Localized Densification during the Stage II–III Transition - Compaction Efficiency at High Pressures

Localized densification and compaction efficiency at high pressures was studied using X-ray computed tomography. Stage III begins with the initiation of a region of uniform average density within the overall high-density zone. No further densification occurs in this region; additional pressure is transmitted into adjacent, less-dense zones and the die wall. This localized increase in wall friction continuously decreases compaction efficiency. Discrete element modeling was used to visualize force transfer. The transition was governed by events that ranged from the microscale to the macroscale. Terapascal levels of pressure were required to produce a uniform compact.     

Compaction Behavior of Spray-Dried Alumina

Compaction behavior and resultant porosity of spray-dried alumina were examined over a range of pressures from 18 to 345 MPa. The variability in pressed density and pore size was measured as a function of spray-dried granule size, binder concentration, and powder moisture content. An exponential behavior was found between pressed density and compaction pressure, density increasing with the logarithm of the compaction pressure. Pore size also displays an exponential behavior, pore diameter decreasing with the log of increasing pressure. The distribution width about the median pore size was noted to decrease approximately in a log-log response to increasing compaction pressure. The behavior of the compaction and porosity curves is related to the degree to which the polymer binder is plasticized which, in turn, affects the pressure at which the spray-dried granules begin to crush. This apparent yield pressure, although dependent on amount of binder and powder moisture, is intrinsically dependent on the water/binder ratio. Finally, by analogy to soil mechanics, an empirical equation relating compact density, yield pressure, and slope is presented.     

Plotting of Compaction Curves of Ceramic Powders on the Basis of One-Parameter Pressing Equations

A one-parameter equation that describes pressing of ceramic powders in the dimensionless coordinates relative pressing pressure – relative density of the compact is suggested. It is shown that the boundary conditions are correctly obeyed for such an equation. The correctness of the initial conditions is provided by the developed method for plotting the compaction curves. The consecutive unloading of the powder body envisaged by the method makes it possible to determine the contribution of the elastic interaction of the particles to compaction. The true compaction curve of ZrO₂ – 5% Y₂O₃ nanopowder is well described by a one-parameter logarithmic equation.     

Pore evolution and compaction behaviour of spray-dried bodies for porcelain stoneware slabs

The compaction behavior of spray-dried powders has turned into concern in porcelain stoneware manufacturing due to the increasing diffusion of large slabs. It is necessary to fill a knowledge gap between the compaction behavior with conventional presses and novel technologies. For this purpose, eighteen industrially-manufactured spray dried bodies were characterized for specific properties connected to the compaction behavior (curves of bulk density, intergranular and intragranular porosity in function of applied load, apparent yield strength). In addition, the firing behavior was investigated in order to reveal any effect of dry bulk density on firing shrinkage and bulk density of fired samples. Powder compressibility is within 50% and 55% (Carr index) and is primarily controlled by moisture. Two regimes are found: low pressure (fast density increasing by granule cave in and closure of intergranular porosity) and high pressure (slow density gain by downsizing microporosity). A peculiar mechanism is unveiled: granules squeeze in the low-P regime and further densification is achieved through microfracture around individual agglomerate. A phenomenological model is illustrated for the compaction of spray-dried powders. In conclusion, the performance of spray dried bodies during compaction is crucial to control the uniformity, in terms of porosity and bulk density, which has important repercussions on the properties of final slabs, especially differential shrinkages and deformation during firing due to density gradients.     

钠基膨润土导热系数测试及误差分析

采用LFA447型激光导热系数测量仪测量了不同含水率、不同压实密度的钠基膨润土在不同温度下的导热系数,得出导热系数和水分、温度、成型压力及湿密度的关系曲线;对测量的准确性和误差进行了分析,并根据误差产生的原因提出改进方案。     

Compaction behavior of Al6061 powder in the semi-solid state

Semi-solid powder processing involves compaction of metallic alloy powders in temperature ranges when both solid and liquid phases coexist. However, modeling of such process has been very scarce. In this study, compaction behavior of Al6061 powder in the presence of liquid phase was investigated. The relationship between compaction pressure and relative density of powder in the semi-solid state was modeled and verified against experimental measurements. The results showed that the powder compaction behavior can be accurately predicted by Shima-Oyane model when the liquid fraction was below 20%. In this range, all the normalized pressure-relative density curves merged into one, which could be expressed in a simple exponential form. When the liquid fraction was higher than 20%, the model failed to predict the compaction behavior because of the non-uniform distribution of the liquid phase. Squeezing-out of the liquid phase and interlocking of the irregular solid phase were speculated to occur during the compaction process.     

