ABSTRACT
ZnO nanoparticles (NPs) have great potential for their use in, e.g., thin film solar cells due to their electro-optical properties adjustable on the nanoscale. Therefore, the production of well-defined NPs is of major interest. For a targeted production process, the knowledge of the stabilization layer of the NPs during and after their formation is of particular importance. For the study of the stabilizer layer of ZnO NPs prepared in a wet chemical synthesis from zinc acetate, only ex situ studies have been performed so far. An acetate layer bound to the surface of the dried NPs was found; however, an in situ study which addresses the stabilizing layer surrounding the NPs in a native dispersion was missing. By the combination of small angle scattering with neutrons and X-rays (SANS and SAXS) for the same sample, we are now able to observe the acetate shell in situ for the first time. In addition, the changes of this shell could be followed during the ripening process for different temperatures. With increasing size of the ZnO core (d(core)) the surrounding shell (d(shell)) becomes larger, and the acetate concentration within the shell is reduced. For all samples, the shell thickness was found to be larger than the maximum extension of an acetate molecule with acetate concentrations within the shell below 50 vol %. Thus, there is not a monolayer of acetate molecules that covers the NPs but rather a swollen shell of acetate ions. This shell is assumed to hinder the growth of the NPs to larger macrostructures. In addition, we found that the partition coefficient µ between acetate in the shell surrounding the NPs and the total amount of acetate in the solution is about 10% which is in good agreement with ex situ data determined by thermogravimetric analysis.
ABSTRACT
Although carotid endarterectomy is the standard procedure, the purpose of this paper is to describe a new surgical technique and indicate its role in the surgery of cervical cerebrovascular disease. The technique consists of resection of the pathologic carotid artery bifurcation followed by its total replacement with a bifurcated thin-wall polytetrafluoroethylene (PTFE) graft constructed by the surgeon. Prosthetic graft reconstruction of the carotid bifurcation (PGRCB) was performed in 21 patients (25 procedures) from November 1984 to May 1987. Many patients were over 70 years old. The indications for surgery were transient ischemic attacks (TIA)'s in 10 patients (high-grade stenosis), established stroke with mild deficit in 3 patients (high-grade stenosis). Eight patients (high-grade stenosis) were asymptomatic and 4 had vertebro-basilar symptoms. In 11 procedures the indications were the same as for standard carotid endarterectomy (significant stenosis and/or ulcerated plaque). In 14 cases, the procedure was justified by extensive lesions involving a long segment of the internal carotid artery (2 restenoses, 12 calcified lesions). In the perioperative period, there were no deaths and no TIA's. The 25 procedures were assessed one month postoperatively by ultrasonic imaging and venous substraction angiography. All the grafts were patent with perfect restoration of the carotid bifurcation anatomy. Systematic follow-up study was carried out in 1987. Three patients had died of non-neurologic causes and one was lost to study. The remaining 19 procedures were evaluated with an average follow-up of 19 months (range 7 to 32 months). The patients were asymptomatic and all the grafts remained patent including one stenosis (greater than 50%) at the common carotid artery anastomosis. Because recent studies have documented a much higher rate of technical defects or recurrent stenosis after carotid endarterectomy, we believe that PGRCB merits consideration, particularly in difficult endarterectomies, and in recurrent carotid stenosis. Furthermore it seems acceptable to advocate PGRCB in aneurysms of the internal carotid artery and in post-irradiation arteritis.
