Decreasing Growth Temperature Research Articles

Sn-doped n-type GaN layer with high electron density of 1020 cm−3 grown by halide vapor phase epitaxy

A Sn-doped n-type GaN layer with a high electron density of 2 × 1020 cm−3 and a low resistivity of 8.7 × 10−4 Ω∙cm was grown by halide vapor phase epitaxy (HVPE). Sn doping was performed through the reaction between Sn metal and HCl gas. The Sn concentration markedly increased with decreasing growth temperature and the activation energy of Sn desorption from the GaN surface was found to be 4.1 eV. Smooth surfaces were obtained by introducing the Sn precursor even though the samples were grown at a low temperature of 905 °C, suggesting that Sn atoms act as surfactants and promote the migration of Ga adatoms. Almost all the Sn atoms act as donors in GaN below the Sn concentration of 2 × 1020 cm−3. These results indicate that using the Sn donor is promising for fabricating low-resistivity n-type GaN substrates by HVPE.

The role of carbon and C-H neutralization in MOCVD β-Ga2O3 using TMGa as precursor

In this Letter, the role of background carbon in metalorganic chemical vapor deposition (MOCVD) β-Ga2O3 growth using trimethylgallium (TMGa) as the Ga precursor was investigated. The quantitative C and H incorporations in MOCVD β-Ga2O3 thin films grown at different growth rates and temperatures were measured via quantitative secondary ion mass spectroscopy (SIMS). The SIMS results revealed both [C] and [H] increase as the TMGa molar flow rate/growth rate increases or growth temperature decreases. The intentional Si incorporation in MOCVD β-Ga2O3 thin films decreases as the growth rate increases or the growth temperature decreases. For films grown at relatively fast growth rates (GRs) (TMGa > 58 μmol/min, GR > 2.8 μm/h) or relatively low temperature (<950 °C), the [C] increases faster than that of the [H]. The experimental results from this study demonstrate the previously predicted theory—H can effectively passivate the compensation effect of C in n-type β-Ga2O3. The extracted net doping concentration from quantitative SIMS {[Si]-([C]-[H])} agrees well with the free carrier concentration measured from Hall measurement. The revealing of the role of C compensation in MOCVD β-Ga2O3 and the effect of H incorporation will provide guidance on designing material synthesis for targeted device applications.

Raman scattering study of photoexcited plasma in GaAsBi/GaAs heterostructures: Influence of carrier confinement on photoluminescence

GaAsBi alloys are potential candidates for near-infrared optoelectronic applications, such as light-emitting diodes, laser diodes, avalanche photodiodes, and solar cells. In this paper, photoexcited plasma in GaAs1-xBix/GaAs (1.7 ≤ x ≤ 8.6%) heterostructures grown by molecular beam epitaxy was investigated using Raman spectroscopy. The strong LO phonon-plasmon coupled mode (LOPC) was generated by continuous-wave laser excitation at room temperature. The upper branch of the LOPC mode (L+ mode) was clearly observed for Bi contents from x = 3.9–8.6%, which indicates that photoexcited carriers with relatively good carrier mobility were confined in the GaAsBi/GaAs heterostructure. The photoexcited carrier density that was obtained by line shape analysis of the Raman spectra based on a dielectric model increased as the Bi content increased from 1.7 to 6.4%. This indicates that carrier overflow from the GaAsBi layer was suppressed because of the larger conduction band offset at the GaAsBi/GaAs heterointerface. In addition, the optical mobility calculated using the damping constant of the photoexcited L+ mode gradually decreased with increasing Bi content. This may be attributed to an increased defect density with decreasing growth temperature. Our findings provide valuable insight into the development of high-performance optoelectronic devices based on GaAsBi alloys and show the potential of Raman spectroscopy for evaluation of carrier confinement in heterostructures.

Evolution of antibiotic resistance impacts optimal temperature and growth rate in Escherichia coli and Staphylococcus epidermidis.

