Molecular Dynamics Study of the Structural Modification of Porous Silica from Low-Energy Recoils

Molecular dynamics simulations are performed to investigate the effects of low-energy recoils on the microscopic structure of porous silica. Exhibiting a logistic growth with the recoil energy, the displacement probability of Si is shown to be smaller than that of O at the same primary knock-on level. Computations of pair distribution functions and bond angle distributions reveal that this material upon irradiation with energies around the displacement thresholds mainly undergoes structural changes in the medium-range order. In the porous network,while the formation of nonbridging oxygen defects tends to induce shorter Si–O bonds than those formed by bridging oxygen atoms, a remarkable increase of inter-tetrahedral bond angles created by multiple recoils can be observed and associated with the rearrangement of ring statistics.

ACL reconstruction with BPTB autograft and irradiated fresh frozen allograft

Objective： To analyze the clinical outcomes of arthroscopic anterior cruciate ligament （ACL） reconstruction with irradiated bone-patellar tendon-bone （BPTB） allograft compared with non-irradiated allograft and autograft. Methods： All BPTB allografts were obtained from a single tissue bank and the irradiated allografts were sterilized with 2.5 mrad of irradiation prior to distribution. A total of 68 patients undergoing arthroscopic ACL reconstruction were prospectively randomized consecutively into one of the two groups （autograft and irradiated allograft groups）. The same surgical technique was used in all operations done by the same senior surgeon. Before surgery and at the average of 31 months of follow-up （ranging from 24 to 47 months）, patients were evaluated by the same observer according to objective and subjective clinical evaluations. Results： Of these patients, 65 （autograft 33, irradiated allograft 32） were available for full evaluation. When the irradiated allografl group was compared to the autograft group at the 31 -month follow-up by the Lachman test, the anterior drawer test （ADT）, the pivot shift test, and KT-2000 arthrometer test, statistically significant differences were found. Most importantly, 87.8% of patients in the autograft group and just only 31.3% in the irradiated allograft group had a side-to-side difference of less than 3 mm according to KT-2000. The failure rate of the ACL reconstruction with irradiated allograft （34.4%） was higher than that with autograft （6.1%）. The anterior and rotational stabilities decreased significantly in the irradiated allograft group. According to the overall International Knee Documentation Committee （1KDC）, functional and subjective evaluations, and activity level testing, no statistically significant differences were found between the two groups. Besides, patients in the irradiated allograft group had a shorter operation time and a longer duration of postoperative fever. When the patients had a fever, the laboratory examinations of all patients were almost normal. Blood routine was normal, the values of erythrocyte sedimentation rate （ESR） were 5-16 mm/h and the contents of C reactive protein （CRP） were 3-10 mg/L. Conclusion： We conclude that the short term clinical outcomes of the ACL reconstruction with irradiated BPTB allograft were adversely affected. The less than satisfactory results led the senior authors to discontinue the use of irradiated BPTB allografl in ACL surgery and not to advocate using the gamma irradiation as a secondary sterilizing method.

Dielectric thin films for GaN-based high-electron-mobility transistors

The effects of dielectric thin films on the performance of GaN-based high-electron-mobility transistors （HEMTs） were reviewed in this work. Firstly, the nonpolar dielectric thin films which act as both the surface passivation layers and the gate insulators of the high-frequency GaN-based high-electron-mobility transistors were presented. Furthermore, the influences of dielectric thin films on the electrical properties of two-dimensional electron gas （2DEG） in the A1GaN/GaN hetero-structures were ana- lyzed. It was found that the additional in-plane biaxial tensile stress was another important factor besides the change in surface potential profile for the device perfor- mance improvement of the A1GaN/GaN HEMTs with dielectric thin films as both passivation layers and gate dielectrics. Then, two kinds of polar gate dielectric thin films, the ferroelectric LiNbO3 and the fluorinated A1203, were compared for the enhancement-mode GaN-based HEMTs, and an innovative process was proposed. At last, high-permittivity dielectric thin films were adopted as passivation layers to modulate the electric field and accordingly increase the breakdown voltage of GaN-based HEMTs. Moreover, the polyimide embedded with Cr particles effectively increased the breakdown voltage of GaNbased HEMTs. Finally, the effects of high-permittivity dielectric thin films on the potential distribution in the drift region were simulated, which showed an expanded electric field peak at the drain-side edge of gate electrode.