Search

The selective laser sintering process has the potential to permanently overcome the boundary between model construction and functional components. The basic prerequisite for this is that components with reproducible quality can be manufactured. For this purpose, the virgin material must already be of the same quality. During the process, the laser sintered powder is heated to temperatures just below the melting temperature and held until the end of the building process. This results in thermal aging of the powder. Powder, which is not sintered, gets refreshed after the production process with new powder and can be reused. Due to the repeated refreshing process of the aged powder, a mixture is produced whose quality is difficult to define. By means of rheometry and differential scanning calorimetry (DSC), both new powder and powder, which was taken from a laser sintering process, are characterized. At the same time, the thermal aging is simulated by means of an oven heat. The rheological investigations show that the storage results in an increase of the viscosity, both in the process and in the oven. The reason is a post-condensation and thus the extension of the molecular chains. In addition, extended molecular chains lead to a faster reaching of the sol-gel transition and thus to a change from viscous to elastic material behavior. The post-condensation also leads to an increase in the melting temperature, which can be detected by means of DSC. The aging of the laser sinter powder can be reproduced by means of the oven storage and permits further investigations to ensure the quality of the virgin material.

In this work, we present new insight into the light and elevated temperature induced degradation (LeTID) by performing isochronal annealing prior to degradation and further investigation on the correlation between LeTID and boron-hydrogen bond concentrations (B-H). Significantly, the maximum LeTID defect density, boron-oxygen regeneration rate and B-H respond similarly with isochronal annealing temperature. This result supports that B-H plays an important role in the formation of LeTID. A new defect model involving atomic hydrogen, B-H and hydrogen dimer is proposed. Finally, a single defect transitioning between two precursor states is proposed to be responsible for the two-step degradation observed in LeTID.

We examine the lifetime degradation in multicrystalline silicon (mc-Si) under illumination at elevated temperature, an effect frequently denoted as ‘LeTID’ (‘Light and elevated Temperature Induced Degradation’). Our lifetime analysis of belt-furnace-fired high-performance mc-Si wafers shows that LeTID is most pronounced on samples with Al2O3/SiNx−stack passivation. In contrast to that, the degradation on samples coated with Al2O3 single-layers is very small. We identify the presence of SiNx to be a key component to trigger the defect activation process. Our measurements suggest that hydrogen released during the high-temperature firing from the hydrogen-rich PECVD-deposited SiNx into the silicon bulk plays a major role in the defect activation process. Additionally, we find that the magnitude of lifetime degradation increases exponentially with increasing peak temperature during fast-firing. Comparing these results with recently published results from the literature, we conclude that hydrogen-metal complexes are possible candidates for the root cause of LeTID.

The changes in effective lifetime for mc-Si under the influence of elevated temperatures ranging from 160°C to 400°C and illumination of 2 suns were investigated. It is shown that Light and elevated Temperature Induced Degradation (LeTID) and its subsequent curing can be observed up to a temperature of ~280°C, but vanishes above. Furthermore, it is shown that lifetimes exceed the initial value during prolonged illumination which is interpreted as advanced hydrogenation. However, even though lifetimes exceed the initial value due to advanced hydrogenation at temperatures >280°C, these lifetimes prove to be at least partially instable at low temperatures under illumination suggesting LeTID is not or at least not completely cured. It is concluded that LeTID and advanced hydrogenation are related to different defect systems. In addition, it is shown that both LeTID and advanced hydrogenation only occur above a peak set firing temperature of 720°C thus indicating that lower peak firing temperatures can successfully suppress LeTID, however, only at the price of losing the benefit from advanced hydrogenation as well. Furthermore, it is shown that increasing illumination intensity speeds up the dynamics of LeTID and advanced hydrogenation, respectively.The changes in effective lifetime for mc-Si under the influence of elevated temperatures ranging from 160°C to 400°C and illumination of 2 suns were investigated. It is shown that Light and elevated Temperature Induced Degradation (LeTID) and its subsequent curing can be observed up to a temperature of ~280°C, but vanishes above. Furthermore, it is shown that lifetimes exceed the initial value during prolonged illumination which is interpreted as advanced hydrogenation. However, even though lifetimes exceed the initial value due to advanced hydrogenation at temperatures >280°C, these lifetimes prove to be at least partially instable at low temperatures under illumination suggesting LeTID is not or at least not completely cured. It is concluded that LeTID and advanced hydrogenation are related to different defect systems. In addition, it is shown that both LeTID and advanced hydrogenation only occur above a peak set firing temperature of 720°C thus indicating that lower peak firing temperatures can success...