Compressibility and sinterability of CeO2–8YSZ powders synthesized by a wet chemical method

The compressibility and sinterability of CeO₂–8YSZ powders prepared by co-precipitation were investigated in detail. It was shown that the compressibility curves are characterized by three linear parts at low, middle and high pressures. The middle and high regions of the applied pressure, as least investigated, were studied in detail. The specific values of the compaction pressure (P_Y2) and density (ρ_Y2) at the intersection point of the compressibility curves were determined for all investigated powders. It was shown that the compressibility curves for all investigated powders can be described by two straight lines by using special coordinates ρ_G/ρ_Y2 and log(P/P_Y2).The sinterability curves of powders after drying and after calcination at 350 °C have a pronounced maximum. The optimum compaction pressures (P*) corresponding to the pressure at the maximal value of sintered density were determined for all investigated powders. It was shown that the region of optimal pressures is in the upper part of the middle-pressure region, whereas the P*/P_Y2 ratio varies between 0.7 and 0.9. The influence of the powder fractionation on the sinterability of the powders was also studied in detail as a function of the compaction pressure and calcination temperature.     

Anodic dissolution phenomena accompanying supersaturation of copper sulfate along a vertical plane copper anode

Vertical plane copper anode was electrochemically dissolved in a stagnant CuSO₄ and CuSO₄-H₂SO₄ electrolyte solution. Measured anodic polarization curve showed three distinct stages: Stage I characterized by an increased current density with increasing in anodic overpotential, Stage II including a maximum peak current followed by an abrupt current decrease caused by CuSO₄ precipitates at higher overpotential and Stage III with much lower and virtually constant current density, that is, the existence of anodic limiting current density over a wide anodic overpotential range. The complete passivation phenomena were not encountered in the present experimental conditions. Supersaturation phenomena of Cu²⁺ near the anode surface in Stage II were in-situ measured by holographic interferometry. The degree of supersaturation more than 0.18 M CuSO₄ was recorded. It was followed by the nucleation phenomena of a kind of CuSO₄ crystal after a certain incubation period. With growth of precipitates on the anode, the degree of supersaturation of Cu²⁺ ion around them slightly decreased. A similar phenomenon was also observed in CuSO₄-H₂SO₄ solution. The measured anodic limiting current density in Stage III agreed well with the calculated one from the correlation equation:

     

Micromechanical model for compaction characteristics of bentonite–sand mixtures

This paper presents a new methodology for predicting the relationship between compaction stress and dry density, referred to herein as the compaction curve, of composite mixtures based on the micromechanics. To examine the effect of sand content on the compaction characteristics, a series of uniaxial compaction tests (UCT) for bentonite–sand mixtures with different sand fractions (by weight) were performed. In addition, a prediction model based on micromechanics for composite mixtures was developed. The developed model can be used to predict the compaction curves of composite mixtures based on experimental compaction curve of the matrix. The accuracy of the proposed model along with its applicability was examined and validated by using experimental data. The results show that the proposed model can accurately predict the compaction curves of the bentonite–sand mixtures in both loading and unloading phases.     

High-pressure compaction modelling of calcite (CaCO3) powder compact

Numerical simulation of manufacturing processes with working conditions at high pressure (above 1 GPa) requires constitutive data of the powder for the whole range of pressure and density. Most of the test apparatuses commonly used to obtain such data are only working in the lower pressure regions. Because of the absence of high-pressure data, many parameters have to be guessed or extrapolated. A material used in high-pressure applications is Calcite (CaCO₃). The material can be used as an insulator in high-pressure capsules it is also a common material in the earth core. An apparatus often used to generate high pressure during compaction is the Bridgman anvil apparatus. In this work experimental tests with a Bridgman anvil set-up using Calcite powder discs with different thicknesses were done. A nonlinear elastic-plastic cap model was developed to model the behaviour of powder material from low pressure and loose state to high pressure and solid state. The constitutive model was implemented in a finite element code. The constitutive data were identified by optimization of experimental data. Validation was done by numerically reproduce the mechanical behaviour of uni-axially pressing Calcite to different pressure (up to 5 GPa) including unloading. The load-displacement curves, density distribution and the surface displacement were measured and compared to the finite element results. The results of the compaction simulations agree reasonably well with the experimental results.     

Dependence of Compaction Efficiency in Dry Pressing on the Particle Size Distribution

The compact densification with pressing pressure (compaction efficiency) was determined to be sensitive to the particle size distribution. For the three types of alumina powders used in this research, the compaction efficiency increased with increasing particle size. It has been demonstrated that if the compact density versus log (pressure) has a linear relationship for any two types of powders, so do the blends of the two powders. A model is proposed which can predict the compaction efficiency of a binary particle system based on the Furnas particle packing model and consider the packing efficiency as a function of forming pressure. The composition of the binary mixture at which the highest density is obtained under high pressures is also the composition having the largest compaction efficiency. When coarse particles were added to this composition, the compaction efficiency slowly decreased, and when fine particles were added, the compaction efficiency rapidly decreased. For a continuous particle size distribution, the highest compaction efficiency is related to the average value of -log (porefraction).     

