Validity of asset pricing models in Istanbul Stock Exchange (ISE) information technology index

Statistical models have been created to understand capital assets’ return and risk. In the empirical studies in which these developed models were tested, it was concluded that the models were valid in some periods and some samples, but not in others. In this study, it is aimed to test whether the developed asset pricing models are valid for the stocks in the Borsa Istanbul (Istanbul Stock Exchange – ISE) Information Technology Index. Model tests were carried out with panel data analysis. The data set consists of the monthly returns of 13 companies traded in the ISE Information Technology Index for the period 2013/January-2019/December. Model tests were performed on both portfolio and stock basis. As a result of the tests, it was concluded that CAPM is valid in firm-based studies in the ISE Information Technology Index, and both CAPM and C4F are valid in portfolio-based studies

Assessment of sustained effects of levosimendan and dobutamine on left ventricular systolic functions by using novel tissue Doppler derived indices in patients with advanced heart failure

Background: Previous studies comparing levosimendan vs. dobutamine have revealed that levosimendan is better in relieving symptoms. Echocardiographic studies have been done using second measurements immediately following a dobutamine infusion or while it was still being administered. The aim of our study was assessment of sustained effects of 24 h levosimendan and dobutamine infusions on left ventricular systolic functions. Methods: A total of 61 patients with acutely decompensated heart failure with New York Heart Association (NYHA) class III or IV symptoms were randomized to receive either levosimendan or dobutamine 2:1 in an open label fashion. Before and 5 days after the initiation of infusions, functional class was assessed, N-terminal prohormone of B-type natriuretic peptide (NT-proBNP) levels and left ventricular ejection fraction (LVEF), mitral inflow peak E and A wave velocity, and E/A ratios were measured; using tissue Doppler imaging, isovolumic myocardial acceleration (IVA), peak myocardial velocity during isovolumic contraction (IVV), peak systolic velocity during ejection period (Sa), early (E’) and late (A’) diastolic velocities, and E’/A’ and E/E’ ratios were measured. Results: The NYHA class improved in both groups, but improvements were prominent in the levosimendan group. NT-proBNP levels were significantly reduced in the levosimendan group. Improvements in LVEF and diastolic indices were significant in the levosimendan group. Tissue Doppler-derived systolic indices of IVV and IVA increased significantly in the levosimendan group. Conclusions: Improvements in left ventricular systolic and diastolic functions continue after a levosimendan infusion.

Evaluation of the effect of mitral stenosis severity on the left ventricular systolic function using isovolumic myocardial acceleration

Background: Isovolumic acceleration (IVA) is a new tissue Doppler parameter in the as­sessment of systolic function of both left and right ventricles. It remains unaffected with the changes in pre- and after-load within the physiological range. The aim of our study was to assess the effect of mitral stenosis degree, which is determined by echocardiography, on the left ventricular (LV) function using IVA. Methods: A total number of 62 patients with mitral stenosis (MS) and 32 healthy controls were examined. The severity of MS (mild, moderate, and severe) was determined on the basis of mitral valve area (MVA) and the mean diastolic mitral gradient findings. The peak myocardial velocities during isovolumic contraction, systole, early diastole and late diastole were measured by using tissue Doppler imaging (TDI). Results: All TDI-derived global LV basal wall systolic (peak myocardial isovolumic contra­ction velocity, peak myocardial systolic velocity and IVA), and diastolic velocities (peak early and late diastolic velocities) were significantly decreased in the patients with MS, compared to the healthy patients (p < 0.001, for all). However, IVA was not different when the degree of MS was evaluated (p = 0.114). In addition, IVA was not correlated with the MVA (r = 0.185, p = 0.150). Conclusions: Left ventricular function is impaired in patients with MS regardless of the severity of the disease.

