Microhardness and friction coefficient of multi-walled carbon nanotube-yttria-stabilized ZrO2 composites prepared by spark plasma sintering

Multi-walled carbon nanotubes (eight walls) are mixed with an yttria-stabilized ZrO2 powder. The specimens are densified by spark plasma sintering. Compared to ZrO2, there is a 3.8-fold decrease of the friction coefficient against alumina upon the increase in carbon content. Examinations of the friction tracks show that wear is very low when the carbon content is sufficient. Exfoliation of the nanotubes due to shearing stresses and incorporation of the debris into a lubricating film over the contact area is probable

FRACTURE MECHANISM IN Si3N4 – GRAPHENE PLATELETS COMPOSITES

Silicon nitride + 1 wt% graphene platelet composites were prepared using various graphene platelets (GPLs) as a filler. Two different sintering routes were applied which resulted in different microstructure: hot isostatic pressing (1700°C/3h/20 MPa) and gas pressure sintering (1700°C/0h/2 MPa). The influence of the GPLs addition and of processing routes on the fracture toughness and fracture mechanism of Si3N4+GPLs was investigated. The main toughening mechanisms, which originated from the presence of the graphene platelets are crack deflection, crack branching and crack bridging. These mechanisms are responsible for the increase of fracture toughness which is higher than that of monolithic Si3N4. The highest value of fracture toughness was obtained in the case of the composite processed by hot isostatic pressing using the GPLs with lowest dimension

Improvement of Carbon Nanofibers/ZrO2 Composites Properties with a Zirconia Nanocoating on Carbon Nanofibers by Sol–Gel Method