Experimental calibration of density-dependent modified Drucker-Prager/Cap model using an instrumented cubic die for powder compact

Theory and experimental calibration of density dependent modified Drucker-Prager/Cap (DPC) model are presented by using a novel instrumented cubic die in powder compaction tests. The cubic die is designed for directly determining the loading and unloading forces and displacements of powder compact inside the die in compaction and transverse directions without any additional calibration. The cap surface parameters and elastic properties are characterized by fitting stress and strain curves recorded during loading and unloading at different green density values and the plastic material parameters for failure surface are obtained by additional radial and axial compressive tests. The experimental data is subsequently used in the simulation of cubic die compaction to verify the results from the density dependent modified DPC model.     

A small-angle X-ray scattering study of local variations within powder compacts

This work used two-dimensional small-angle X-ray scattering (2D-SAXS) to investigate the compaction behaviour of pre-gelatinised starch (PGS) and microcrystalline cellulose (MCC), which are commonly used as pharmaceutical excipients. By analysing azimuthal variations in scattering intensity, reproducible relationships were found between the compaction pressure, relative density and changes in the shapes of 2D-SAXS patterns for each material. These results indicated differences in the compaction mechanisms between PGS and MCC.The relationships also provided a means for investigating local variations in compaction behaviour within specimens prepared using different materials and compaction conditions. Relative density results from 2D-SAXS were consistent with expectations based on the effects of friction during compaction and appeared similar to data from other methods. In addition, however, 2D-SAXS measurements revealed local variations in the effective direction in which compaction occurred, with significant radial components observed near the die walls. This appeared to be consistent with the transfer of some compaction pressure to friction on the die wall. These observations represent an important advance, since other experimental methods do not easily reveal the direction of force transmission within the powder compact.     

Compaction of Aggregated Alumina Powder

The behavior of aggregated powders during compaction was studied using powders differing only in their aggregate size distribution. The relations between aggregate size distribution, compaction pressure, and green density were determined. The crushing of aggregates during compaction was followed by determining the changes in the aggregate size distribution. The presence of aggregates retards the compaction at all stages in the compaction process.     

聚苯乙烯/铜粉温压成型的研究

对添加聚苯乙烯（ＰＳ）作粘结剂的铜粉进行了温压成型试验,研究了ＰＳ含量、温压工艺参数等对温压压坯密度的影响。结果表明,ＰＳ的加入可以明显提高铜粉温压压坯的致密化程度,提高压坯的相对密度;同时温压压坯致密化还同温压温度有关;温压成型时压坯密度与温压压力的关系仍可用经典的粉末压制方程来描述。     

Bulk density of solid polymer resins as a function of temperature and pressure

In a plasticating extruder, solid polymers are heated and are subjected to high pressures before they are melted and delivered to a die. In both the solids conveying and melting sections, these temperature and pressure increases will compact the unmelted polymer bed as it moves down the screw channel. Performance of the extruder depends in part on how well the screw design matches the compaction behavior of the resin for a given set of process conditions. The design of these screw sections, however, is often done based on past experience and with little knowledge of the resin compaction behavior. A much improved design would include screw performance prediction using variable bulk density and computer simulations. Computer simulations, however, are often performed using constant solid bulk density because of the lack of reliable density data as a function of both pressure and temperature. An instrument was developed for studying the compaction behavior of pellet and powder resins. Bulk densities and storage friction coefficients are reported for several important thermoplastic resins as a function of temperature and pressure. The bulk density data were fitted to a semi-empirical model.     

Cold compaction study of Armstrong Process® Ti-6Al-4V powders

The Armstrong Process® developed by Cristal US, Inc./International Titanium Powder, is an innovative, low-cost technology for producing Ti and Ti alloy powders in a one-step, continuous process. In this work, Armstrong Ti-6Al-4V powders were characterized and the cold compaction behavior of the powders were investigated in detail. As-received as well as milled powders were uniaxially die-pressed at designated pressures up to 690 MPa to form disk samples with different aspect ratios. Samples with high aspect ratio exhibited non-uniform density along the pressing axis and the density distribution was consistent with the result predicted by finite element analysis. The model developed from the linear regression analysis on the experimental density data can be used to predict density of compacts with different aspect ratios. In the studied pressure range, an empirical powder compaction equation was applied to linearize the green density — pressure relationship. Cold compaction parameters were obtained for the as-received and milled Armstrong Ti-6Al-4V powders.     

Performance evolution of NiO/yttria-stabilized zirconia anodes fabricated at different compaction pressures

Nickel oxide (NiO)/yttria-stabilized zirconia (YSZ) anode substrates were fabricated at four compaction pressures, 70, 200, 500 and 1000 MPa, the particle size distributions of NiO and YSZ were investigated with the powders treated at different compaction pressures, and the effects of compaction pressure on the performance of anodes with and without pore-formers were investigated by studying the effects of compaction pressure on the sintering shrinkage, compaction density, sintered density and electrical conductivity of anodes and the performance of cells. Experimental results demonstrated that the mismatch in the sintering shrinkages of YSZ films and the anodes compacted at 70 and 1000 MPa caused gas leakage across the films and thus a higher local temperature than the furnace temperature. The single cell with the anode using pore-former and compacted at 500 MPa exhibited the best output performance of 2.66 W cm⁻² at 800 °C.