Roy Kaplow

Measurement of minority carrier lifetime in solar cells from photo-induced open-circuit voltage decay

We present an experimental technique for determining the excess minority carrier lifetime within the base region of p-n junction solar cells. The procedure is to forward-bias the solar cell with a flash from a stroboscope and then to monitor the decay of the open-circuit voltage. Results are given for conventional horizontal-junction devices, as well as for vertical single- and multijunction solar cells. Lifetimes obtained with this technique are compared with those obtained from a method based on open-circuit voltage decay following the abrupt termination of a forward current, and with results obtained from a traveling light spot measurement of base minority carrier diffusion length in vertical-junction solar cells, from which the lifetime can be inferred. It is found that the forward current method does not yield a reliable lifetime estimate.

Phase Transitions and Shape Memory in NiTi

The “premartensitic” transformation of the B2 phase in NiTi was studied using electrical resistance measurement, X-ray diffraction, and shape-change effects. The degree of rhombohedral distortion of that transformation and the electrical resistance change show similar temperature dependence. In particular, whenTR » Ms, this encompasses a narrow temperature region of rapid increase just belowTR, followed by an extended range of levelling off. The onset of the martensitic transformation is not necessarily preceeded by the rhombohedral distortion, and the latter proceeds continuously belowTR and persists throughout the martensite transformation range. Thus three kinds of phase transformation can occur: B2 ai R, B2 ai M, R ai M; B2 ai R is not a precursory phenomenon. Observations on the two-way memory (TWM) and its correlation to resistance changes in NiTi leads to the conclusion that whenTR >Ms, the TWM is comprised of two stages. On cooling, the B2 → R transformation is responsible for the initial stage, which is thermally reversible without hysteresis. The second stage of the TWM, which is due to martensite formation, is reversible in shape change, but with a thermal hysteresis. It is also suggested that on heating, the R → B2 transformation can contribute to the primary shape memory effect in NiTi.

Interstitial atom configurations in stable and metastable Fe-N and Fe-C solid solutions

Mössbauer Fe57 spectroscopy allows comparison of Fe−N and Fe−C interstitial solid solutions. The spectra of Fe−N retained austenite indicate that nitrogen atoms are randomly distributed on octahedral sites in the austenite and in the virgin martensite. On heating, austenite decomposes directly to the equilibrium phases α iron and Fe4N at temperatures above 160°C. Virgin martensite ages at room temperature by local ordering of nitrogen atoms. In that process, three new iron atom environments develop, characteristic of the Fe16N2 (α″) structure. However, the excessive width of the peaks indicate the perfect order of the Fe16N2 precipitate is not achieved, except after very long times. Further aging at 100°C leads to the complete decomposition of the virgin martensite to the discrete phases α iron and Fe16N2. This two phase structure is stable up to 160°C, above which the precipitation of Fe4N occurs. These results are in contrast to Fe−C data. Carbon atoms in retained austenite tend to be far apart in their octahedral sites, and this nonrandom distribution is inherited by the virgin martensite. Fe−C austenite decomposes by the formation of ∈ carbide below 160°C and precipitation of Fe3C above 180°C. The carbon atoms in virgin martensite agglomerate at room temperature and regions of ordered Fe4C are believed to result. Subsequently ∈ carbon is formed at 80°C and Fe3C precipitates above 160°C.1

MOSSBAUER EFFECT IN IRON-CARBON AND IRON-NITROGEN ALLOYS.

Abstract Interstitial solid solutions of carbon and nitrogen in iron have been analyzed by Mossbauer spectroscopy. In both allotropie forms, austenite (gamma) and martensite (alpha), electric quadrupole effects have been observed, arising from a repopulation of electrons among the d-orbitals of iron atoms adjacent to the interstitial atoms. In carbon-austenite, ¦e 2 q Q 2 ¦=0.625 mm/sec for iron atoms which are first neighbors of carbon atoms. In nitrogen-austenite, the quadrupole effects are smaller but an appreciable positive isomer shift is observed for iron atoms which are first neighbors of nitrogen atoms. This suggests that these iron atoms have more d-electrons due to some covalent bonding with nitrogen as exists in the isomorphous Fe4N phase. Carbon shows a positive and nitrogen a negative ion behavior in both martensite and in austenite. The c-axis is a direction of difficult magnetization in both Fe-C and Fe-N martensites. In carbon-martensite, two six-peak spectra are clearly resolved: a high intensity spectrum, with an unresolved fine structure, and an average effective field slightly larger than in α-iron; and a low intensity spectrum, with a quadrupole effect of ¦e 2 q Q 2 ¦=− 0.8 mm/sec and an effective field 80% that of the main spectrum. This low-intensity spectrum, which disappears on tempering, is attributed to the two iron nuclei pushed apart along the c-axis by the carbon atom. In nitrogen-martensite, the same low-intensity spectrum appears, though not as well resolved, and shows an effective field larger than in carbon martensite and a quadrupole effect of opposite sign. The indications are that a strong σ bond develops between carbon and iron along the “dipole distortion” but not between nitrogen and iron. This is in agreement with estimates from crystal field theory, since for the two iron atoms of the “dipole” the dz2 orbital should be in the lowest state in carbon-martensite, but not in nitrogen-martensite.