The development of new carbon nanofibers (CNFs)–ceramic nanocomposite materials with excellent mechanical, thermal, and electrical properties is interesting for a wide range of industrial applications. Among the ceramic materials, zirconia stands out for their excellent mechanical properties. The main limitations in the preparation of this kind of nanocomposites are related with the difficulty in obtaining materials with homogeneous distribution of both phases and the dissimilar properties of CNFs and ZrO2 which causes poor interaction between them. CNFs-reinforced zirconia nanocomposites ZrO2/xCNFs (x=1–20 vol%) were prepared by powder mixture and sintered by spark plasma sintering (SPS). ZrO2-reinforced CNFs nanocomposites CNFs/xZrO2 (x=20 vol%) were prepared by powder mixture and a surface coating of CNFs by the wet chemical route with zirconia precursor is proposed as a very effective way to improve the interaction between CNFs and ZrO2. After SPS sintering, an improvement of 50% in fracture strength was found for similar nanocomposite compositions when the surface coating was used. The improved mechanical properties of these nanocomposites are caused by stronger interaction between the CNFs and ZrO2.This work was financially supported by National Plan Projects MAT2006-01783 and MAT2007-30989-E and the Regional Project FICYT PC07-021. A. Borrell, acknowledges the Spanish Ministry of Science and Innovation for her research grant BES2007-15033.Borrell Tomás, MA.; Rocha, VG.; Torrecillas, R.; Fernandez, A. (2011). Improvement of Carbon Nanofibers/ZrO2 Composites Properties with a Zirconia Nanocoating on Carbon Nanofibers by Sol–Gel Method. Journal of the American Ceramic Society. 94(7):2048-2052. https://doi.org/10.1111/j.1551-2916.2010.04354.xS20482052947Iijima, S. (1991). Helical microtubules of graphitic carbon. Nature, 354(6348), 56-58. doi:10.1038/354056a0Merkoçi, A. (2005). Carbon Nanotubes in Analytical Sciences. Microchimica Acta, 152(3-4), 157-174. doi:10.1007/s00604-005-0439-zUchida, T., Anderson, D. P., Minus, M. L., & Kumar, S. (2006). Morphology and modulus of vapor grown carbon nano fibers. Journal of Materials Science, 41(18), 5851-5856. doi:10.1007/s10853-006-0324-0Hvizdoš, P., Puchý, V., Duszová, A., & Dusza, J. (2010). Tribological behavior of carbon nanofiber–zirconia composite. Scripta Materialia, 63(2), 254-257. doi:10.1016/j.scriptamat.2010.03.069Balázsi, C., Kónya, Z., Wéber, F., Biró, L. P., & Arató, P. (2003). Preparation and characterization of carbon nanotube reinforced silicon nitride composites. Materials Science and Engineering: C, 23(6-8), 1133-1137. doi:10.1016/j.msec.2003.09.085Tatami, J., Katashima, T., Komeya, K., Meguro, T., & Wakihara, T. (2005). Electrically Conductive CNT-Dispersed Silicon Nitride Ceramics. Journal of the American Ceramic Society, 88(10), 2889-2893. doi:10.1111/j.1551-2916.2005.00539.xHirota, K., Hara, H., & Kato, M. (2007). Mechanical properties of simultaneously synthesized and consolidated carbon nanofiber (CNF)-dispersed SiC composites by pulsed electric-current pressure sintering. Materials Science and Engineering: A, 458(1-2), 216-225. doi:10.1016/j.msea.2006.12.065Dusza, J., Blugan, G., Morgiel, J., Kuebler, J., Inam, F., Peijs, T., … Puchy, V. (2009). Hot pressed and spark plasma sintered zirconia/carbon nanofiber composites. Journal of the European Ceramic Society, 29(15), 3177-3184. doi:10.1016/j.jeurceramsoc.2009.05.030Lee, S.-Y., Kim, H., McIntyre, P. C., Saraswat, K. C., & Byun, J.