Bacterial response to temperature changes can influence their pathogenicity to plants and humans. Changes in temperature can affect cellular and physiological responses in bacteria that can in turn affect the evolution and prevalence of antibiotic-resistance genes. Yet, how antibiotic-resistance genes influence microbial temperature response is poorly understood. We examined growth rates and physiological responses to temperature in two species-E. coli and Staph. epidermidis-after evolved resistance to 13 antibiotics. We found that evolved resistance results in species-, strain- and antibiotic-specific shifts in optimal temperature. When E. coli evolves resistance to nucleic acid and cell wall inhibitors, their optimal growth temperature decreases, and when Staph. epidermidis and E. coli evolve resistance to protein synthesis and their optimal temperature increases. Intriguingly, when Staph. epidermidis evolves resistance to Teicoplanin, fitness also increases in drug-free environments, independent of temperature response. Our results highlight how the complexity of antibiotic resistance is amplified when considering physiological responses to temperature. Bacteria continuously respond to changing temperatures-whether through increased body temperature during fever, climate change or other factors. It is crucial to understand the interactions between antibiotic resistance and temperature.

Morphological and optical characterizations of different ZnO nanostructures grown by mist-CVD

In this study, the effects of growth temperatures, post-growth treatment, and substrate cleaning recipes on structural, morphological, luminescence, and growth kinetics of ZnO nanostructures grown by mist Chemical Vapor Deposition method (mist-CVD) on SiO2/Si substrates are investigated. The grown ZnO nanostructures have polycrystalline wurtzite (wz) hexagonal crystal structure according to X-ray Diffraction (XRD) measurements. However, the lower growth temperature is found to lead to the formation of ZnO nanostructures oriented in the [0002] direction. From Atomic Force Microscope (AFM) measurements, it is seen that the Root Mean Square (RMS) values decrease with decreasing growth temperature. In addition, it is shown that the surface covering is tightly dependent on decreasing growth temperature of the ZnO nanostructures. The hexagonal and triangular pyramid-like ZnO nanostructures are grown on the surface according to the Scanning Electron Microscope (SEM) measurements. The wurtzite type of crystallization is also demonstrated by confocal Raman measurements. The characteristic vibration modes related to Zn and O motion in the lattice are found to be ∼435 cm−1 and ∼100 cm−1, respectively. The luminescence properties are studied by low-temperature (10 K) Photoluminescence (PL) measurements. The spectra are dominated by the transition related to the donor-bound exciton (DBE) with a peak position typically observed at ∼369 nm (3.362 eV) in hexagonal ZnO structures. Defect-related transitions are observed to be strongly correlated with growth temperature and post-growth treatment.

Hik28-dependent and Hik28-independent ABC transporters were revealed by proteome-wide analysis of ΔHik28 under combined stress

Synechocystis histidine kinase, Sll0474: Hik28, a signal protein in a two-component signal transduction system, plays a critical role in responding to a decrease in growth temperature and is also involved in nitrogen metabolism. In the present study, under combined stress from non-optimal growth temperature and nitrogen depletion, a comparative proteomic analysis of the wild type (WT) and a deletion mutant (MT) of Synechocystis histidine kinase, Sll0474: Hik28, in a two-component signal transduction system identified the specific groups of ABC transporters that were Hik28-dependent, e.g., the iron transporter, and Hik28-independent, e.g., the phosphate transporter. The iron transporter, AfuA, was found to be upregulated only in the WT strain grown under the combined stress of high temperature and nitrogen depletion. Whereas, the expression level of the phosphate transporter, PstS, was increased in both the WT and MT strains. Moreover, the location in the genome of the genes encoding Hik28 and ABC transporters in Synechocystis sp. PCC6803 were analyzed in parallel with the comparative proteomic data. The results suggested the regulation of the ABC transporters by the gene in a two-component system located in an adjacent location in the genome.

Enhancement of two-dimensional electron gas mobility using strain reduced AlGaN barriers grown by metalorganic vapor phase epitaxy under nitrogen atmosphere

We demonstrated high-electron-mobility transistors (HEMTs) with enhanced two-dimensional electron gas (2DEG) mobility using a low-strain AlGaN barrier grown by metalorganic vapor phase epitaxy under a nitrogen atmosphere. We investigated the effects of the growth temperature under a nitrogen atmosphere on the electrical properties of AlGaN-HEMT structures, focusing on 2DEG mobility. At growth temperatures below 855 °C, the 2DEG mobility decreased with decreasing growth temperature owing to an increase in the threading dislocation density. However, at growth temperatures above 855 °C, the 2DEG mobility decreased with increasing growth temperature. This finding was attributed to the compressive strain in the GaN channel, which increased with increasing growth temperature owing to the increased tensile strain in the AlGaN barriers. We concluded that temperatures around 855 °C are suitable for AlGaN barrier growth under nitrogen atmosphere. Finally, we achieved the highest 2DEG mobility of 2182 cm2 V−1 s−1 with a low sheet resistance of 406 Ω sq−1. using an Al0.41Ga0.59N barrier.

