Phase Diagrams and Defect Thermodynamics to Devise Doping Strategies in Lead Chalcogenide Thermoelectric Materials and its Alloys

This thesis discusses the application of phase diagrams and the associated thermodynamics to semiconductor materials through theoretical computational calculations. The majority of work is focused on thermoelectric semiconducting materials that enable direct inter-conversion between electrical and thermal energy. First, one of the most efficient thermoelectric material, PbTe, is picked to demonstrate the assessment of unknown phase diagrams by combining two methods - DFT and CALPHAD. Since there had been no previous investigations of defect stability in this material using computations, DFT is used to deduce the stability of various intrinsic point defects, and in turn attribute origins of n- and p-type conductivity to the most stable defects. Then, the calculated defect formation energies are used in the Pb-Te thermodynamic model built using the CALPHAD method to compare the estimated solubility lines and non-stoichiometric range of the PbTe phase with experimental data. Next, another lead chalcogenide, PbSe, is picked to explore the phase stability of the PbSe phase upon the addition of dopants (Br, Cl, I, Na, Sb, Bi, In), which is a common strategy to make thermoelectric materials and devices more efficient. The range of efficiencies and thermoelectric properties as functions of composition and temperature that can be achieved depends on the amount of dopant that can be added without precipitating secondary phases. Also, depending on the system and its phase diagram, there can be more than one way of doping a material. To help detail which method(s) of doping into PbSe will result in maximum dopant solubility, a procedure similar to the above for PbTe is followed by using DFT in combination with Boltzmann statistics to map solvus boundaries of the PbSe phase, but now in the ternary phase space of composition and temperature. This method also helps predict electrical conductivity, n- or p-type, in each region of the phase diagrams that represent different doping methods. Lastly, the role of surface energy contributions in changing phase stability at nano-dimensions is explored. The CALPHAD approach is employed to investigate these changes in three systems by calculating their phase diagrams at nano dimensions and comparing them with their bulk counterparts.</p

Phase Stability and Thermodynamic Assessment of the Np-Zr system

Metallic fuels have an important role to play in "fast breeder" Gen-IV type nuclear reactors, and U-Pu-Zr is one of the prototypical systems. Because of the variability in fuel chemistry during burn-up, it is important to understand the effect of minor actinides and fission products on phase stability. Within this framework, we present a study on phase equilibria in the binary Np-Zr alloy system on which little work has been published. To resolve the contradictory reports on the ordering/ clustering trends of the bcc phase, a thermodynamic study is performed using the CALPHAD method. The calculated Np-Zr phase diagram is consistent with two sets of data: formation enthalpies of the bcc phase that are calculated with ab initio KKR-ASA-CPA electronic-structure method and lattice stabilities of solution phases obtained from first-principles technique. Another important feature in the Np-Zr alloy system is the non-stoichiometric delta-NpZr2 phase that forms in a hexagonal C32 structure similar to the delta-phase in the U-Zr system and the w-phase in pure Zr. An increase in the homogeneity range of the delta-phase when going from Pu to Np and to U is attributed to a lowering of its heat of formation that is caused by an increase in d-band occupation. Two different possibilities for the stability of the delta- and w- phases have been proposed in the present work. Additionally, calculated changes in enthalpy versus temperature are plotted for two alloy compositions of the Np-Zr system to guide future experimental work in resolving important issues in this system. Finally, an ab initio study, implemented with the L(S)DA U formalism, is performed for pure Np that reveals a transition from a non-magnetic to a magnetic state at a critical U parameter

Calculation of dopant solubilities and phase diagrams of X–Pb–Se (X = Br, Na) limited to defects with localized charge