Investigating the Frequency of Stent Fracture and its Impact on in-Stent Restenosis in Patients Undergoing Carotid Artery Stenting

Objective: This single-center study aimed to assess the incidence and predictors of carotid artery Xact stent fractures (SF) and their impact on in-stent restenosis (ISR) during long-term follow-up. Methods: A cohort of 108 patients (97 males, median age 69.4±8 months) who underwent Xact stent placement for internal carotid artery stenosis between 2013 and 2021 and were diagnosed with SFs through fluoroscopy in 2022 were included. SFs were categorized as types I-V based on fracture characteristics. Follow-up included duplex ultrasound examinations to assess stent patency. Results: The average follow-up duration was 49.2±24.3 months, with ISR observed in 10 patients. Twenty-three SFs (21.3%) were identified: type I (5 patients), type II (7 patients), type III (3 patients), type IV (6 patients), and type V (2 patients). Calcification and stent length significantly predicted SFs (p<0.001; p<0.028). Conclusion: Calcification and stent length are associated with Xact SFs, but SFs do not impact ISR during long-term follow-up

PRODUCTION OF ALUMINUM BASED SPHERICAL FOAMS BY POWDER METALLURG ROUTE AND INVESTIGATION OF THEIR PRODUCTION PARAMETERS

Bu çalışmada, toz metalurjisi yöntemi ile alüminyum esaslı küresel şekilli metalik köpük üretimi gerçekleştirilmiştir. Üretim parametrelerinden köpürtme sıcaklığı, köpürtme süresi, köpürtücü madde miktarı ve deformasyon miktarının küresel şekilli metalik köpük üretimi üzerine etkileri belirlenmiştir. Yapılan deneysel çalışmada Alümix 231 tozlarına köpürtücü madde miktarının köpürmeye etkirsini incelemek amacıyla %0,5-0,75-1 ve 1,5 oranlarında TiH2 ilave edilerek 30 dakika karıştırılmıştır. Daha sonra karışım tozlar kalıp içerisinde tek yönlü preste 600 MPa basınç uygulanarak preslenmiş ve belirli ölçülerde numuneler elde edilmiştir. Elde edilen numuneler 550oC sıcaklıkta ısıtılıp 180 dakika süreyle bekletilmiştir. Fırın içerisinden alınan numuneler deformasyonun köpürmeye etkisini incelemek amacıyla %10-30-50 ve 70 oranlarda deforme edilip, belirli boyutlarda kare şeklinde kesilerek köpürmeye hazır numuneler elde edilmiştir. Daha sonra kesilmiş numuneler fırın içerisine ayrı ayrı yerleştirilip 650oC, 670oC, 690oC ve 710oC'da 2 ile 5 dakika arasında 30 saniye aralıklarla bekletilerek köpürtme işlemine tabii tutulmuştur. Bu işlem sırasında herhangi bir kalıp kullanılmadan serbest köpürtme yapılmıştır. Elde edilen Al köpük numunelerde, üretim parametrelerinin lineer genişlemeye, yoğunluğa ve gözenek yapısına etkileri araştırılmıştır. Deneysel çalışmalar sonucunda deformasyon miktarındaki artış (%70) ile beraber gözenek sayısının azaldığı, gözenek boyutunun, lineer genişleme oranın ve boşluk miktarının ise arttığı tespit edilmiştir. Köpürtme sıcaklığının, köpürtme süresinin ve köpürtücü madde miktarının aynı anda artırılması numunelerde homojen olmayan gözenek yapısı oluşumuna, yer yer çöküntülere, boşluk ve lineer genişleme oranlarının azalmasına neden olduğu belirlenmiştir. Dolayısıyla bu parametrelerin ara değerlerde seçilmesi (690oC, %1 TiH2, 3-3,5 dak) köpürebilirliğe olumlu yönde etki etmektedir.In this study, aluminum based spherical shape foams were produced by using powder metallurgy techniques. Effect of production parameters such as; foaming temperature, foaming time, amount of foaming agent and deformation rate on the production of foam were determined. In experimental study, various amount of powdered foaming agent (0.5, 0.75, 1, and 1.5% TiH2) were added to Alumix 231 powder and mixed for 30 minutes. Mixed powders were compacted at 600 MPa pressure by using a uniaxial action press to produce powder blanks. Blanks were then preheated to 550 oC in a furnace for 180 min and then deformed by 10, 30, 50 and 70% and then cut into square shape to produce spherical foams. Samples were foamed without any mold at 650oC, 670oC, 690oC and 710oC between 2 to 5 minutes with the interval of 30 seconds. Effect of production parameters on the pore size, shape, linear expansion and density of spherical shape aluminum foam were investigated. It was found that pore size, linear expansion rate and the amount of porosity increased with increasing the deformation rate (70%) however, it reduced the pore number. Increasing the foaming temperature, foaming time and the amount of foaming agent together, resulted in inhomogeneous pore structure together with collapsing of foam structure. This also reduced the amount of porosity and linear expansion rate of the foams. Choosing a suitable parameters (such as 690 oC foaming temperature, 1% TiH2 foaming agent and 3-3.5 min foaming duration) resulted in a positive effect on foamabilit