Van der Waals Template‐Assisted Low‐Temperature Epitaxial Growth of 2D Atomic Crystals

AbstractTo date, the synthesis of high‐quality 2D crystals using vapor deposition methods usually requires high temperature, hindering the integration of 2D materials with Si circuits and exacerbating energy consumption. Exploring low‐temperature growth strategies and understanding synthesis mechanism are critical for the practical application of 2D materials. Herein, a van der Waals (vdW) template‐assisted growth of 2D crystals (including PbI2, CdI2, BiI3, CuI, Sb2Te3,and Bi2Se3) is reported, the growth temperature decreases by maximum 35% compared with traditional vapor deposition. The low‐temperature 2D growth process resulting from the low surface diffusion barrier of precursors on vdW surfaces is proposed, confirmed by the density functional theory and molecular dynamics calculations. Particularly, the grown 2D crystals can be peeled off from vdW templates easily and transferred to arbitrary substrates for functional applications and the exfoliated vdW templates can be reused for another round of growth. Although the growth temperature is reduced greatly, the excellent photoelectric performance of grown 2D crystals is demonstrated, benefitting from high crystalline quality. These findings provide a universal method for the low‐temperature synthesis of high‐crystallinity 2D materials toward applications in flexible electronics.

Cubic InxGa1−xN/GaN quantum wells grown by Migration Enhanced Epitaxy (MEE) and conventional Molecular Beam Epitaxy (MBE)

We report the growth of InxGa1−xN/GaN quantum wells (QWs) in metastable-cubic phase through two well-controlled methods, Migration Enhanced Epitaxy (MEE) and conventional Molecular Beam Epitaxy (conventional MBE) on GaAs substrate. An increment of In mole fraction in the c-InxGa1−xN QWs was found with the decrease of growth temperature for both methods. The MEE implemented in this work, which consists of alternating the atomic flux periods at temperatures, approximately 100 ∘C lower than in conventional MBE, successfully addresses the cubic InxGa1−xN/GaN QW growth challenge. The QWs grown by this method present lower In segregation than those grown using conventional MBE. Excitonic transitions in the visible spectrum range from violet (414 nm) to green (544 nm) wavelengths were obtained in the QWs, by varying In content for both methods. These excitonic emissions are in good agreement with the theoretical calculations performed. Likewise, we identified the chemical bonds present in each c-InxGa1−xN QW, which corresponded to In–N and Ga–N, and their binding energies through In 3d, Ga 3d, and N 1s core levels by x-ray photoelectron spectroscopy (XPS). Therefore, this work provides an important understanding of In segregation, In incorporation mechanism, and radiative excitonic transitions of cubic QWs, that can be considered in complex heterostructures for novel optoelectronic applications.

Iridium‐Catalyzed Single‐Walled Carbon Nanotube Synthesis by Alcohol‐Gas‐Source Method Under Low Ethanol Pressure: Growth Temperature Dependence

AbstractSingle‐walled carbon nanotubes (SWCNTs) are grown via an Ir‐catalyzed gas‐source‐type cold‐wall chemical vapor deposition (CVD) method under high vacuum. A nozzle injector is used to supply ethanol gas. The grown SWCNT surface shows a colored concentric morphology. This corresponds to the in‐plane distribution of SWCNT lengths, which is induced by in‐plane variations in the ethanol pressure. Raman spectroscopy and scanning electron microscopy are used to investigate the dependence of the SWCNT yield on the growth temperature and ethanol pressure. The ethanol pressure that gives the highest SWCNT yield decreases with decreasing growth temperature. Optimization of the ethanol pressure enables SWCNT growth between 500 °C and 800 °C, and vertically aligned SWCNTs are grown at 700 °C and 800 °C. The SWCNT diameters are small at all growth temperatures, namely less than 0.9 nm for growth at 500 °C and 600 °C, and less than 1.0 nm for growth at 700 °C and 800 °C. The estimated activation energy for SWCNT growth with an Ir catalyst is 0.97 eV, which is much smaller than that with a Co catalyst.

