an impulse pressure measurement method was developed which is based on the estimation of the punch movement velocity during MI-compaction. The idea of this method is based on the estimation of the
velocity of solid bodies [6]. From our point of view the method presented below is a feasible measuring technique for dynamic compaction.
Determination of punch movement velocity under the influence of magnetic field was done by means of an apparatus which scheme is presented in Fig. 2.
Fig. 2. Scheme of Ml-press for punch movement velocity measurement 1 - external cylinder, 2 - internal cylinder, 3 - punch, 4 - concentrator, 5 - satellite, 6 - inductor, 7 - clamp bolts, 8 - oscillograph, 9 - back plate, 10 - LED, 11 - photodiode
The given device represents a part of a single-action MI-press where the press-tool is substituted for a measuring cell consisting of a external cylinder, internal cylinder and two photoelectric barriers. When a current impulse flows through the inductor (6), then the satellite (5), concentrator and punch (4) begin moving under the influence of the force F. The internal cylinder (2) retards the concentrator and the satellite from moving. The light fluxes in the photoelectric barriers are recorded by an oscillograph while the punch block is passing by. This allows an experimental determination of the punch traveling time between both photoelectric barriers and therefore the determination of the punch velocity.
Fig. 3 shows a typical response of the measuring circuits during the punch traveling between the photo sensors for an inductor current of 80 kA. The punch velocity estimated on the basis of this oscillogram was 7.9 m/s.
'*♦' «Л Л л X.f' J.
****** » *******
\ Г \J
100 mks i------1
Fig. 3. Oscillogram of signals from optical pairs in punch moving
The reliability of the obtained data was estimated by means of the widely known velocity determination technique. If a body performs a free fall from the height h the final velocity v can be
calculated by the formula v = 2gh , where g is the acceleration of gravity. The analogous punch
velocity was registered when it was thrown into the measuring cell from 3 meters.
By this means the proposed technique to measure the punch velocity could be calibrated and therefore gives reliable values which can be used to determine pressures in the Ml-press as follows.
With the help of the apparatus presented on Fig.2 experiments have been conducted in order to define the punch velocity under various discharge currents. The pressure P in a single-action compaction was estimated in accordance with the law of conservation of momentum where the final expression is the following:
P=m Vs t •
where m is the mass of a system “punch - concentrator - satellite”, t is time of the magnetic force F effect, v is the punch velocity, S is the cross-sectional area of the punch. The effective time t of the magnetic force which was determined from oscillograms of the discharge currents was 300 p,s.
The determination of the pressures developing during a double-action Ml-compaction is done in the same way as in case of the single-action mode, but considering a doubling of the kinetic energy since in this process two identical systems “punch - concentrator - satellite” are involved which are moving towards each other. A summary of the measured pressure levels achieved a different discharge currents is presented in Table 1 for single-action and double-action MI-compaction It can be seen that in case of double-action MI-compaction the pressures are practically two times stronger than in case of the single-action mode. This can be explained by a doubling of the moving mass in case of double-action mode.
In order to examine the accuracy of pressure measurements there have been done measurements by means of a resistance strain gage based on the technique given in reference [5]. At a discharge current of 150 kA in the circuit the pressure received was 1.2 GPa what is in accordance with the data presented in Table.
A I
\ I
\
Table
Dependence of MI-compaction pressure from discharge current of MI-press in various modes
Discharge current, kA
Mode 80 100 124 150
Compaction pressure, MPa
Single-action MI-compaction 190 320 430 590
Double-action 390 650 860 1200
MI-compaction
Thus, the proposed technique provides reliable results for the measurement of pressure pulses and can be applied in research and practical work on MI-compaction of powder masses.
3. Experimental results and discussions
For compaction experiments alumina nanopowders were used obtained by plasmochemical methods [7]. This powder synthesis technology is based on the thermal decomposition of an aqueous solution of the corresponding metal salt in a high-frequency discharge plasma. About 15-20% of the so produced powder particles consist of hollow spheroids. The shell of spheres has polycrystalline
structure. The average crystallite size is 20-30 nm. Under the influence of weak adhesive forces
crystallites agglomerate into particles with 50 to 1000 nm in size [8].
Alumina powder compaction has been done in a single-action and a double-action MI-
presses. The density of the compacts was determined using the Archimedes Principle.
Fig. 4 illustrates the obtained densities with alumina powder as a function of the applied pressure, in comparison with corresponding data found in literature for single-action MI-pressing [2] and static compaction [9].
3,5
E 2S
bJJ
2
Сfl
С
CD
Q
1.5
1
—f
2 3
1 ^ 4
tX
0
0,4 0,8 1,2 1,6 2
Compaction pressure, GPa
Fig. 4 Dependence of AI2O3 compacts density from compaction pressure for various pressing modes and techniques: 1 - single-action MI, 2 - double-action MI, 3 - single-action MI [2], 4 -static pressing [9]
A comparison of the various compaction methods allows the following conclusions:
1. Dynamic methods of compaction provide higher compact densities as compared to static
ones.
2. Literature data for densities of alumina compacts obtained by means of a single-action MI-press [2] coincide with the data of the presented investigations in the pressure range of about 0,6-0,8 GPa. The observed deviation from a linear pressure dependence at pressure levels below 0,6 GPa can be explained by the following. The given Ml-presses have been designed for production of disk shaped compacts with 5 mm in diameter. In this case, when the compact sizes are rather small, the influence of friction caused by interaction of powder particles with the die is increasing. In order to overcome such lateral friction forces it is necessary to use 30-40% of axial pressure. But this decreases the compaction efficiency in particular at low pressures which results in a deviation from linear dependence “compact density - compaction pressure” as it is shown in Fig. 4. At the same time this effect becomes stronger with decreasing compact diameters [10].
3. The double-action MI-compaction provides maximum green densities as compared with other compaction techniques. As calculations show, by this compaction method energy of about 1 eV in average is transferred to each powder particle at the moment of collision. Under the influence of such excitation agglomerates are destructed, some particles are crushed and point and linear imperfections appear. All this consequently leads to powder activation. Such activation effects become even stronger due to adiabatic heating during impulse compaction. These processes are followed by the growth of compacts density. Pressure doubling in double-action compaction leads to considerable growth of compacts density as presented in Fig. 4. Thus the double-action MI-compaction technique provides considerable activation of the powder that positively affects the quality of compaction and the subsequent sintering of nanoceramics.
4. Conclusions
The present paper is devoted to the construction of a double-action MI-press and its application in nanoceramics production technology.
A new technique was proposed for measurement of impulse compaction pressure based on the determination of the punch velocity under MI-compaction. The given technique possesses good metrological characteristics and is easy to use.
The main advantage of double-action MI-compaction in comparison with single-action MI-compaction lies in doubling of the compaction pressures and in the ability to gain high density compacts at low pressures. This effect is explained by dispersion processes going on due to opposite movement of powder environments as well as by adiabatic heating of the compacted masses. Furthermore during the compaction process additional imperfections are generated that also cause compacts density growth and will later lead to a reduction of sintering temperatures.
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Рецензент: Гынгазов Сергей Анатольевич, доктор технических наук, ведущий научный сотрудник Проблемной научно-исследовательской лаборатории электроники, диэлектриков и полупроводников института неразрушающего контроля Национального исследовательского Томского политехнического университета