Hydrogen is well known for its ability to effectively passivate defects in silicon. However, it has been recently proposed that it can also be detrimental, with susepected involvement in light and elevated temperature induced degradation (LeTID). The role of rapid firing processes in the passivation of B-O defects and activation of LeTID indicate a similar involvement of hydrogen in the passivation of B-O defects and LeTID related reactions. While B-O passivation rate is suppressed by dark annealing in Czochalski mono-crystalline silicon, both mono-crystalline silicon and multi-crystalline wafer types showed a similar formation rate of LeTID. It has been recently demonstrated that interstitial hydrogen can migrate to the surface by annealing. Thermal annealing suppressed B-O passivation rate significantly while the LeTID formation rate was not changed significantly. The implication is that as the B-O defect is present uniformly in the bulk of silicon, while hydrogen must either be readily present in bulk, or diffuse to the entire bulk region to passivate B-O defect, the LeTID may not necessarily occur throughout the bulk like B-O passivation. During the first 300 hours of light soaking on mc-Si samples, Cz-Si samples showed improvement in lifetime, which can be explained by the passivation of the B-O defect. However, the full recovered lifetime was similar to the lifetime after firing. The lifetime curve showed no shockley-read-hall component when the lifetime was fully recovered, which confirms the LeTID, which is independent defect to B-O defect, was no longer present. This implies that while B-O defect was passivated, the LeTID defect may have also been recovering at the same rate. It may be that hydrogen initially contributing to the recombination as LeTID, is contributing to B-O passivation, which leads to a recovery in lifetime with a reduction in LeTID and B-O defect.

The pressure and temperature dependent mechanical properties as Young modulus, Thermal expansion coefficient of rare earth REX (RE = La, Pr, Eu; X = O, S, Se, and Te) chalcogenides are studied. The rare earth chalcogenides showed a structural phase transition (B1–B2). Pressure dependence of Young modulus discerns an increase in pressure inferring the hardening or stiffening of the lattice as a consequence of bond compression and bond strengthening. Suppressed Young modulus as functions of temperature infers the weakening of the lattice results in bond weakening in REX. Thermal expansion coefficient demonstrates that REX (RE = La, Pr, Eu; X = O, S, Se, and Te) chalcogenides is mechanically stiffened, and thermally softened on applied pressure and temperature.

The pressure and temperature dependent mechanical properties as melting temperature, hardness and brittle nature of XBi (X = Am and Cf) are studied. The rare earth actinides pnictides showed a structural phase transition (B1–B2) at a transition pressure (PT) of 14.3GPa (AmBi) and 10.8GPa (CfBi). Pressure dependence of melting temperature (Tm) discerns an increase inferring the hardening or stiffening of the lattice as a consequence of bond compression and bond strengthening. Suppressed TM as functions of temperature infers the weakening of the lattice results in bond weakening in XBi (X = Am, Cf). Vickers Hardness (HV), Poisson’s and Pugh ratio of XBi (X = Am and Cf) demonstrates that XBi (X = Am and Cf) is mechanically stiffened, thermally softened and brittle on applied pressure and temperature.

The thermoelectric properties of La-excess alloys of composition La1+δ(Fe0.85Si0.15)13 (δ = 0.06 and 0.09) were studied. As temperature increased from near 0K to room temperature we observed a corresponding increase of electrical resistivity, as a result of continuous metallic behavior of the alloys. On the other hand, the increase of temperature induced an increase of thermal conductivity. The decrease of the thermopower was also observed on increasing temperature and reached the minimum at around the TC then increased again until room temperature.

The ferromagnetic shape memory alloys of composition Ni46Mn41In13 have been studied through transport and magneto-transport measurements. We observe giant negative magnetoresistance (MR) (∼−64% at 50 kOe) around the martensitic transition (MT) temperature. The observed MR is found to be highly irreversible in the region of MT, which is connected to the field and temperature induced first order magneto-structural transition.

Proton resonance frequency shift (PRFS)‐based MR thermometry is hampered by temporal field changes. Temporal changes in the susceptibility distribution lead to field changes and are therefore a possible source of errors. The susceptibility of fat, χfat, is temperature dependent, in the same order of magnitude as the temperature dependence of the chemical shift of water (0.01 ppm/° C). PRFS‐based temperature measurements may therefore be corrupted by non‐local field effects due to temperature induced susceptibility changes in fatty tissue. We performed simulations to quantify the influence of dχfat/dT on PRFS‐based MR temperature maps during thermal interventions in the breast. Susceptibility distributions were calculated for a 3D breast model without and with a stationary Gaussian temperature distribution and a maximum temperature of 57° C. Subsequently, the magnetic field was calculated in Fourier‐domain from these susceptibility distributions. Changes in the magnetic field distribution were quantified b...

U‐Nb(6‐wt%) exhibits plastic yield strength of a few‐tenths of a GPa that can vary depending upon the starting microstructure and heat‐treatment. However, when a several‐mm thick specimen of U‐Nb(6‐wt%) is shock loaded in the range between 1.5 and 10 GPa, no elastic precursor is observed in interferometer measurements at the rear free surface. The absence of the elastic precursor and other features of the compression and release measurements are explained by assuming shear stress‐relaxation rate is dependent upon the shear stress. The resulting stress waves are unsteady and broaden so that shear stress relaxation can occur in the front preventing the plastic yield point from being reached. U‐Nb(6‐wt%) is known to twin in quasi‐static compression and shear‐induced, rate‐dependent twinning is likely the underlying cause for shear stress relaxation in our experiments. Recent experiments in which U‐Nb(6‐wt%) was heavily cold‐rolled ( work‐hardening to ∼25% strain) display no evidence of a precursor, admitting the possibility of a pressure or temperature induced stress‐relaxation process.

We report thermally induced roughness at the nanometer scale, formed in the process of obtaining the (7×7) reconstruction on Si(111). C contamination seems to be the precursor in the formation of the different structures associated to this roughness. The large scale rearrangements, observed after the SiO2 high temperature induced desorption, show various shapes, namely: pyramids, raspberry shapes and tori.