The manufacture of sintered glasses and glass-ceramics, glass matrix composites, and glass-bounded ceramics or pastes is often affected by gas bubble formation. Against this background, we studied sintering and foaming of barium silicate glass powders used as SOFC sealants using different powder milling procedures. Sintering was measured by means of heating microscopy backed up by XPD, differential thermal analysis, vacuum hot extraction (VHE), and optical and electron microscopy. Foaming increased significantly as milling progressed. For moderately milled glass powders, subsequent storage in air could also promote foaming. Although the powder compacts were uniaxially pressed and sintered in air, the milling atmosphere significantly affected foaming. The strength of this effect increased in the order Ar ≈ N2 < air < CO2. Conformingly, VHE studies revealed that the pores of foamed samples predominantly encapsulated CO2, even for powders milled in Ar and N2. Results of this study thus indicate that foaming is caused by carbonaceous species trapped on the glass powder surface. Foaming could be substantially reduced by milling in water and 10 wt% HCl.
Glass powders are widely used in fabricating sintered glasses, sintered glassÀceramics, glass matrix composites, seals, and glass-bonded ceramics or pastes. [1][2][3] For many of these applications, glasses with low crystallization tendency are used, for example, for low-temperature cofired ceramics, [4,5] pastes, [6] or sealants [7,8] to ensure sufficient sinterability.Due to the low viscosity required for joining and gas tight sealing, gas bubble formation and subsequent swelling (foaming) often occur, even when organic aids are not used in powder processing. In contrast to the extensive and ongoing study of desired foaming phenomena utilized for foam glasses or glassÀceramic foams, [9][10][11][12][13][14][15] this nondesired foaming effect and its underlying mechanisms are rarely reported. Lucchini [16] observed bubble formation for sodium and calcium lead silicate glass-bonded barium hexaferrites and discussed glass volatilization as the underlying mechanism. Pore formation was also found in porcelain stoneware tiles [17] and lead borosilicate glass frits, [18] where effusing oxygen or water, physically or chemically adsorbed to the powder surface, was discussed as the foaming source. Lara et al. [19] reported foaming during the sintering and crystallization of Ca, Mg, and Zn alumosilicate glass powders used for solid oxide fuel cell (SOFC) sealing. The authors believed that density changes or gas evolution during crystallization are responsible for foaming. Undesired porosity was also reported to occur during sintering of BaO-B 2 O 3 -SiO 2 , [20] lead-free Bi 2 O 3 -B 2 O 3 -SiO 2 solder glass, [21] LTCC glass powders; [22] as well as during porcelain tiles production. [23] More recently, Agea-Blanco et al. [24] studied sintering and foaming of commercial barium zinc alumino boro silicate glass powders, used for SOFC sealing, with heating microscopy, differential thermal analysis (DTA), and vacuum hot extraction (VHE). It was shown that foaming intensity increases with milling duration. For moderately milled glass powders, consecutive storage in air could also promote foaming. Although powder compacts were pressed and sintered in air, foaming was affected by different milling atmospheres among which CO 2 proved to enhance foaming most strongly. Similarly, VHE studies revealed that foaming is predominantly driven by C, CO, and CO 2 , even for powders milled in Ar and N 2 . It was therefore concluded that foaming is caused by carbonaceous species adsorbed on the glass powder surface during milling and later storage.The present study is focused on the nature of these species and the mechanisms keeping them trapped during heating to the temperature of foaming. To ensure detectable amounts of foaming species, K01 glass powders were milled for several hours. Recrushed powder compacts, or those heated to different temperatures and quenched in air were studied with heating
Sintering and foaming of barium and calcium silicate glass powder compacts have been studied for different powder milling. Sintering was measured by means of heating microscopy backed up by XRD, DTA, Vacuum Hot Extraction (VHE) and electron microscopy. Foaming intensity strongly increased with decreasing glass particle size. Although powder compacts were uniaxially pressed and sintered in ambient air, foaming was affected by the milling atmosphere and most intensive for milling in CO2. Conformingly, VHE studies revealed that foaming of fully sintered samples was mainly driven by CO2, even for powders milled in technical air, Ar and N2. Prolonged storage of air milled barium silicate glass powders in ambient air before pressing and sintering caused further increase of foaming intensity. These findings indicate that carbonaceous species are preferentially trapped to or close beneath the powder surface during milling and later storage. The temperature range of CO2 degassing from fully sintered barium and calcium silicate glass powder compacts fits the temperature ranges of decomposition of BaCO3 and CaCO3 mix-milled with the respective barium and calcium silicate glass powders.
In reality, the load bearing capacity of the diamond structure shown is not only based on its overall design and cell geometry, but on the exact connection of the individual struts, also. Thinking of different length scales, one may consider the use of related structures as a metamaterial which is leading to the question of its manufacture. Moreover, there is the demand of locally programming material functionalities within a structure making it "smart," that is, enabling an autonomous sensing and reacting to changes in its environment. State of the art manufacturing technologies are typically based on a computer-aided design and manufacturing (CAD-CAM) process chain, starting with the design of a model on a computer. Despite the parts function, design needs to consider available technologies for the parts manufacture and realization. [2,3] Within this respect, additive manufacturing (AM) technologies are enabling the highest freedom which has, in turn, stimulated new design strategies. Not all of them are exactly new, e.g., biomimetic or bionic design, but with the vision of realizing extremely complex structures with minimum restriction in design these concepts have gained new momentum. The key question is how to make designs that are using less material and energy and which shape or form is the best? In this context, minimal surface membranes appear particularly attractive, as, per definition, they are established with minimum use of material and their shape is a result of minimization and homogenization of tensile stresses. In this context "Tensile Stress Design" is a prominent example from engineering, which has become popular even before modern computing has enabled advanced modeling because related structures can be obtained by selforganization, e.g., by the use of soap bubble models. [4,5] Self-organization is the dynamic and adaptive process through which systems achieve and, also, maintain structure without any additional external control. Self-organization is not restricted to a certain size regime, but its use for the manufacture of objects requires a precise knowledge about how to trigger self-organization processes, how to guide them into the right direction for obtaining the desired shape and how to rest them in order to obtain structural integrity of the final object.The present communication will introduce a strategy for 3D printing of structures which partially melt, form a transient liquid phase and self-organize governed by minimizing their A shape evolution approach based on the thermally activated self-organization of 3D printed parts into minimal surface area structures is presented. With this strategy, the present communication opposes currently established additive manufacturing strategies aiming to stipulate each individual volumetric element (voxel) of a part. Instead, a 3D structure is roughly defined in a 3D printing process, with all its advantages, and an externally triggered self-organization allows the formation of structural elements with a definition greatly exceeding the volume...
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