-S. (2003). Atomic layer deposition of ZrO2 on W for metal–insulator–metal capacitor application. Applied Physics Letters, 82(17), 2874-2876. doi:10.1063/1.1569985Kobayashi, S., & Kawai, W. (2007). Development of carbon nanofiber reinforced hydroxyapatite with enhanced mechanical properties. Composites Part A: Applied Science and Manufacturing, 38(1), 114-123. doi:10.1016/j.compositesa.2006.01.006Sun, J., Gao, L., Iwasa, M., Nakayama, T., & Niihara, K. (2005). Failure investigation of carbon nanotube/3Y-TZP nanocomposites. Ceramics International, 31(8), 1131-1134. doi:10.1016/j.ceramint.2004.11.010Ukai, T., Sekino, T., Hirvonen, A. T., Tanaka, N., Kusunose, T., Nakayama, T., & Niihara, K. (2006). Preparation and Electrical Properties of Carbon Nanotubes Dispersed Zirconia Nanocomposites. Key Engineering Materials, 317-318, 661-664. doi:10.4028/www.scientific.net/kem.317-318.661Duszová, A., Dusza, J., Tomášek, K., Morgiel, J., Blugan, G., & Kuebler, J. (2008). Zirconia/carbon nanofiber composite. Scripta Materialia, 58(6), 520-523. doi:10.1016/j.scriptamat.2007.11.002Wang, X., Padture, N. P., & Tanaka, H. (2004). Contact-damage-resistant ceramic/single-wall carbon nanotubes and ceramic/graphite composites. Nature Materials, 3(8), 539-544. doi:10.1038/nmat1161Zhan, G.-D., Kuntz, J. D., Garay, J. E., & Mukherjee, A. K. (2003). Electrical properties of nanoceramics reinforced with ropes of single-walled carbon nanotubes. Applied Physics Letters, 83(6), 1228-1230. doi:10.1063/1.1600511Yucheng, W., & Zhengyi, F. (2002). Study of temperature field in spark plasma sintering. Materials Science and Engineering: B, 90(1-2), 34-37. doi:10.1016/s0921-5107(01)00780-2Haase, F., & Sauer, J. (1998). The Surface Structure of Sulfated Zirconia:  Periodic ab Initio Study of Sulfuric Acid Adsorbed on ZrO2(101) and ZrO2(001). Journal of the American Chemical Society, 120(51), 13503-13512. doi:10.1021/ja9825534Matsui, K., Suzuki, H., Ohgai, M., & Arashi, H. (1995). Raman Spectroscopic Studies on the Formation Mechanism of Hydrous-Zirconia Fine Particles. Journal of the American Ceramic Society, 78(1), 146-152. doi:10.1111/j.1151-2916.1995.tb08374.xGateshki, M., Petkov, V., Williams, G., Pradhan, S. K., & Ren, Y. (2005). Atomic-scale structure of nanocrystallineZrO2prepared by high-energy ball milling. Physical Review B, 71(22). doi:10.1103/physrevb.71.224107Pyda, W., Haberko, K., & Bulko, M. M. (1991). Hydrothermal Crystallization of Zirconia and Zirconia Solid Solutions. Journal of the American Ceramic Society, 74(10), 2622-2629. doi:10.1111/j.1151-2916.1991.tb06810.xDell’Agli, G., & Mascolo, G. (2000). Hydrothermal synthesis of ZrO2–Y2O3 solid solutions at low temperature. Journal of the European Ceramic Society, 20(2), 139-145. doi:10.1016/s0955-2219(99)00151-xTai, C. Y., Hsiao, B.-Y., & Chiu, H.-Y. (2007). Preparation of silazane grafted yttria-stabilized zirconia nanocrystals via water/CTAB/hexanol reverse microemulsion. Materials Letters, 61(3), 834-836. doi:10.1016/j.matlet.2006.05.068Tai, C. Y., Lee, M.-H., & Wu, Y.-C. (2001). Control of zirconia particle size by using two-emulsion precipitation technique. Chemical Engineering Science, 56(7), 2389-2398. doi:10.1016/s0009-2509(00)00454-1Tai, C. Y., & Hsiao, B.-Y. (2005). CHARACTERIZATION OF ZIRCONIA POWDER SYNTHESIZED VIA REVERSE MICROEMULSION PRECIPITATION. Chemical Engineering Communications, 192(11), 1525-1540. doi:10.1080/009864490896133Ci, L., Wei, J., Wei, B., Liang, J., Xu, C., & Wu, D. (2001). Carbon nanofibers and single-walled carbon nanotubes prepared by the floating catalyst method. Carbon, 39(3), 329-335. doi:10.1016/s0008-6223(00)00126-3Choi, S. R., & Bansal, N. P. (s. f.). Alumina-Reinforced Zirconia Composites. Handbook of Ceramic Composites, 437-457. doi:10.1007/0-387-23986-3_18Li, W., & Gao, L. (2000). Rapid sintering of nanocrystalline ZrO2(3Y) by spark plasma sintering. Journal of the European Ceramic Society, 20(14-15), 2441-2445. doi:10.1016/s0955-2219(00)00152-7Borrell, A., Fernández, A., Merino, C., & Torrecillas, R. (2010). High density carbon materials obtained at relatively low temperature by spark plasma sintering of carbon nanofibers. International Journal of Materials Research, 101(1), 112-116. doi:10.3139/146.110246Dusza, J., Morgiel, J., Tatarko, P., & Puchy, V. (2009). Characterization of interfaces in ZrO2–carbon nanofiber composite. Scripta Materialia, 61(3), 253-256. doi:10.1016/j.scriptamat.2009.03.052Lauwers, B., Kruth, J. P., Liu, W., Eeraerts, W., Schacht, B., & Bleys, P. (2004). Investigation of material removal mechanisms in EDM of composite ceramic materials. Journal of Materials Processing Technology, 149(1-3), 347-352. doi:10.1016/j.jmatprotec.2004.02.01

