mechanisms of dry powder net-shaping under...

MECHANISMSOF DRY POWDER

NET-SHAPING UNDERULTRASONIC VIBRATION

AND BY THE COLLECTORMETHOD

Oleg Khasanov, Edgar Dvilis, Vitaly Sokolov and Yury PokholkovTomsk Polytechnic University, Tomsk, Russia

Global Roadmap for Ceramics – ICC2 ProceedingsVerona, June 29-July 4, 2008© ISTEC-CNR, Institute of Science and Technology for Ceramics-National Research Council

ABSTRACT

The methods of compacting dry polydisperse and nanometric powders into requiredshapes with improved density distribution have been developed: pressing under pow-erful ultrasonic action and collector pressing by the control of friction forces redistrib-ution. Modeling and experimental investigation of the deformation process of a pow-der body by different methods show that in the case of ultrasonic pressing, the relativedifferential of density along the pressing height has decreased by 35% as compared tothe conventional uniaxial pressing. Collector pressing promotes decreasing dispersionof density distribution by 10 times, and reducing the relative density differential alongthe sample height by 60%.

INTRODUCTION

The present-day requirements imposed on physical, mechanical, optical, electrical andother properties of the ceramics issue the challenge to develop the different novel fab-rication methods. One of the main stream is development of nanostructured ceramics.However the effective methods to manufacture nanoceramics and nanocomposites inlarge scale are not developed yet.

To fabricate the bulk nanostructured ceramic/composite articles it is necessary touse nanopowder having suitable chemical and phase composition, to compact thenanopowder into desirable shape and to sinter the article with remaining nanometricgrains, i.e. preventing the grain growth.

360 MECHANISMS OF DRY POWDER NET-SHAPINGUNDER ULTRASONIC VIBRATION AND BY THE COLLECTOR METHOD

The essential fabrication difficulties are associated with shaping of homoge-neous green nano- and micro-structured powder compacts. The green compacthomogeneity as well as porosity and pore distribution are critical parameters thatdetermine opportunity to reduce sintering temperatures and durations, to preservethe nanoscale structure in the fully-dense ceramics and therefore to achieve therequired properties.

Conventional methods of dry powder compaction, being applied to nanopowder,encounter an obstacle due to high agglomeration/aggregation forces as well as highinterparticle and particle-die wall frictional forces. These properties are distinctive fornanopowders owing to their high specific surface area and of numerous interparticlecontacts. These features give rise to local density differences and internal stress gradi-ents within the green body especially with complex shape followed by differences inthe local sintering rates and differential shrinkage, and eventually, to the ceramicswarping. Albeit some of these defects may be healed by high temperature sintering, thenanostructure character in the dense product may easily be lost. Therefore, uniformcompaction of nanometric ceramic powders especially via dry compaction is yet achallenge.

It is known that to overcome the difficulties associated with the dry compaction ofceramic nanopowders usually one can use the liquid-based techniques. But thisapproach includes application of many chemicals agents (dispersant, binders, lubri-cants, plasticizers, etc.) during the powder processing and compaction which makethese techniques non-friendly to the environment. Besides, the additional chemicalscan contaminate the final product during high temperature treatment of chemicallyactive nanopowders or cause the residual porosity due to burning-out.

So the dry compaction of powders is still the most advantageous technique from allaspects such as simplicity, cost and time effectiveness, as well as the flexibility in theapplied pressure range and the sample size. Undoubtedly, this technique should urgent-ly be upgraded by novel approaches to fit the demands for desirable compaction ofnanometric powders.

Recently, we have developed and patented the techniques allowing uniform drypacking of nanometric and poly-disperse powders into green compacts having complexshapes. These compacts were characterized by homogeneous microstructure and den-sity with low degree of powder aggregation within the bulk and near the die-walls. Thiswas achieved through two different approaches:

• application of powerful ultrasonic action on the nanopowder during the com-paction;

• frictional forces control within the dry nanopowder body using dies with specialdesign (“the collector method”).