The control of defects, particularly impurities, to tune the concentrations of electrons and holes is of utmost importance in the use of semiconductor materials. To estimate the amount of dopant that can be added to a semiconductor without precipitating secondary phases, a detailed phase diagram is needed. The ability of ab initio computational methods to predict defect stability can greatly accelerate the discovery of new semiconductors by calculating phase diagrams when time-consuming experimental ones are not available. DFT defect energy calculations are particularly successful in identifying doping strategies by determining the energy of multiple defect charge states in large band gap semiconductors and insulators. In metals, detailed phase diagrams can be determined from such calculations but only one, uncharged defect is needed. In this work, we have calculated dopant solubilities of Br and Na in the thermoelectric material PbSe by mapping its solvus boundaries in different regions of the respective ternary phase diagrams using DFT defect energy calculations. The narrow gap PbSe provides an example where defects with nominal charge state (based on valence counting) have properly-localized charge states. However, defects with unexpected charge states produce delocalized electrons, which are then, in effect, defects with the expected charge state. Simply applying the methods for calculating multiple defect charge states in PbSe and treating them as separate defects fails to predict properties measured by experiments. Performing thermodynamic calculations using only the expected charge states, excluding others, enables accurate prediction of experimentally measured doping efficiencies and phase diagrams. Identifying which defect charge states to include in thermodynamic calculations will expedite the use of such calculations for other semiconductors in understanding phase diagrams and devising effective doping strategies

Phase stability in nanoscale material systems: extension from bulk phase diagrams

Phase diagrams of multi-component systems are critical for the development and engineering of material alloys for all technological applications. At nano dimensions, surfaces (and interfaces) play a significant role in changing equilibrium thermodynamics and phase stability. In this work, it is shown that these surfaces at small dimensions affect the relative equilibrium thermodynamics of the different phases. The CALPHAD approach for material surfaces (also termed “nano-CALPHAD”) is employed to investigate these changes in three binary systems by calculating their phase diagrams at nano dimensions and comparing them with their bulk counterparts. The surface energy contribution, which is the dominant factor in causing these changes, is evaluated using the spherical particle approximation. It is first validated with the Au–Si system for which experimental data on phase stability of spherical nano-sized particles is available, and then extended to calculate phase diagrams of similarly sized particles of Ge–Si and Al–Cu. Additionally, the surface energies of the associated compounds are calculated using DFT, and integrated into the thermodynamic model of the respective binary systems. In this work we found changes in miscibilities, reaction compositions of about 5 at%, and solubility temperatures ranging from 100–200 K for particles of sizes 5 nm, indicating the importance of phase equilibrium analysis at nano dimensions

Diabetes changes the outcome of tuberculosis?

Background: Diabetes has become a global epidemic affecting children, adolescents, and adults. It is recognized as a group of heterogeneous disorders with the common elements of hyperglycemia and glucose intolerance, due to insulin deficiency, impaired effectiveness of insulin action, or both. Diabetes mellitus (DM) is classified on the basis of etiology and clinical presentation of the disorder into four types: type 1 diabetes, type 2 diabetes, gestational diabetes, and other specific types. Failures, deaths, relapse rates and favorable outcomes (cured/treatment completed) were comparable in pulmonary tuberculosis (TB) patients with or without DM. It is also documented that in well-controlled diabetes the course of pulmonary tuberculosis is not different from that in patients without diabetes.Methods: Diabetic patients visiting the outpatient department/diabetic clinic of our facility were enrolled after taking written informed consent. The data on socio-demographic and diabetic parameters and examination findings were recorded on proforma as attached.Results: X-ray findings at start of treatment showed that proportion of patient of group I was higher than group II in left site (26.00% versus 8.00%) and proportion of patient of group II was higher than group I in right site (58.00% versus 48.00%) and bilateral (34.00% versus 26.00%), though left side was affected in higher proportion of group I patients as compared to group II but this difference was not found to be statistically significant.Conclusions: Our study concluded that even though the state in which patient presented that is diabetic or non-diabetic the outcome of treatment didn’t change but the earlier one was more associated with complications and also the healing took more time in patients with diabetes

Unravelling the unseen ‘C’ with the cone beam computed tomography: a rare case report