Effect of CNFs content on the tribological behaviour of spark plasma sintering ceramic-CNFs composites

Alumina-carbon nanofibres (CNFs) and silicon carbide-CNFs nanocomposites with different volume fraction of CNFs (0-100vol.%) were obtained by spark plasma sintering. The effect of CNFs content on the tribological behaviour in dry sliding conditions on the ceramic-carbon nanocomposites has been investigated using the ball-on-disk technique against alumina balls. The wear rate of ceramic-CNFs nanocomposites decreases with CNFs increasing content. The friction coefficient of the Al 2O 3/CNFs and SiC/CNFs nanocomposites with high CNFs content was found to be significantly lower compared to monolithic Al 2O 3 and SiC due to the effect of CNFs and unexpectedly slightly lower than CNFs material. The main wear mechanism in the nanocomposite was abrasion of the ceramic and carbon components which act in the interface as a sort of lubricating media. The experimental results demonstrate that the addition of CNFs to the ceramic composites significantly reduces friction coefficient and wear rate, resulting in suitable materials for unlubricated tribological applications. © 2011.This work has been carried out with financial support of National Plan Projects MAT2006-01783 and MAT2007-30989-E and the Regional Project FICYT PC07-021. A. Borrell acknowledges the Spanish Ministry of Science and Innovation for her FPI Ph.D. grant. We would like to thank the people from Institute Technological of Materials (ITM) of the Polytechnic University of Valencia for helping us with the tribology experiments during A. Borrell's short stay in 2009.Borrell Tomás, MA.; Torrecillas, R.; Rocha, VG.; Fernandez, A.; Bonache Bezares, V.; Salvador Moya, MD. (2012). Effect of CNFs content on the tribological behaviour of spark plasma sintering ceramic-CNFs composites. Wear. 274:94-99. https://doi.org/10.1016/j.wear.2011.08.013S949927