Investigation of the foamability and the joinability of al based foamable materials reinforced with or without addition of B4C by friction stir welding

Bu çalışmada, toz metalurjisi yöntemi ile köpürebilir metalik malzemeler üretilmiş ve köpürtme öncesi sürtünme karıştırma kaynak yöntemiyle birleştirilmiştir. Bu amaçla matrisi oluşturacak Al tozlarına, %7 oranında Si, %1 oranında TiH2 (köpürtücü madde) ve değişik oranlarda B4C tozları ilave edilerek karıştırılmıştır. Daha sonra karışım tozlar, 400 MPa basınç altında preslenip, 550oC'de ekstrüzyon ve haddeleme işlemlerinden geçirilerek köpürebilir numuneler haline getirilmiştir. Elde edilen numuneler sürtünme karıştırma kaynağı (SKK) ile farklı dönme hızlarında (1250, 1600 ve 2000 dev/dak) ve kaynak ilerleme hızlarında (40, 63, 80 mm/dak) birleştirilmiştir. Numuneler daha sonra 690oC ile 750oC sıcaklıklar arasında köpürtme işlemine tabi tutularak kapalı gözenekli Al köpükler üretilmiştir. Deneysel çalışmalar sunucunda, B4C takviyeli preform malzemelerin genleşme oranlarının AlSi7 köpüklere oranla daha düşük olduğu tespit edilmiştir. Genleşme değerleri köpürtme sıcaklığındaki artışa bağlı olarak artmıştır. B4C miktarı ve sıcaklıkla birlikte numunelerde gözenek boyutları artmış, gözenek sayısının ise azalmıştır. SKK ile birleştirilmiş %6 B4C takviyeli ve takviyesiz preform malzemeler için maksimum çekme dayanımı 1600 dev/dak dönme hızında ve 40 mm/dak ilerleme hızında elde edilmiştir. Buna karşın malzemelerin mukavemet değerlerinin ana malzemeye oranla daha düşük olduğu tespit edilmiştir. Serbest köpürtme neticesinde malzemelerde kaynak bölgesi ana malzemeye göre daha zayıf genleşme davranışı sergilemiştir. Fakat kalıpta köpürtme işlemi sonucu yapılarında belirgin bir kaynak bölgesi veya birleşme izi gözlenmemiştir. %6 B4C takviyeli ve takviyesiz AlSi7 köpüklerde bağıl yoğunluktaki artış ile birlikte eğme dayanımları artmıştır. Kaynaklı ve kaynaksız köpük malzemelerde B4C ilavesi eğme dayanımını artırırken, SKK köpüklerin eğme dayanımlarına azaltıcı yönde etki etmiştir.In this study, foamable metallic preform materials were produced and joined by friction stir welding before foaming. For this, 7% Si, 1% TiH2 (foaming agent) and B4C at various rates powders were added to the Al powders and then were mixed properly. Mixed powders were compacted at 400 MPa and extruded and rolled at 550oC so that precursor material was obtained. These precursor materials were joined by friction stir welding (FSW) at different rotation speeds (1250, 1600 ve 2000 rev/min) and welding speeds (40, 63, 80 mm/min). Then welded samples were foamed at temperatures between 690oC and 710oC to produce closed cell metallic foams. As a result of experimental studies, the expansion rate of B4C reinforced preform materials has been found to be lower than AlSi7 foams. Expansion values were increased with increase in foaming temperature. Pore size was increased with foaming temperature and the amount of B4C in the samples but the number of pores was decreased. The maximum tensile strength for welded 6% reinforced and unreinforced materials were obtained in 1600 rev/min rotation speed and 40 mm/min welding speed. In contrast to the base material of strength values of welded materials have been found to be lower. In a result of free foaming, the welding zone compared to the base material was showed weaker expansion behavior. However, a significant welding region or welding track in their structure with the foaming process in the mold was unwatched. The bending strength of 6% reinforced and unreinforced foam materials was increased with increase relative density. The bending strength was increased with B4C addition on the other hand was decreased by FSW