PRINCIPLE OF DRY POWDER PRESSING UNDER POWERFULULTRASONIC ACTION

The die-wall friction forces can be decreased without application of lubricants andplasticizers by intensive mechanical ultrasonic vibrations of the shape-forming sur-faces of a die. In this case in the friction pair “die wall-powder body” the surface of the

PRINCIPLE OF DRY POWDER PRESSING UNDER POWERFUL ULTRASONIC ACTION

powder particles will periodically detach from the die wall and the friction forces willoperate only when the surfaces are in contact. Thus, the die-wall friction coefficientwill decrease in proportion to the ratio of time periods of the contact and detachment[1].

Compaction of dry powder under powerful ultrasonic action (PU-action) is carriedout in a die joined with a resonance-size waveguide simultaneously with the uniaxialpressing at room temperature [2] (Fig.1). Standing wave is induced in the die to get themaximal amplitude in a region of powder compacting.

At the initial stage of compaction the powder has low apparent density and a soundattenuation is high. But with increasing the compaction pressure the interparticle bondsbecome stronger and conditions of sound propagation through the powder will beimproved. Due to vibration interaction of particles and agglomerates the ultrasonicaction also decreases the interparticle friction forces [1].

The ultrasonic vibrations of the die have fixed resonance frequency. By creatingquasi-resonance conditions it is possible to pack nanopowder uniformly. Such condi-tions mean that PUA amplitude is proportional or equal to multiples of the particle oragglomerate mean size. In this case at the initial compacting stages the uniform pack-ing of nanoparticles is observed and at the following stages the minimal gradients ofmechanical stresses are achieved. Moreover, a powerful ultrasonic action provides theconditions for deagglomeration and mechano-activation of the nanoparticles duringcompaction immediately before the sintering [3]. Using the known relations the mainparameters of PU-action on a compacted powder can be calculated: vibratory velocityof particle displacement, amplitude of sound pressure, intensity of ultrasonic vibra-tions, acoustic speed, acoustic impedance, coefficient of sound attenuation and elasticmodules of powder body [2, 4].

Figure 1. Ultrasonic die for powder compacting connected with magnetostrictivetransducers.


PRINCIPLE OF THE COLLECTOR METHOD OF COMPACTION

To achieve the uniformity of density distribution in dry powder green compact alongthe pressing axis the partial redistribution of wall friction forces in pressing directioncan be used even without decreasing of these forces. Such approach of powder com-pacting can be used when the lateral shape-forming surface is divided along the com-paction axis in some parts which are moved during compacting in different directions

Figure 2. Dies for conventional single-action pressing (a) and for collector pressing(b).

Figure 3. Distribution diagrams of the wall friction forces along the compact heightfor conventional (a) and collector pressing (b).

CHARACTERIZATION OF THE DENSITY DISTRIBUTION IN THE DRY GREEN COMPACT

relatively to a powder body. This technique we called as the “collector method” ofcompaction [5, 6].

Fig. 2 shows the schemes of the cylindrical dies for conventional single-actionpressing with top pressing punch (a) and for the collector pressing with two opposite-ly moved parts of a passive shape-forming surface (b). The directions of movement ofthe die parts are denoted by the arrows. Along the conjugation lines of shape-formingparts which move in opposite directions the wall friction forces have a maximum.

Fig. 3 shows the distribution diagrams of the wall friction forces (and also the den-sity distribution) along the green compact height for conventional single-action press-ing (a) and collector pressing (b).

In a case of the conventional compaction (Fig. 2a, Fig. 3a) the maximal value ofwall friction force and the corresponding maximal value of densification degree of apowder body are achieved in the region of conjugation of a movable top punch and adie. Because of loss of the compaction force for overcoming the wall friction forcesinside of powder body the minimal densification degree is observed in the most distantfrom the top pressing punch. The distributions of the wall friction forces and the den-sity in a height of powder body are axisymmetric and non-uniform.

In a case of the collector pressing (Fig. 2b, Fig. 3b) the bottom punch moves joint-ly with one part of the lateral surface, and the top punch jointly with its second part.The distributions of the wall friction forces and the density of a powder body are cen-trally symmetrical, and the average density in the horizontal cross section is invariablealong the green compact height.