Effect of graphene and CNFs addition on the mechanical and electrical properties of dense alumina-toughened zirconia composites

Fully dense carbon/alumina-toughened zirconia composites were prepared by using a combination of aqueous colloidal powder processing and spark plasma sintering technique (SPS). Various carbon elements were introduced in alumina-toughened zirconia matrix (ZA) as filler; carbon nanofibers (CNFs) and graphene oxide (GO). The influence of the addition of different carbon forms on the microstructure and on the mechanical and electrical properties was investigated. In the case of the ZAGO composites, the SPS technique allowed, in one-step, the in situ reduction of the graphene oxide during the sintering process. The fracture toughness increases for ZAGO composites in comparison to the ZA material while the hardness decreases slightly with carbon elements addition. The electrical conductivity of the ZA composite drastically increased with the addition of graphene oxide, and it reached 10 Omega cm at 2 vol%. CrownA. Borrell acknowledges the Spanish Ministry of Economy and Competitiveness for her Juan de la Cierva contract (JCI-2011-10498) and the Generalitat Valenciana by the financial support for the GV/2014/009 project. M.D. Salvador thanks to CAPES - Programa Ciencias sem Fronteiras (Brazil) for the concession of a PVE project No. A086/2013. A.S.A. Chinelatto thanks to CAPES for the concession of a post-doctoral fellowship in ICV-CSIC.Rincón, A.; Moreno, R.; Chinelatto, ASA.; Gutierrez, CF.; Salvador Moya, MD.; Borrell Tomás, MA. (2016). Effect of graphene and CNFs addition on the mechanical and electrical properties of dense alumina-toughened zirconia composites. Ceramics International. 42(1):1105-1113. https://doi.org/10.1016/j.ceramint.2015.09.037S1105111342

Colloidal processing of fully stabilized zirconia laminates comprising graphene oxide-enriched layers

Multilayer materials have demonstrated to provide an efficient mechanism for toughening by deflection of a propagating crack by weak interlayers. Therefore, the aim of this work is to study the colloidal processing of 8 mol% yttria stabilized zirconia (8YSZ) based laminates by intercalating thin layers of graphene enriched with 8YSZ, and to evaluate the advantages of such multilayered structure in the propagation of cracks induced by indentation. Green tapes of 8YSZ and graphene-oxide with YSZ were obtained by aqueous tape casting and sintered in one-step by spark plasma sintering at 1400 degrees C. Microindentation results showed that the indentation cracks propagate within the horizontal direction within the ceramic layer, but in the cross-sectional direction the presence of the GO-rich layers stops the cracks without deflection or bifurcation. The hardness and elastic modulus values were higher than 17.6 GPa and 230 GPa, respectively, and similar for all layers.This work has been supported by the Spanish Ministry of Economy and Competitiveness (project MAT2015-67586-C3-R) and the Generalitat Valenciana by the financial support for the GV/2014/009 project. M.D. Salvador thanks to CAPES-Programa Ciencias sem Fronteiras (Brazil) for the concession of a PVE project No. A086/2013. A. Borrell, acknowledges the Spanish Ministry of Economy and Competitiveness for her Juan de la Cierva-Incorporacion contract (IJCI-2014-19839).Rincón, A.; Moreno, R.; Gutierrez González, FC.; Sainz, R.; Salvador Moya, MD.; Borrell Tomás, MA. (2016). Colloidal processing of fully stabilized zirconia laminates comprising graphene oxide-enriched layers. Journal of the European Ceramic Society. 36(7):1797-1804. https://doi.org/10.1016/j.jeurceramsoc.2016.01.035S1797180436