Mössbauer measurements on the aging of iron-carbon martensite

Abstract Mossbauer spectroscopy data, analyzed directly in terms of difference-spectra, indicate that in a high-carbon iron-base alloy (1.86 wt.%C) virgin martensite ages at room temperature by agglomeration of carbon atoms into higher carbon regions, in which two different iron sites are evidenced (with magnetic moments of ~2μB and ~3μB), consistent with the formation of an ordered atomic arrangement structurally similar to Fe4N. Further aging at 82°C leads to continued agglomeration of carbon atoms, at the expense of those arrangements present after 3 months of room-temperature aging. An additional Mossbauer spectrum, developed after 62 h at 82°C and characterized by a hyperfine field of 240 kOe. may be due to e-carbide. In the virgin martensite itself, three separate iron-atom spectra can be distinguished, and are interpreted as being due to first-, second- and third-nearest neighbors of carbon interstitials (with hyperfine fields of 280, 334 and 364 kOe, respectively). The data also indicate that the carbon atoms are widely separated, rather than randomly arranged, in the virgin martensite as well as in the retained austenite.

Stress-Induced Shape Changes and Shape Memory in the R and Martensite Transformations in Equiatomic NiTi

Equiatomic NiTi wire was cooled below TR, the critical temperature for theBR transition, and then stressed in situ in a custom-built X-ray diffraction stage to study the shape memory phenomenon. Tensile stressing the specimen causes a shifting of X-ray intensity from lR to -lR. This is rationalized in terms of domain growth under external stress, resulting in a preferred arrangement of domains in theR -phase, which dimensionally accommodates the applied force. A recoverable strain of ~1.37 pet is accommodated by theR-phase, 0.56 pct of which is recovered immediately after the stress is removed and the remaining 0.81 pct on heating to above TR. This amount of strain is rationalized in terms of the difference in d-spacings and multiplicity factors between {111}R and {-111}R. The springback is primarily associated with the reversal of domain alignment while the shape memory on heating is primarily due to the return of the phase to cubic symmetry. Straining beyond 1.37 pct induces stress-assisted marteniste formation up to 5.47 pct Δl/l, the maximum strain achieved in this series of experiments. This results in a second stage of shape recovery on heating through theA, — Af temperature range. Only 15 vol pct of stress-assisted martensite accounts for nearly all of the additional ~4 pct change in Δl/l. This emphasizes the important role of the martensitic transformation in achieving large changes in macroscopic length in the shape memory phenomenon.

Mössbauer spectroscopy of hexagonal iron-nitrogen alloys

In Fe−N alloys, the hexagonal-close-packed phase can be completely retained metastably at room temperature by rapid quenching from 700°C, with nitrogen contents ranging from about 17 to 27 at. pct N; (between the latter composition and 33 at. pct N, the hexagonal phase is stable at room temperature). The phase is ferromagnetic; the Curie temperature is a sharp function of nitrogen content, with the maximum Curie point (about 300°C) occurring at 24 at. pct N. The Curie point is below room temperature in the hexagonal phase for nitrogen contents less than about 17.5 at. pct N. For alloys of the Fe3N composition quenched from various temperatures, Mössbauer spectroscopy indicates that the hexagonal phase undergeos ordering of nitrogen atoms on interstitial sites.

Structure of Amorphous Silicon Monoxide

The pair distribution function in amorphous deposited silicon monoxide has been measured at room temperature. The result is not in good agreement with expectations for a mixture of well‐defined amorphous silicon and amorphous SiO2. Other possibilities, for which the average number of nearest‐neighbor Si–O bonds per atom deviates significantly from 2, can be ruled out. A ring‐type structure, consistent with the diffraction results, and also (apparently) with optical data, is suggested as the most likely alternative.

The Kinetics of Carbon Clustering in Martensite.

The rate of clustering of carbon atoms into regions of ordered Fe4C has been studied by Mössbauer spectroscopy. The activation energy associated with this process is 89,500 ± 12,000 J per mole (21.4 kcal per mole).

The distinctness of magnetic hyperfine fields associated with structurally unique iron sites in Fe3B

Mössbauer spectra were measured of Fe3B, which is isostructural to orthorhombic Fe3C. The metastable Fe3B compound was formed by splat cooling Fe-1.5 wt pct B alloys. Two hyperfine spectra are found, corresponding to effective fields of 235 KOe for one type of iron atom and 264 KOe for the other. Isomer shifts were positive with respect to α-iron, also present in the sample.