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Changjun Zhang

Publications and source records attributed to Changjun Zhang.

10 recordsLinked to original sources
PubMed2006

A first-principles investigation of LiNH(2) as a hydrogen-storage material: effects of substitutions of K and Mg for Li.

Li-N-H compounds hold promise as novel hydrogen-storage materials with high gravimetric hydrogen densities. Because the dehydriding reaction caused by the decomposition of LiNH(2) requires a higher temperature than desired, much effort has been devoted to the destabilization of LiNH(2) to decrease the decomposition temperature. In particular, there has been recent experimental evidence for lowering the temperature by partial substitution of Li by Mg. However, the reason is not clear. In this study, we have employed density functional theory to investigate LiNH(2) and partially Li-substituted systems aiming to understand the effects of the substitution on the destabilization of the NH(2) species. K, a more electropositive element, and Mg, a more electronegative element, have been chosen as two probes to illustrate the effects. We have focused on the investigation of effects of substitutions on the N-H bond strength that is regarded as a qualitative indicator of the decomposition temperature. We have found that in both cases the N-H bonds are weakened, in particular, the Mg substitute appears to be more effective in the destabilization of the NH(2). The relative strength of the metal-N ionic bonding has been found to be a key factor to explain the effects of the substitutes. These have been discussed in detail in terms of Wannier function analyses.

Journal Article↗
PubMed2005

Quantum delocalization of hydrogen in the Li2NH crystal.

By mapping out potential energy surfaces from density-functional theory (DFT) and solving a protonic Schrödinger equation, we find that the H atom in a unit cell of the Li2NH crystal shows remarkably strong quantum behavior, leading to the delocalization of H over six octahedral sites around each N. This can be rationalized in terms of rapid coherent tunneling among these equivalent octahedral sites. Structural and dynamical consequences of our finding are discussed. Since the Li-N-H compounds are considered promising candidates for H-storage, understanding of these fundamental properties will be useful toward improving the performance of the material.

Journal Article↗
PubMed2005

First-principles study of superabundant vacancy formation in metal hydrides.

Recent experiments have established the generality of superabundant vacancies (SAV) formation in metal hydrides. Aiming to elucidate this intriguing phenomenon and to clarify previous interpretations, we employ density-functional theory to investigate atomic mechanisms of SAV formation in fcc hydrides of Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au. We have found that upon H insertion, vacancy formation energies reduce substantially. This is consistent with experimental suggestions. We demonstrate that the entropy effect, which has been proposed to explain SAV formation, is not the main cause. Instead, it is the drastic change of electronic structure induced by the H in the SAV hydrides, which is to a large extent responsible. Interesting trends in systems investigated are also found: ideal hydrides of 5d metals and noble metals are unstable compared to the corresponding pure metals, but the SAV hydrides are more stable than the corresponding ideal hydrides, whereas opposite results exist in the cases of Ni, Rh, and Pd. These trends of stabilities of the SAV hydrides are discussed in detail and a general understanding for SAV formation is provided. Finally, we propose an alternative reaction pathway to generate a SAV hydride from a metal alloy.

Journal Article↗
PubMed2004

A density functional theory study of the coadsorption of water and oxygen on TiO2(110).

The behavior of adsorbed water on oxides is of fundamental interest in many areas. Despite considerable attention received recently, our understanding of water chemistry is still short of needs and expectations, particularly on the topic of the coadsorption of water and other species. In this study we carry out density functional theory calculations to investigate the coadsorption of water and oxygen on the TiO(2)(110) surface. We show that oxygen exerts profound influences on the water adsorption, altering the mechanism of water dissociation. On the one hand, the possible dissociation route along [-110] is prohibited due to the weakening of the H bond between water and the lattice bridging oxygen in the presence of the coadsorbed oxygen, and on the other hand the coadsorbed oxygen induces dissociation along [001]. These results lead to a consistent interpretation of experiments. Furthermore, several possible final states and the related formation mechanisms are discussed in detail.

Journal Article↗
PubMed2004

The interdependence of defects, electronic structure and surface chemistry.

In this article we present three diverse applications of first-principles simulations to problems of materials chemistry and chemical physics. Their common characteristic is that they are essentially problems of the relationships among atomic structures and the properties they promote in real materials and real applications. The studies are on transition-metal oxide surface chemistry, the reactivity and electronic structure of sp(2)-bonded carbon systems, and defects and electrochromic properties in WO(3). In these demanding applications we must have concern for how realistic our model systems are and how well current implementations of DFT perform, and we comment on both.