Preparation and characterization of carvedilol and indomethacin amorphous solid dispersions

Uporaba amorfnih trdnih disperzij je en izmed načinov izboljšanja hitrosti raztapljanja v vodi slabo topnih zdravilnih učinkovin. Amorfne trdne disperzije vsebujejo amorfno zdravilno učinkovino, ki je dispergirana v polimernem matriksu. Prisotnost polimera je potrebna zaradi stabilizacije amorfne oblike zdravilne učinkovine, ki je nagnjena h kristalizaciji, kar ogroža stabilnost končnega produkta. Razlog termodinamske nestabilnosti amorfne oblike je v njenih lastnostih: izkazuje višjo Gibbsovo prosto energijo, entropijo, entalpijo in prosti volumen. Amorfno obliko namreč največkrat pripravimo s hitro ohladitvijo kapljevine snovi, ki se je stalila pri temperaturi tališča. V primeru, da staljeno snov ohlajamo dovolj hitro, ne pride do kristalizacije, saj molekule niso imele dovolj časa, da bi se uredile in oblikovale kristale. Takšno tekoče agregatno stanje imenujemo podhlajena kapljevina. Z nadaljnjim zniževanjem temperature prihaja do strukturne prerazporeditve molekul kapljevine. Pri dovolj nizki temperaturi je gibanje molekul onemogočeno in celotna struktura nekako zamrzne. Ta dogodek se zgodi pri temperaturi steklastega prehoda, ki je lastnost amorfnih snovi. Gibanje molekul pod temperaturo steklastega prehoda je minimalno, nad njo je pa dovolj izrazito, da lahko pride do kristalizacije snovi. Veliko amorfnih zdravilnih učinkovin ima nizko vrednost temperature steklastega prehoda in iz tega razloga jo poskušamo zvišati z dodatkom polimera, ki s svojo višjo temperaturo steklastega prehoda deluje kot protimehčalo. Dodatno pa lahko polimer stabilizira amorfno zdravilno učinkovino preko tvorbe vodikovih vezi tako, da je gibanje molekul zdravilne učinkovine oteženo. Stabilnost amorfnih trdnih disperzij je torej velik in pomemben tehnološki izziv in je tudi možen razlog za majhno število tovrstnih produktov na tržišču. Namen naloge je bila izdelava in ovrednotenje amorfnih trdnih disperzij karvedilola in indometacina s polimeri HPMC-AS in Eudragit® L100-55, ter ovrednotenje njihove stabilnosti v času enomesečne stabilnostne študije. Z metodo iztiskanja talin smo izdelali 4 amorfne trdne disperzije s polimerom HPMC-AS z 20% in 40% deležem zdravilne učinkovine. Amorfne trdne disperzije so bile v obliki praška. Delež zdravilne učinkovine v trdni disperziji smo izbrali na podlagi predhodno izdelanih filmov zdravilne učinkovine in polimera pri različnih deležih zdravilne učinkovine. Vsakemu filmu smo s pomočjo diferenčne dinamične kalorimetrije določili temperaturo steklastega prehoda, ki se je nahajala med temperaturama steklastega prehoda zdravilne učinkovine in polimera. Na XIII podoben način smo želeli določiti vrednosti temperatur steklastega prehoda filmov s polimerom Eudragit® L100-55, vendar smo bili neuspešni. Vrednosti smo zatem poskusili določiti praškastim zmesem, ampak so bili rezultati nerazložljivi. Iz tega razloga smo se odločili, da polimer Eudragit® L100-55 opustimo v nadaljnjih raziskavah. Pri HPMC-AS amorfnih trdnih disperzijah smo ugotovili, da se eksperimentalno dobljene vrednosti razlikujejo od napovedanih izračunanih vrednosti, kar je potrjevalo, da so medmolekularne interakcije med karvedilolom in HPMC-AS močnejše kot med posameznimi molekulami zdravilne učinkovine. V primeru indometacina smo izmerili nekoliko znižane vrednosti temperatur steklastega prehoda, kot smo napovedovali, kar nakazuje na šibkejše medmolekularne interakcije med indometacinom in polimerom. Posledično zato obstaja večja verjetnost za tvorbo kristalizacijskih jeder, ker bodo molekule indometacina težile k združevanju in kristalizaciji oz. nestabilnosti. Za vrednotenje amorfnih trdnih disperzij smo poleg dinamične diferenčne kalorimetrije uporabili še infrardečo spektroskopijo s Fourierovo transformacijo, in sicer tehniko oslabljenega totalnega odboja in metodo malokotnega in širokokotnega rentgenskega sipanja. S pomočjo analiznih metod smo potrdili, da nam je uspelo izdelati amorfne trdne disperzije. Izjema je bila amorfna trdna disperzija s 40% vsebnostjo indometacina, kjer so bili poleg amorfne oblike prisotni tudi kristali zdravilne učinkovine. Rezultat je posledica presežene topnosti indometacina v polimeru, kar nakazuje, da je pri višjih odmerkih indometacina potrebno uporabiti več polimera, ali pa drugi polimer. Pri ostalih amorfnih trdnih disperzijah ni bilo tovrstnih težav in prisotnost amorfne zdravilne učinkovine je bila razvidna iz rezultatov meritev. Potrdili smo, da je uporabljen postopek primeren za izdelavo, z infrardečo spektroskopijo pa smo potrdili prisotnost vodikovih vezi med zdravilno učinkovino in polimerom. V enomesečni stabilnostni študiji nas je zanimalo ali so amorfne trdne disperzije sposobne ohraniti svoje lastnosti v primeru, če so shranjene pri temperaturi 40 °C in 75% relativni vlažnosti. Taki pogoji imajo lahko različen vpliv na posamezne amorfne trdne disperzije, saj ima vsaka amorfna trdna disperzija svojo vrednost temperature steklastega prehoda in shranjevanje amorfne trdne disperzije blizu te vrednosti lahko pospeši kristalizacijo. Prisotnost vlage ni zaželena, ker prav tako lahko zniža vrednost temperature steklastega prehoda (deluje kot mehčalo) in je zato ogrožena stabilnost amorfne zdravilne učinkovine. Za namene študije stabilnosti so bili vzorci shranjeni v odprtih in zaprtih steklenih vsebnikih. Ugotovili smo, da sta obe amorfni trdni disperziji s karvedilolom ohranili svoje XIV lastnosti v času celotne stabilnostne študije. Rezultati nakazujejo, da je bila zdravilna učinkovina prisotna v amorfni obliki ne glede na to, ali je bil vzorec shranjen v zaprtem ali