Beyond metals: theoretical discovery of semiconducting MAX phases and their potential application in thermoelectrics.

MAX phase is a family of ceramic compounds, typically known for their metallic properties. However, we show here that some of them may be narrow bandgap semiconductors. Using a series of first-principles calculations, we have investigated the electronic structures of 861 dynamically stable MAX phases. Notably, Sc2SC, Y2SC, Y2SeC, Sc3AuC2, and Y3AuC2 have been identified as semiconductors with band gaps ranging from 0.2 to 0.5 eV. Furthermore, we have assessed the thermodynamic stability of these systems by generating ternary phase diagrams utilizing evolutionary algorithm techniques. Their dynamic stabilities are confirmed by phonon calculations. Additionally, we have explored the potential thermoelectric efficiencies of these materials by combining Boltzmann transport theory with first-principles calculations. The relaxation times are estimated using scattering theory. The zT coefficients for the aforementioned systems fall within the range of 0.5 to 2.5 at temperatures spanning from 300 to 700 K, indicating their suitability for high-temperature thermoelectric applications.

Susceptibility of Patients with Inflammatory Bowel Disease to COVID-19 Compared with Their Households.

BACKGROUND: Immunosuppressive agents used in the treatment of inflammatory bowel diseases (IBDs) could potentially increase the risk of coronavirus disease 2019 (COVID-19). We aimed to compare COVID-19 frequency in patients with IBD with their households and identify the related risk factors. METHODS: Firstly, a multi-centered, observational study on 2110 patients with IBD and 2110 age-matched household members was conducted to compare COVID-19 frequency. Secondly, the data of patients with IBD and COVID-19 who had called the COVID-19 hotline were added. Multivariable logistic regression was used to evaluate the effect of age, type and severity of IBD, the number of comorbidities, and medications on the frequency of COVID-19 among the patients with IBD. RESULTS: The prevalence of COVID-19 in patients with IBD and household groups was similar (34 [1.61%] versus 35 [1.65%]; P = 0.995). The prevalence of COVID-19 increased from 2.1% to 7.1% in those with three or more comorbidities (P = 0.015) and it was significantly higher in those with severe IBD (P = 0.026). The multivariable analysis only showed a significant association with anti-TNF monotherapy (OR: 2.5, CI: 0.97-6.71, P = 0.05), and other medications were not associated with COVID-19. CONCLUSION: The prevalence of COVID-19 in patients with IBD was similar to the household members. Only patients with IBD receiving anti-TNF monotherapy had a higher risk of COVID-19 susceptibility. This finding could be attributed to the higher exposure to the virus during administration in health care facilities.

Invasion front dynamics in disordered environments.

Invasion occurs in environments that are normally spatially disordered, however, the effect of such a randomness on the dynamics of the invasion front has remained less understood. Here, we study Fisher's equation in disordered environments both analytically and numerically. Using the Effective Medium Approximation, we show that disorder slows down invasion velocity and for ensemble average of invasion velocity in disordered environment we have [Formula: see text] where [Formula: see text] is the amplitude of disorder and [Formula: see text] is the invasion velocity in the corresponding homogeneous environment given by [Formula: see text]. Additionally, disorder imposes fluctuations on the invasion front. Using a perturbative approach, we show that these fluctuations are Brownian with a diffusion constant of: [Formula: see text]. These findings were approved by numerical analysis. Alongside this continuum model, we use the Stepping Stone Model to check how our findings change when we move from the continuum approach to a discrete approach. Our analysis suggests that individual-based models exhibit inherent fluctuations and the effect of environmental disorder becomes apparent for large disorder intensity and/or high carrying capacities.

Achieving an Ultrahigh Power Factor in Sb2Te2Se Monolayers via Valence Band Convergence.

An efficient approach to improve the thermoelectric performance of materials is to converge their electronic bands, which is known as band engineering. In this regard, lots of effort has been made to further improve the thermoelectric efficiency of bulk and exfoliated monolayers of Bi2Te3 and Sb2Te3. However, ultrahigh band degeneracy and thus significant improvement of the power factor have not yet been realized in these materials. Using first-principles methods, we demonstrate that the valley degeneracy of Bi2Te3 and Sb2Te3 can be largely improved upon substitution of the middle-layer Te atoms with the more electronegative S or Se atoms. Our detailed analysis reveals that in this family of materials, two out of four possible valence band valleys merely depend on the electronegativity of the middle-layer chalcogen atoms, which makes the independent modulation of the valleys' position feasible. As such, band alignment of Bi2Te3 and Sb2Te3 largely improves upon substitution of the middle-layer Te atoms with more electronegative, yet chemically similar, S and Se ones. A superior valence band alignment is attained in Sb2Te2Se monolayers where three out of four possible valleys are well aligned, resulting in a giant band degeneracy of 18 that holds the record among all thermoelectric materials. As a result, an outstanding power factor for the hole-doped monolayers is achieved, indicating a highly efficient p-type thermoelectric material.

Helicobacter pylori colonization in dental plaque and gastric infection.

BACKGROUND: With regard to the presence of Helicobacter pylori (H.pylori ) in dental plaque and its association with gastric infection, data from the literature are controversial. The aim of this study was to determine the prevalence of H.pylori in dental plaque of dyspeptic patients who underwent upper gastrointestinal endoscopy and to investigate its relationship to gastric infection. METHODS: In a cross sectional study, over one year ( February 2002-February 2003) samples were taken with separate sterile curretes from tooth surfaces of molars, premolars and incisors of 180 consecutive dyspeptic patients referred to the Endoscopy Department of Emmam Khomeini Hospital. Dental plaque samples were subjected to the rapid urease test, Giemsa staining, PCR test and culture for identification of H.pylori. During endoscopy, six biopsies were taken from the gastric antrum and corpus and histological examination, Giemsa stainings, Rapid Urease test and culture were performed. Patients were defined as infected when both rapid urease test and Giemsa stain, or when cultures were positive for H.pylori. Information concerning oral hygiene protocol were recorded. For data analysis, the chi square test was used and P < 0.05 was defined statistically significant. RESULTS: Our data proved the presence of H. pylori in dental plaque of infected and uninfected patients. There was no significant association between H. pylori colonization in dental plaque and gastric infection. In this study, oral hygiene (the frequency of dental visits and teeth cleaning) did not have a significant influence on the presence of H. pylori in dental plaque. CONCLUSION: Dental plaque is the reservoir of H.pylori with no relationship to gastric infection.