Growth of InPBi on InP(311)B Substrate by Molecular Beam Epitaxy

The growth of InPBi by molecular beam epitaxy on an InP(311)B substrate is investigated. Bi atoms are incorporated into the samples grown at or below 350 °C. A Bi concentration of up to 5% is estimated by X‐ray diffraction and secondary mass spectroscopy measurements. Although the surface roughness increased with decreasing growth temperature and at a high Bi flux, Bi atoms are found to have surfactant effects on the surface flatness of the samples grown at a low Bi flux.

Lipopolysaccharide of Pseudomonas mandelii, Isolated from Antarctica

Representatives of the Pseudomonas mandelii species are able to exist and multiply in places where the temperature is constantly low. The optimum growth temperature for P. mandelii is 25–30°C, although this bacterium can grow at 4°C but not at 37°C. Therefore, P. mandelii is an excellent example of psychrotolerant bacterium which like psychrophilic bacteria is characterized by a number of structural and functional adaptations that facilitate survival at low temperatures. To understand these microorganisms’ role in Antarctica the characterization of its biopolymers is vital. One of these biopolymers is lipopolysaccharide (LPS), composition and structure of which are diagnostically significant. This determines the aim of the work – to isolate lipopolysaccharides from the cells of Antarctic strain of P. mandelii, grown at different temperatures, to characterize them chemically, and to study their functional and biological activity. Methods. The object of the study was Pseudomonas sp. U1, isolated from moss on Galindez Island in Antarctica. Lipopolysaccharides were extracted from dried cells by 45% phenol water solution at 65–68°С by Westphal and Jann method. The amount of carbohydrates was determined by phenol-sulfuric method. Carbohydrate content was determined in accordance to the calibration curve, which was built using glucose as a standard. The content of nucleic acids was determined by Spirin, protein − by Lowry method. Serological activity of LPS was investigated by double immunodiffusion in agar using the method of Ouchterlony. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS-PAAG electrophoresis) was performed according to Laemmli. Results. As a result of phylogenetic analysis (programs ClustalX 2.1, Tree view, Mega v. 6.00) it was shown that the Antarctic bacterial strain Pseudomonas sp. U1 associated with green moss has a 99.4% homology with the type strain from the GenBank database NR024902 P. mandelii CIP 105273T. According to these data and proximity to the corresponding cluster of species, the studied isolate can be identified as P. mandelii. A characteristic feature of LPS isolated from P. mandelii cells, grown at different temperatures (20°C and 4°C) is their heterogeneity. This is evidenced by the data of the monosaccharide composition, electrophoretic distribution, which showed that P. mandelii produces S- and SR-forms of LPS, differed in the length of the O-specific polysaccharide chains. The R-form of LPS is also present, which does not contain an O-specific polysaccharide chains. Structural heterogeneity is also inherent in LPS lipid A. This is evidenced by the data of the fatty acid composition. In LPS grown at 4°C no unsaturated fatty acids were found, while such ones are synthesized in LPS of other bacteria grown in the cold, in response to a decrease in growth temperature. The study of the immunochemical properties of LPS was carried out using polyclonal O-antisera as antibodies, and LPS as antigens indicated that in homologous systems LPS exhibited serological activity. LPS obtained from P. mandelii U1 cells, grown at 20°C, had a complex antigenic composition and gave two clear lines of precipitation at a concentration of 1 mg/mL. LPS obtained from P. mandelii U1 cells, grown at 4°C, gave one line, which indicates their serological homogeneity. Conclusions. For the first time lipopolysaccharides were isolated from cells of P. mandelii U1, grown at 4°C and 20°С. A characteristic feature of these LPS is their heterogeneity. This is evidenced by the data of the monosaccharide and fatty acid composition, electrophoretic distribution, which showed that P. mandelii produces S- and SR-forms of LPS, differed in the length of the O-specific polysaccharide chains. LPS, obtained from cells, grown at different temperatures, are differed by serological activity.

Correlation of Morphology Evolution with Carrier Dynamics in InN Films Heteroepitaxially Grown by MOMBE

In this study, we report the catalyst-free growth of n-type wurtzite InN, along with its optical properties and carrier dynamics of different surface dimensionalities. The self-catalyzed epitaxial growth of InN nanorods grown by metal–organic molecular-beam epitaxy on GaN/Al2O3(0001) substrates has been demonstrated. The substrate temperature is dominant in controlling the growth of nanorods. A dramatic morphological change from 2D-like to 1D nanorods occurs with decreasing growth temperature. The InN nanorods have a low dislocation density and good crystalline quality, compared with InN films. In terms of optical properties, the nanorod structure exhibits strong recombination of Mahan excitons in luminescence, and an obvious spatial correlation effect in phonon dispersion. The downward band structure at the nanorod surface leads to the photon energy-dependent lifetime being upshifted to the high-energy side.