But the non-uniformity of density distribution remains in the horizontal layers of acompact. The increasing the quantity of alternate oppositely moved parts of the passiveshape-forming surface results in minimization of the non-uniformity of density distri-bution in the horizontal cross sections of a powder body.

CHARACTERIZATION OF THE DENSITY DISTRIBUTION IN THE DRYGREEN COMPACT

The dependence of the compact density on compaction pressure in dimensionless formcan be described by compaction equation of the logarithmic type [2]:

(1)

where ρ is the relative density of the pressed material; P=P0/Pcr is the relative com-paction pressure; P0 is the current compaction pressure; Pcr is the pressure at which thetheoretical density can be achieved; a, b are the constants.

The physical meaning of the constant a is the relative density of the compact atcompaction pressure which is equal to the critical one (P0 = Pcr). Thus, a equals to therelative theoretical density of a compacted material tacking into account all its chemi-cal and physical components (a = 1).

The coefficient b characterizes the densification intensity (dρ/dP0) of a powdergreen body and also reflects the non-uniformity of density distribution along the greencompact height.


The pressure distribution along the height h in the cylindrical green compacts withhydraulic radius R during single-compression action is given by [7]:

(2)

This equation was derived under the assumption that the product of the wall frictioncoefficient f by the lateral pressure coefficient ξ (the hydrostatic coefficient) is con-stant.

Using equations (1) and (2) the equation for relative density differential along thegreen compact height can be expressed as

(3)

Thus, the density differential depends on values of the coefficients of the wall fric-tion and the lateral pressure as well as on the ratio of a height to hydraulic radius of agreen compact.

Evidently, the main causes for the non-uniform density distribution throughout thedry powder compact in closed rigid die are the frictional processes. The uniformity ofthe density distribution throughout the compacted powder can be achieved by elimina-tion of the die-wall friction effects.

In a case of collector pressing the density differential along the height h of a greencompact having hydraulic area S0 and the parts of hydraulic perimeter Π ’ and Π’’ is

(4)

Thus, the vertical distribution of a green compact density depends not only on thecoefficient of lateral pressure and geometric parameters of a compact but also on therelation of the values of friction forces in differently directed parts of lateral shape-forming surface.

According to (4) to minimize the density differential it is necessary to meet a con-dition Π ’f ’ = Π’’f’’. In a case when Π ’ / Π’’ ≠ 1 it is possible to change the ratio (f’’/f ’)via minimization of the friction forces by PU-action on the corresponding parts of thedie (Fig. 4).

Using the finite-element method the modeling of powder deformation at the uniax-ial single-action static compaction and at the collector pressing taking into account thewall friction was carried out (Fig. 5). The comparison of the pictures shows the char-acter of deformation in long-length powder body of cylindrical form under the studiedconditions.

In a case of the conventional compacting (Fig. 5a) there is essential distortion of thedeformation isolines in the form of generatrixes of a cone. For collector pressing (Fig.5b) the distortion of isolines comes to minimum.

CHARACTERIZATION OF THE DENSITY DISTRIBUTION IN THE DRY GREEN COMPACT

To get the patterns of density distribution in the volume of compacted powder bodyexperimentally the special green compacts consisting of identical horizontal layers offreely and uniformly filled powder divided by thin contrast soot layers have been pre-pared [8]. By analysis of the matrix of vertical deformation values in the local regionsof green compact the diagram of density distribution can be obtained. Fig. 6 shows thediagrams of density distribution in the cylindrical green compacts from granulatedBaTi4O3-BaWO4 nanopowder compacted by the conventional uniaxial single-actionmethod (Fig. 6a), under PU-action (Fig. 6b) as well as by the collector method (Fig.6c). The vertical axis presents the values of relative distance from pressing punch (h/R)

Figure 4. Die combined the collector method and compaction under PU-action.

Figure 5. Deformation of layers of dry green compacts pressed by conventional sin-gle-action pressing (a) and by collector method (b).


along the height of a sample; the horizontal axis is the relative distance (r/R) from thecompaction axis in a sample radius.

