Static Pressing Research Articles

Dynamic preparation of TiO 2 films for fabrication of dye-sensitized solar cells

Preparation of nanocrystalline porous titanium dioxide (TiO 2) films with roll-to-roll compatible methods was studied. Gravure printing was used for spreading TiO 2 paste and paper calendering for pressing TiO 2 nanoparticle films. Influence of different preparation methods on performance of fabricated dye-sensitized solar cells (DSSCs) was investigated. The attained light into electricity conversion efficiency was compared with DSSCs fabricated on conductive plastic substrates by doctor-blading spreading with subsequent static pressing. The latter method achieved a light conversion efficiency as high as 5.1%. The efficiency of the cells with a gravure printed TiO 2 film was found to be rather low. The highest conversion efficiency was 1.7%; this is concluded to be due to insufficient thickness of the gravure printed TiO 2 films. Calendering of the doctor-bladed films on the other hand attained a light conversion efficiency of 4.7%.

The densification of powder mixtures containing soft and hard components under static and cyclic pressure

The effect of pressure cycling on the densification and ejection of Pb–Al 2O 3 powder mixtures is investigated and compared to static loading. The volume fraction of Al 2O 3 is varied between 0 and 80%. The density of all mixtures increases with time under constant pressure even at room temperature. This mechanical behavior is the result of the creep of lead. This effect diminishes with increasing volume fraction of Al 2O 3. Pressure cycling does not have any influence on the final density of lead samples in the absence of Al 2O 3. The influence of cycling becomes, however, noticeable in the presence of Al 2O 3 in the mixture. In this case, the density and strength of specimens are higher than the corresponding ones obtained after static pressing for the same dwell time. This effect is enhanced with increasing Al 2O 3 percentage. A static pressure approximately 50–80% larger than the cyclic one is required to achieve the same density. Moreover, the strength of compacts with high Al 2O 3 content is substantially higher after cyclic pressing than under static pressing. The ejection force is higher after cyclic compaction at a given pressure level owing to the higher density achieved.

Self-diffusion and magnetic properties in explosion densified nanocrystalline Fe

The enhanced diffusivity due to the high volume fraction of grain boundaries (GB's) is a key feature for the modified physical and chemical properties of nanocrystalline (n-) materials in contrast to the chemically identical coarse-grained solids. The authors previously performed tracer diffusion experiments on high-density n-Pd (relative density {rho}/{rho}{sub 0} {approximately} 97%, where {rho}{sub 0} is the theoretical density) and n-Fe ({rho}/{rho}{sub 0} {approximately} 92%) prepared by conventional static compaction of the gas-condensed powders at elevated temperatures, showing that the diffusivity of the n-metals is similar to the conventional GB diffusion in polycrystalline (poly-) metals. The specimens used in these studies, however, were not fully dense, with defects such as pores or cracks which may act as fast diffusion paths. The dynamic compaction technique utilizing the explosion or collide of a high speed projectile has also been applied for consolidation of fine powders of metals and ceramics. Recently, n-ceramics prepared by static pressing of fine powders were successfully further densified to more than 90% of the theoretical density by explosive compaction suppressing grain growth and contamination. In order to provide additional experimental information on interface structure and properties in n-metals, the density of n-Fe was further increased by explosive compactionmore » and effects of the densification on both self-diffusivity and bulk magnetic properties were examined.« less

Industrial Use of the High-Rate (Impact) Pressing of Metallic Powders

Features of the high-rate (impact) pressing of metal powders are discussed. It is shown that impact pressing compacts powders to a relative density which is 97% of the theoretical value after a single loading. The pressure which exists during high-rate pressing is usually higher than the static pressure for the same density of the pressed powder. The force which presses the products out of the die is 1.5-2.5 times lower than in static pressing. Preliminary pressing of the powder reduces friction losses and ensures a more uniform distribution of density over the height of the product. Recommendations are given on the loading rate in high-rate pressing and on the use of this operation in relation to the shape of the product. An example is given to illustrate the use of high-rate pressing for making large-diameter gaskets from copper powder.

Materials based on highly concentrated ceramic binding suspensions (HCBS). Molding processes and strength of silica mixtures based on plasticized hcbs of quartz sand

A composition of a ramming silica mixture based on quartzite filler and HCBS of quartz sand plasticized by an additive of refractory clay and having a total clay content of at most 2% is presented. Three methods of molding (compaction) are considered, namely, pneumo(vibro)ramming, vibropressing, and static pressing. The first is shown to give maximum compaction. The effects of some process factors on the properties of the new refractory material are described. The suggested mixture promises to serve well in rammed monolithic linings of heating units

Structure and superconducting parameters of composites YBaCuO-Ag prepared by static, dynamic and hot pressing

To investigate effect of a technological scheme on the structure and superconducting parameters of powder composites YBaCuO-Ag, three types of pressing of powder mixtures were used, i.e. static, hot, and dynamic pressing. Silver content changed in the range c=0-50 vol.%. Orthorhombicity degree, b/a, of the YBaCuO-ceramics proved to be practically independent on c up to c=30 vol.% for static and hot pressing, and it decreased appreciably even at small c values in the case of dynamically pressed samples. Critical temperature T/sub c/ and transport critical current density j/sub c/ show similar behaviour with maximum about c=10 vol.% for static and hot pressing. Dynamic pressing results in significant deterioration of these parameters. >

Refractory concretes of a new generation. Vibrorheology. Vibration methods of compacting and forming

Problems of vibrorheology are discussed as applied to the manufacture of refractory concretes. The principal schemes of vibrocompaction (vibroforming) processes are considered. The general characteristics of forming systems and the main parameters and regularities of the process are analyzed. It is shown that an estimate of the efficiency of vibroforming should include the combination of the amplitude and frequency (for example, the intensity criterion). Some related problems are touched upon, namely, the vibrotransport effect, “airtightness” of the mold, variation of the amplitude along the height of the formed item. Silica-based ceramic concretes are used as an example for analyzing the efficiency of vibrocompaction as compared with static pressing. In the former case the same degree of compaction is attained at pressures four orders of magnitude lower than in the latter case.