Optical and Electrical Properties of InxGa1−xSe Mixed Crystal Grown from Indium Flux by Traveling Heater Method

InxGa1−xSe mixed crystals have been successfully grown from an indium flux by the traveling heater method at three growth temperatures. The thickness of the grown InxGa1−xSe mixed crystal perpendicular to (001) was more than 3 mm. The lattice constant, and optical and electrical properties of the InxGa1−xSe mixed crystals and undoped GaSe crystals were investigated and compared. The indium content of the InxGa1−xSe mixed crystals was observed to increase with decreasing growth temperature, while the lattice constant along the c-axis was observed to follow Vegard’s law. It was confirmed that a bandgap of In0.020Ga0.980Se is narrower than that of undoped GaSe according to the photoluminescence (PL) spectra. Compared with undoped GaSe crystal, the carrier concentration p was decreased by the incorporation of indium (In0.020Ga0.980Se, p = 6.4 × 1014 cm−3 at 257 K; In0.037Ga0.963Se, p = 2.6 × 1014 cm−3 at 257 K). In addition, it was suggested that the dominant carrier scattering mechanism of high-indium-content crystals at low temperature is ionized impurity scattering.

Temperature Differentially Affects Gene Expression in Antarctic Thraustochytrid Oblongichytrium sp. RT2316-13.

Oblongichytrium RT2316-13 synthesizes lipids rich in eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The content of these fatty acids in the total lipids depended on growth temperature. Sequencing technology was used in this work to examine the thraustochytrid’s response to a decrease in growth temperature from 15 °C to 5 °C. Around 4% (2944) of the genes were differentially expressed (DE) and only a few of the DE genes (533 upregulated; 206 downregulated) had significant matches to those in the SwissProt database. Most of the annotated DE genes were related to cell membrane composition (fatty acids, sterols, phosphatidylinositol), the membrane enzymes linked to cell energetics, and membrane structure (cytoskeletal proteins and enzymes). In RT2316-13, the synthesis of long-chain polyunsaturated fatty acids occurred through ω3- and ω6-pathways. Enzymes of the alternative pathways (Δ8-desaturase and Δ9-elongase) were also expressed. The upregulation of the genes coding for a Δ5-desaturase and a Δ5-elongase involved in the synthesis of EPA and DHA, explained the enrichment of total lipid with these two long-chain fatty acids at the low temperature. This molecular response has the potential to be used for producing microbial lipids with a fatty acids profile similar to that of fish oils.

Value-added utilization of pyrolysis heavy distillate for the synthesis of nitrogen doped graphene with chemical vapor deposition

The synthesis of graphene and nitrogen doped graphene (NG) with inexpensive pyrolysis heavy distillate (PHD) as a carbon source and urea as a nitrogen source by chemical vapor deposition on a copper substrate under hydrogen flow has been studied. The variables in synthesis were process temperature and hydrogen flow rate. The PHD is rich in aromatic compounds. These cyclic structured molecules resemble the basic unit of graphene and there is no need to break C-C atomic bonds and allows for graphene synthesis at a lower temperature. The morphology and properties of synthesized graphene and NG were characterized by Raman spectroscopy, TEM, XRD, BET and XPS. Results indicated that the increasing hydrogen flow rate and decreasing growth temperature, increase defects in the graphene, which are favorable for the catalyst support applications. From the Raman and XRD analysis, the NG and graphene derived from these carbon sources are few layers. The average nitrogen content was 5.89% in the structure of NG with three different types of nitrogen atoms namely; pyridinic (32.03%), pyrrolic (51.70%) and graphitic (16.97%). The defects are mainly of vacancy type because pyridinic nitrogen creates vacancies in the structure of graphene. We believe that these results will play an important role in the utilization of inexpensive carbon sources to convert high value added materials.

Fatty Acid and Hopanoid Adaption to Cold in the Methanotroph Methylovulum psychrotolerans.