Recognition of aberrant root canal configurations is critical to successfully negotiate and treat a canal and one of such aberrancies is a C shape root canal system. The fins and webs present in a C shape canal system presents a challenge to debridement and obturation. Knowledge of variations through advanced imaging modalities like CBCT, rotary and hand instruments assisted with ultrasonics and modified obturation techniques aid in effective management of C shaped root canals. This case report presents a successful endodontic management of a rare c shape canal configuration in a mandibular second premolar with the aid of the cone‑beam computed tomography (CBCT) scanning imaging technique

p-Type Co Interstitial Defects in Thermoelectric Skutterudite CoSb_3 Due to the Breakage of Sb_4-Rings

Skutterudite CoSb_3 based thermoelectric devices have high potential for engineering applications because both n- and p-type doped CoSb_3 demonstrate excellent thermoelectric performance. A crucial point concerning the application of CoSb_3 is to understand and control its defect chemistry. To reveal the native conductivity behavior of nonstoichiometric CoSb_3, we investigated the intrinsic point defects in CoSb_3 using density functional theory. We found CoSb_3 is p-type in either Co or Sb rich regions of phase stability. Interstitial Co (Co_i) and interstitial Co-pair (Co_(i-p)) are the dominant point defects in the Co rich region. However, Co_(i-p) will be difficult to form because the formation temperature of Co_(i-p) is much lower than the synthesis temperature of CoSb_3. The unexpected acceptor nature of the Co_i or Co_(i-p) defects is explained by the breakage of multiple Sb_4-rings. Co vacancy (Co_v) is found to be the p-type defect in the Sb rich region. Furthermore, the solubility of excess Co in CoSb_3 is expected to be larger than that of Sb because of the lower formation energy and higher carrier concentration of Co_i compared with those of Co_v

p-Type Co Interstitial Defects in Thermoelectric Skutterudite CoSb_3 Due to the Breakage of Sb_4-Rings

Skutterudite CoSb_3 based thermoelectric devices have high potential for engineering applications because both n- and p-type doped CoSb_3 demonstrate excellent thermoelectric performance. A crucial point concerning the application of CoSb_3 is to understand and control its defect chemistry. To reveal the native conductivity behavior of nonstoichiometric CoSb_3, we investigated the intrinsic point defects in CoSb_3 using density functional theory. We found CoSb_3 is p-type in either Co or Sb rich regions of phase stability. Interstitial Co (Co_i) and interstitial Co-pair (Co_(i-p)) are the dominant point defects in the Co rich region. However, Co_(i-p) will be difficult to form because the formation temperature of Co_(i-p) is much lower than the synthesis temperature of CoSb_3. The unexpected acceptor nature of the Co_i or Co_(i-p) defects is explained by the breakage of multiple Sb_4-rings. Co vacancy (Co_v) is found to be the p-type defect in the Sb rich region. Furthermore, the solubility of excess Co in CoSb_3 is expected to be larger than that of Sb because of the lower formation energy and higher carrier concentration of Co_i compared with those of Co_v

Computational and experimental investigation of TmAgTe_2 and XYZ_2 compounds, a new group of thermoelectric materials identified by first-principles high-throughput screening

A new group of thermoelectric materials, trigonal and tetragonal XYZ_2 (X, Y: rare earth or transition metals, Z: group VI elements), the prototype of which is TmAgTe_2, is identified by means of high-throughput computational screening and experiment. Based on density functional theory calculations, this group of materials is predicted to attain high zT (i.e. ∼1.8 for p-type trigonal TmAgTe_2 at 600 K). Among approximately 500 chemical variants of XYZ_2 explored, many candidates with good stability and favorable electronic band structures (with high band degeneracy leading to high power factor) are presented. Trigonal TmAgTe_2 has been synthesized and exhibits an extremely low measured thermal conductivity of 0.2–0.3 W m^(−1) K^(−1) for T > 600 K. The zT value achieved thus far for p-type trigonal TmAgTe_2 is approximately 0.35, and is limited by a low hole concentration (∼10^(17) cm^(−3) at room temperature). Defect calculations indicate that Tm_(Ag) antisite defects are very likely to form and act as hole killers. Further defect engineering to reduce such XY antisites is deemed important to optimize the zT value of the p-type TmAgTe_2