Some properties of aluminum nitride compacts prepared by shock-wave loading

The prescribed initial density of the material before loading was achieved by different methods: free pouring of powder, prior Static pressing, and prior explosive pressing. The original material density, loading conditions, and density of the compacts obtained are indicated in Table i. In experiment 4 after free pouring of the aluminum nitride powder it was preliminarily compressed by an explosion by loading scheme I with a peak pressure amplitude of 2 GPa.

Explosive pressing of a TiC-TiNi powder composite

Explosive pressing enables specimens of relative densities up to 95% to be obtained, whereas the maximum attainable density after static pressing does not exceed 70%. There exist optimum sintering temperatures and compact densities, which are different for the static and explosive pressing methods. It is important that the optimum temperature of sintering after explosive pressing is 150°C lower than that of sintering after static pressing. This enables the sintering of the hard metal to take place virtually in the absence of a liquid phase. Explosive pressing is performed without a plasticizer, which simplifies the preparation of the powder mixture, enables the rate of heating in sintering to be increased, and improves the conditions of operation of the sintering furnace. Compared with static pressing, the explosive method ensures an about 50% lower residual porosity, a more fine-grained structure, and higher transverse rupture strength.

Pressability of refractory materials on hydrodynamic machines

During hydrodynamic pressing the densification of refractory materials occurs somewhat differently than during static pressing, which is due to the action of high rates of impulse pressure application. The compactability of powders is described by the well-known Bal'shin relationship with the insertion of a dynamic coefficient. The hydrodynamic method is suitable for obtaining high-density articles from various refractory materials with an adequately uniform density distribution over the volume.

Kinetics of joint formation between a powder layer and a steel backing plate in hot pressing

1. A study was made of the kinetics of joint formation between a powder layer and a steel backing plate during static hot pressing. 2. It is demonstrated that the formation of such a joint is controlled by a dislocation climb mechanism leading to steady-state creep, in which the rate of deformation is a function of the fourth power of stress. 3. Estimates have shown that the energy of activation for steady-state creep in the process under consideration corresponds to either the energy of self-diffusion or the energy of vacancy motion. 4. It is demonstrated that the same phenomenon occurs in the main volume of a powder layer during densification and in the contact zone during the formation of a joint between a powder layer and a substrate.

Oxidation behavior of porous blanks during heating before dynamic hot pressing

1. The surface oxidation experienced by the powder particles in a porous blank during heating in a protective atmosphere before DHP is due to the presence of closed pores in the blank. 2. Four fields have been discovered differing in the magnitude of closed porosity and in scope for its elimination through the heating of the blank. 3. Conditions have been established for the cold static pressing and heating of porous blanks before DHP ensuring that the surface oxidation of the pores in the blanks during heating in a protective atmosphere is a minimum.

Adhesion of a powder layer applied by the hot pressing method to a dense steel basis

1. A study was made of the process of bond formation between a powder layer and a dense basis during static hot pressing. 2. The mechanism responsible for mass transport in the bond zone during the reaction of a powder layer with a dense metal has been elucidated.

Forces in the static and dynamic pressing of metal powders and chips

1. The ΣPfr vs Ppr and Pb vs Ppr relationships in static and dynamic pressing are linear, and for this reason (Kfr)st and (Kfr)dyn are indepent of load within the pressure range investigated. 2. To attain a given compact density in static and dynamic pressing, the force in the latter should be 1.3–1.5 times higher than the force in static pressing, and (Kfrst < (Kfr)dyn. 3. Increasing the ductility of materials being densified and decreasing their coefficient of friction against the die walls reduces the value of Kfr. In hot pressing, the values of Kfr are lower than in cold pressing.

Static pressing of ferrite plates

1. Over the compaction-pressure range investigated the mechanical properties of multihole ferrite plates increase with rising pressure. The best results were recorded at a pressure of 8 tons/cm2. 2. The coercive force of ferrite plates is strongly affected by compaction pressure. At constant chemical composition corresponding to ferrite 2VT it is possible to alter the coercive force from 2.2 to 1.5 Oe by varying compaction pressure. 3. The coefficients of rectangularity and squareness of the hysteresis loop are strongly affected by compaction-pressure variation at low pressures (up to 3.5 tons/mm2, but remain virtually constant at higher pressures.

THE COMPACTION OF METAL POWDERS BY ROLLING. II.–AN EXAMINATION OF THE COMPACTION PROCESS

The compaction process is examined in detail. It is shown that, where particle deformation is concerned, compaction by rolling is similar to compaction by static pressing, with the addition of elongation of the particles in the rolling direction when the rolling pressure is sufficiently high. A method for determining the average roll pressure is described. A comparison of the rolling of a metal powder with the rolling of a solid bar, and the determination of the effect of particle shape and mean size, indicates that not only roll/powder friction but also the slip between particles plays an important role in the compaction process. This leads to an examination of the flow properties of powders, which are measured in terms of a “powder-viscosity factor” that indicates whether and at what order of rolling speed a powder can be coherently compacted. Finally, a mechanism of compaction is proposed on the basis of the present findings and on the authors’ earlier work.