Three strains of aerobic psychrotolerant methanotrophic bacteria Methylovulum psychrotolerans, isolated from geographically remote low-temperature environments in Northern Russia, were grown at three different growth temperatures, 20, 10 and 4°C and were found to be capable of oxidizing methane at all temperatures. The three M. psychrotolerans strains adapted their membranes to decreasing growth temperature by increasing the percent of unsaturated fatty acid (FAs), both for the bulk and intact polar lipid (IPL)-bound FAs. Furthermore, the ratio of βOH-C16:0 to n-C16:0 increased as growth temperature decreased. The IPL head group composition did not change as an adaption to temperature. The most notable hopanoid temperature adaptation of M. psychrotolerans was an increase in unsaturated hopanols with decreasing temperature. As the growth temperature decreased from 20 to 4°C, the percent of unsaturated M. psychrotolerans bulk-FAs increased from 79 to 89 % while the total percent of unsaturated hopanoids increased from 27 to 49 %. While increased FA unsaturation in response to decreased temperature is a commonly observed response in order to maintain the liquid-crystalline character of bacterial membranes, hopanoid unsaturation upon cold exposition has not previously been described. In order to investigate the mechanisms of both FA and hopanoid cold-adaption in M. psychrotolerans we identified genes in the genome of M. psychrotolerans that potentially code for FA and hopanoid desaturases. The unsaturation of hopanoids represents a novel membrane adaption to maintain homeostasis upon cold adaptation.

Engineering V-shaped pits in InGaN layers grown by PA-MBE toward optimizing the active region of green LEDs

Surface morphology of thick InGaN with higher indium content, grown on GaN/sapphire templates using plasma-assisted molecular beam epitaxy (PA-MBE) under metal-rich growth condition, is investigated as a function of growth temperature and III–V ratio using atomic force microscopy. V-shaped pits on the InGaN surface are found to diminish with the decrease in growth temperature and remain unchanged with variation in III–V ratio, while exhibiting a significant impact on surface roughness, crystal quality, and optical properties. Fluctuation in density and depth of V-shaped pits is discussed, taking into account the low growth temperature of PA-MBE.

Cold Acclimation of the Thermoacidophilic Red Alga Galdieria sulphuraria: Changes in Gene Expression and Involvement of Horizontally Acquired Genes.

Galdieria sulphuraria is a unicellular red alga that lives in hot, acidic, toxic metal-rich, volcanic environments, where few other organisms survive. Its genome harbors up to 5% of genes that were most likely acquired through horizontal gene transfer. These genes probably contributed to G.sulphuraria's adaptation to its extreme habitats, resulting in today's polyextremophilic traits. Here, we applied RNA-sequencing to obtain insights into the acclimation of a thermophilic organism towards temperatures below its growth optimum and to study how horizontally acquired genes contribute to cold acclimation. A decrease in growth temperature from 42�C/46�C to 28�C resulted in an upregulation of ribosome biosynthesis, while excreted proteins, probably components of the cell wall, were downregulated. Photosynthesis was suppressed at cold temperatures, and transcript abundances indicated that C-metabolism switched from gluconeogenesis to glycogen degradation. Folate cycle and S-adenosylmethionine cycle (one-carbon metabolism) were transcriptionally upregulated, probably to drive the biosynthesis of betaine. All these cold-induced changes in gene expression were reversible upon return to optimal growth temperature. Numerous genes acquired by horizontal gene transfer displayed temperature-dependent expression changes, indicating that these genes contributed to adaptive evolution in G.sulphuraria.

Thermal Analysis for the Prediction of Grain Refinement: An Experimental Investigation on an AlSiMg Foundry Alloy

Thermal analysis is widely used as a prediction tool for the quality of Al alloys before casting. In the present work, the effects of different grain refiners on the characteristic temperatures and on the grain size of α-Al phase were studied by thermal analysis and metallographic investigations. The response of an AlSiMg alloy towards grain refiners, added in form of rods and tabs, was investigated. In foundry practice, the fading phenomenon of grain refiners is well-known but not completely understood. For these reasons, the fading effect of each refiner at 60 minutes and 120 minutes holding times was also studied. Cooling curves and their derivatives were obtained during solidification of the alloys in a metallic crucible. Experimental data indicated the increase of the temperature associated with the nucleation of α-Al dendrites for the grain-refined alloys in comparison to the untreated ones. Simultaneously, a decrease in primary Al growth temperature, that led to the disappearance of the minimum temperature, were observed. Microstructural features revealed that an increase of 6 ÷ 7 °C of the nucleation temperature, compared to the not refined alloy, corresponds to a significant decrease in average grain size.