Journal Article↗
PubMed2002

Gestational diabetes mellitus in women of single gravidity in Tianjin City, China.

OBJECTIVE: The aim of this study was to investigate the prevalence of gestational diabetes mellitus (GDM) and risk factors for the development of GDM in pregnant women in Tianjin, China, where the prevalence of GDM is still unknown. STUDY DESIGN AND METHODS: A total of 9,471 pregnant women living in the six urban districts of Tianjin, China, took part in the initial screening between December 1998 and December 1999. The screening test consisted of a 50-g 1-h glucose test. Women with a reading > or =7.8 mmol/l at the initial screening were invited to undergo a 75-g 2-h oral glucose tolerance test (OGTT). GDM was confirmed using the World Health Organization's diagnostic criteria. RESULTS: At the initial screening test, 888 women had a glucose reading of > or =7.8 mmol/l. A total of 701 (79%) women took a subsequent OGTT. Of these, 174 women were confirmed to have GDM (154 with impaired glucose tolerance [IGT] and 20 with diabetes). The prevalence of GDM was 2.31% (2.03% for IGT and 0.28% for diabetes), adjusting for serum glucose levels at the initial screening test. Independent predictors for GDM were maternal age, stature, prepregnancy BMI, weight gain in pregnancy before screening, diabetes in first-degree relatives, and habitual cigarette smoking during pregnancy. Women who smoked or had a short stature are more likely to develop GDM than their counterparts. CONCLUSIONS: The prevalence of GDM in pregnant women in the city of Tianjin, China, was 2.31%. Short stature and smoking in pregnancy were additional risk factors for GDM.

Adult↗
PubMed2002

Women with impaired glucose tolerance during pregnancy have significantly poor pregnancy outcomes.

OBJECTIVE: This article tests the hypothesis that women with impaired glucose tolerance (IGT) have the same pregnancy outcomes as those of their counterparts with normal glucose tolerance. RESEARCH DESIGN AND METHODS: From December 1998 to December 1999, 84 of 90 antenatal care base units (ACBUs) under the Tianjin Antenatal Care Network in China participated in the first screening program for gestational diabetes mellitus (GDM). A total of 9,471 pregnant women under the care of participating ACBUs were screened. Of the women screened, 154 were positive for IGT. Of the 154 women, 102 opted for conventional obstetric care. The comparison group was 302 women of normal glucose tolerance (NGT). The initial screening consisted of a 50-g 1-h glucose test, and was carried out at 26-30 gestational weeks. Women with a serum glucose > or =7.8 mmol/l were followed up with a 75-g 2-h oral glucose tolerance test. The World Health Organization's diagnostic criteria for GDM were used. RESULTS: Women with IGT were at increased risk for premature rupture of membranes (P-ROM) (odds ratio [OR] 10.07; 95% CI 2.90-34.93); preterm birth (6.42; 1.46-28.34); breech presentation (3.47; 1.11-10.84); and high birth weight (90th percentile or 4,000 g) (2.42; 1.07-5.46); adjusting for maternal age, pregravid BMI, hospital levels, and other confounding factors. CONCLUSIONS: The presence of IGT in pregnancy is predictive of poor pregnancy outcomes.

Adult↗
PubMed2002

[Mutation analysis of Cx26 gene in Chinese hereditary nonsyndromic deafness sufferers].

OBJECTIVE: To analyze the mutations in the code region of Cx26 gene in Chinese hereditary nonsyndromic hearing impairment (NSHI) sufferers. METHODS: Thirty-three cases (29 cases in the families of 8 students who were picked out from the Deafness and Muteness School of Tianjin, 2 cases as control and 2 normal cases of genetic counseling) were included in this study. The blood samples were obtained to distill the DNA templates. Using polymerase chain reaction (PCR), the code region of Cx26 gene was amplified. The mutations were screened by restriction endonucleases fingerprinting-single strand conformation polymorphism (REF-SSCP). Afterwards we inspected the polymorphous changes or mutations of these segments with DNA sequence. RESULTS: There were 30 cases with the nucleotide changes in the Cx26 code region. The rate was 90.9% (30/33). Eight kinds of mutations were found, 79G-->A, 109G-->A, 161A-->T, 235delC, 240G-->A, 341A-->G, 571T-->C and 608T-->C. 161A-->T, 240G-->A and 571T-->C were detected primarily. There were 3 cases with 235delC in 22 deafness sufferers and the rate was 13.64% (3/22). CONCLUSIONS: The 235delC of Cx26 gene is the main mutation in Chinese NSHI sufferers, and in NSHI cases many polymorphous changes exist.

Adolescent↗