In a case of conventional dry compacting (Fig. 6a) one can observe the sharplyinhomogeneous density distribution in the green compact volume, at which the maxi-mum densification is achieved in a ring zone of conjugation of the pressing punch withthe matrix wall and near the central zone of the moving top punch.

In a case of compacting under PU-action (Fig. 6b) the density distribution behaviorremains the same, but because of decreasing the wall friction forces under the actionof PU-vibrations the uniformity of density distribution is more high and the relativedensity differential along the compact height reduced by 35 % in comparison with theconventional dry pressing.

The collector pressing (Fig. 6c) promotes decreasing dispersion of density distribu-tion by 10 times, and reducing the relative density differential along the sample heightby 60%. The behavior of density distribution cardinally differs from two previous casesand is like the pattern of density distribution at double-action pressing but with essen-tially smaller gradients.

EXAMPLES OF PRACTICAL APPLICATION

Using the method of dry compaction under PU-action the optical transparentNd3+:Y2O3 ceramics have been fabricated (Fig. 7). Ultrasonic compacting decreasesthe volume concentration of pores in comparison with conventional pressing from135 ppm to 94 ppm as well as leads to increasing the transparency of the sinteredceramic: the scattering coefficient (at λ=1.06 μm) comes down from 8.26 cm-1 to2.53 cm-1 [9].

The collector method can be used for fabrication of uniformly dense powder arti-

Figure 6. Density distribution in the green compacts from Ba-Ti-W-O nanopowdercompacted by conventional single-action method (a), PU-action (b) and collectormethod (c).

cles of different shapes (Fig.8) including impellers, ceramic cases for RF-duplexerswith precision manufacturing tolerance [5], etc.

SUMMARY

Two methods of uniform packing of dry nano- and poly-dispersed powders intorequired shapes have been developed.

SUMMARY

Figure 7. Optical transparent Nd3+:Y2O3 ceramics sintered after conventional drypressing of nanopowder (left) and after compaction under PU-action (right, bottom).

Figure 8. Designed powder articles which can be compacted by the collectormethod.


The first one is based on the application of ultrasonic vibration to a special die(acoustic waveguide filled by dry powder) simultaneously with uniaxial compactionpressure at room temperature to reduce inter-particle and die-wall frictional forces. Bycreating quasi-resonance conditions when the amplitude of ultrasonic vibration is pro-portional or equal to multiples of the particle or agglomerate mean size, it is possibleto uniformly pack particles during pressing.

The second method involves specially designed dies, where active and passive shap-ing surfaces are joined in one shaping part according to the principle of minimizingdie-wall friction forces and specific rules of collective motion of the shaping parts.

ACKNOWLEDGEMENTS

This work is supported by the RFBR grants N° 06-08-00512, N° 06-08-96932, ISTCproject #3719.

The authors thank very much professor R.Chaim (Technion Institute of Technology,Haifa) for discussion and comments.

REFERENCES

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4. O. L. Khasanov, Yu. P. Pokholkov, V. M. Sokolov, E. S. Dvilis, Z. G. Bikbaeva and V V.Polisadova, Particularities of powerful ultrasound action on nanostructured powders, MRSSymp. Proc., Vol.520, 77 – 82, 1998.

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8. O. Khasanov, E. Dvilis, V. Sokolov, Yu. Pokholkov and G. Loutts. Nanopowder net-shapingfor manufacturing nanostructured ceramics, Proc. of the 2006 Nanotechnology Conferenceand Trade Show, NSTI, Vol.1, 23-26, 2006.

9. V. V. Osipov, O. L. Khasanov, V. A. Shitov, E. S. Dvilis, M. G. Ivanov, A. N. Orlov, V. V.Platonov, I. V. Vyukhina, A. A. Kachaev and V. M. Sokolov, Optical Nd3+Y2O3 ceramicsfrom nanopowders, compacted by static pressure under ultrasonic action, Nanotechnologiesin Russia, 3, 2008 (to be published).

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