odprtem vsebniku. Vrednosti temperatur steklastega prehoda amorfnih trdnih disperzij se niso bistveno spremenile. Tudi v tej študiji smo potrdili prisotnost intermolekularnih vodikovih vezi. Primerljivost rezultatov med vzorci v odprtih in zaprtih vsebnikih nakazuje, da je polimer HPMC-AS primeren za izdelavo stabilnih amorfnih trdnih disperzij z različnimi odmerki karvedilola. Tovrstni polimer je stabiliziral tudi amorfno trdno disperzijo s 40% vsebnostjo karvedilola, čeprav je njena temperatura steklastega prehoda nižja zaradi večje vsebnosti zdravilne učinkovine, kar bi lahko v pogojih študije, 40 °C in 75% relativni vlažnosti močno pospešilo kristalizacijo. Amorfna trdna disperzija, ki je vsebovala 20% indometacina je ostala nespremenjena v času enomesečnega opazovanja. Nobena od uporabljenih analiznih metod ni zaznala prisotnost kristalov zdravilne učinkovine. Amorfna trdna disperzija z višjo vsebnostjo indometacina (40%) je že v začetku vsebovala kristale zdravilne učinkovine, ki smo jih detektirali z dinamično diferenčno kalorimetrijo, ne pa z metodo rentgenskega sipanja. Metoda rentgenskega sipanja je potrdila prisotnost kristalov v vzorcih v odprtih vsebnikih, in sicer že po dveh tednih. Predpostavljamo, da je temu vzrok nižja temperatura steklastega prehoda in predvsem vpliv vlage. V nadaljevanju nas je tudi zanimalo, kako se spekter infrardeče spektroskopije spreminja s temperaturo. Preko tega se da zaznati prisotnost vodikovih vezi med obema zdravilnima učinkovinama in HPMC-AS. Meritve smo izvedli pred in po temperaturah steklastega prehoda in potrdili smo, da je prišlo do prekinitve interakcij po prehodu temperatur steklastega prehoda pri vseh koncentracijah zdravilne učinkovine. Gre za potrditev pomembnosti upoštevanja temperature shranjevanja glede na temperaturo steklastega prehoda vzorca, saj je nad njo velika verjetnost za združevanje molekul zdravilne učinkovine zaradi večje gibljivosti molekul ter zaradi prekinitve medmolekularnih interakcij s polimerom. Znano je, da amorfne snovi izkazujejo višjo Gibbsovo prosto energijo, kar termodinamsko ni ugodno in jo zato želijo znižati, kar se manifestira z relaksacijo. Ta se začne takoj po izdelavi amorfne oblike, je pa lahko hitrost relaksacije različna. Zniževanje Gibbsove proste energije lahko doseže končno vrednost, ko amorfna oblika doseže vrednosti ravnotežne krivulje podhlajene tekočine. Relaksacijo v praksi lahko določamo z dinamično diferenčno kalorimetrijo, kjer določamo entalpijo vzorca. Vzorec v obdobju relaksacije entalpijo izgublja. Dinamika relaksacije napoveduje mobilnost molekul pod temperaturo XV steklastega prehoda. Pri naši študiji smo določili daljše relaksacijske čase amorfnih trdnih disperzij s karvedilolom. Daljši povprečni relaksacijski časi nakazujejo, da se amorfna oblika bolj počasi relaksira, o čemer odločajo interakcije med zdravilno učinkovino in polimerom. Na podlagi dveh modelnih sistemov smo pokazali kateri dejavniki vplivajo na lastnosti in stabilnost amorfnih trdnih disperzij. Hkrati smo ugotovili, da vsaka zdravilna učinkovina potrebuje za stabilizacijo specifičen polimer. Zato ni univerzalnih rešitev stabilizacije amorfnih trdnih disperzij s polimernimi matriksi.Amorphous solid dispersions provide one of the few approaches available for improving the solubility of poorly water-soluble active pharmaceutical ingredients. They are mainly 2-component systems consisting of drug and polymer, where the amorphous drug is molecularly dispersed in an amorphous polymer matrix. The presence of polymer helps to maintain the drug in an amorphous state, which is thermodynamically unstable due to the possession of excess Gibbs free energy, enthalpy and entropy. To delay or prevent crystallization, the molecular mobility of the amorphous glass should be sufficiently low to avoid nuclei formation and crystal growth and is achieved by the maintaining the amorphous solid dispersion at a specific storage temperature and conditions, together with strong drug-polymer interactions. One of the major preparation processes for amorphous solid dispersions involves hot melt extrusion, producing solid dispersions at elevated temperatures without solvents. Four amorphous solid dispersions of 20% and 40% (w/w) carvedilol and indomethacin were manufactured using HPMC-AS as a polymeric carrier. Solid dispersions were characterized as freshly manufactured powders, as they were during a 1-month stability study using various analytical methods. Attention was paid to the molecular interactions in solid dispersions, miscibility, phase separation, crystallinity and molecular mobility. Solid dispersions of carvedilol exhibited satisfactory stability, which was reflected in preservation of amorphous carvedilol due to the sufficiently high glass transition temperature of the solid dispersions and the drug-polymer interactions. Indomethacin solid dispersions demonstrated the importance of drug loading in solid dispersions, together with the moderate or weak intermolecular interactions between drug and polymer. The enthalpy relaxation provides information regarding the lower molecular mobility of carvedilol in solid dispersions, indicating sufficient stabilization of amorphous drug by the selected polymer. Moreover, the intermolecular interactions were studied below and higher than the glass transition of the mixtures with different drug loadings, using temperature-dependent infrared spectroscopy. During this experiment, it was found that the intermolecular hydrogen bonds varied with the composition and measured temperature, resulting in disruption of intermolecular hydrogen bonds after passing the glass transition temperature