High-density Gas Research Articles

Crack-Free High-Composition (>35%) Thick-Barrier (>30 nm) AlGaN/AlN/GaN High-Electron-Mobility Transistor on Sapphire with Low Sheet Resistance (<250 Ω/□)

In this article, a high-composition (>35%) thick-barrier (>30 nm) AlGaN/AlN/GaN high-electron-mobility transistor (HEMT) structure grown on a sapphire substrate with ultra-low sheet resistivity (<250 Ω/□) is reported. The optimization of growth conditions, such as reduced deposition rate, and the thickness optimization of different epitaxial layers allowed us to deposit a crack-free high-composition and thick AlGaN barrier layer HEMT structure. A significantly high two-dimensional electron gas (2DEG) density of 1.46 × 1013 cm−2 with a room-temperature mobility of 1710 cm2/V·s was obtained via Hall measurement using the Van der Pauw method. These state-of-the-art results show great potential for high-power Ga-polar HEMT design on sapphire substrates.

Atomic-Level Description of Chemical, Topological, and Surface Morphology Aspects of Oxide Film Grown on Polycrystalline Aluminum during Thermal Oxidation—Reactive Molecular Dynamics Simulations

Oxidation results in the formation of an oxide film whose properties and structure can be tailored by controlling the oxidation conditions. Reactive molecular dynamics simulations were performed to study thermal oxidation of polycrystalline Al substrates as a function of O2 density and temperature. The structural, chemical, and topological aspects of polycrystalline Al (poly-Al) substrates and oxide films formed upon oxidation were studied. The studies were supported by surface topography and morphology analyses before and after oxidation. An analysis of Al–O atomic pair distribution showed the development of long-range order in the oxide films grown upon exposure to low-density (0.005 g/cm3) and high-density (0.05 g/cm3) O2 gas. The long-range order was more apparent for the high-density environment, as the oxide films formed in low-density O2 gas did not fully cover the poly-Al surface. The dominance of over-coordinated polyhedral units in a tightly packed structure was indicative of medium- and long-range atomic order in the oxide films. The two-phase structure of the oxide was found in the films, with a crystalline phase at the metal/oxide interface and an amorphous phase at the oxide/O2 interface. The combination with topological analyses supported the conclusions of the chemical analysis and enabled us to capture an amorphous-to-crystalline phase transformation in the oxide films with increasing oxygen density and temperature. An important effect of Al surface roughness before oxidation on the behavior of the metal/oxide interface and on the oxide film structure was observed.

3D modeling of positive streamers in air with inhomogeneous density

We study the effect of an inhomogeneous gas density on positive streamer discharges in air using a 3D fluid model with stochastic photoionization, generalizing earlier work with a 2D axisymmetric model by Starikovskiy and Aleksandrov (2019 Plasma Sources Sci. Technol. 28 095022). We consider various types of planar and (hemi)spherical gas density gradients. Streamers propagate from a region of density towards a region of higher or lower gas density , where corresponds to and . We observe that streamers can always propagate into a region with a lower gas density. When streamers enter a region with a higher gas density, branching can occur at the density gradient, with branches growing in a flower-like pattern over the gradient surface. Depending on the gas density ratio, the gradient width and other factors, narrow branches are able to propagate into the higher-density gas. In a planar geometry, we find that such propagation is possible up to a gas density slope of , although this value depends on a number of conditions, such as the gradient angle. Surprisingly, a higher applied voltage makes it more difficult for streamers to penetrate into the high-density region, due to an increase of the primary streamer’s radius.

The incorporation of xenon at point defects and bubbles in uranium mononitride

Uranium mononitride (UN) has been proposed as an accident tolerant fuel for nuclear fission reactors and offers enhanced performance during accident scenarios relative to the current fuel, uranium dioxide. However, its performance in reactor is significantly less well understood than for the oxide. Therefore, this work explores incorporation of Xe into UN using density functional theory to understand the early stages of fission gas evolution. These results are used to derive a new potential for Xe in UN, which is then employed to simulate the growth of xenon bubbles in spherical voids of various sizes at 300 K and 1200 K. At sufficiently high gas densities, the xenon was found to mainly crystallise in an fcc arrangement. Loop punching was observed at 10.2 GPa and above for larger bubbles of 4.8 nm radius, significantly so for higher temperatures. This work suggests that no Xe undergoes thermal resolution at temperatures up to 1200 K and that the UN lattice prefers to undergo deformation instead.

High-density 2D hole gas in p-GaN/AlN/AlGaN on a silicon substrate with polarization-enhanced Mg ionization

Improving the hole concentration in p-GaN specimens has posed a major challenge due to the high activation energy of Mg doping in GaN. Recently, a delta doping technique for modulating the valence band and increasing the Mg ionization efficiency in GaN has been proposed. However, the memory effect of Mg in the reaction chamber greatly increases the difficulty of epitaxial growth; additionally, the Mg ionization efficiency remains too low. In this study, we demonstrate a novel p-GaN/AlN/AlGaN structure with an increased Mg ionization efficiency, from 5.16% to 14.22%, relative to the conventional p-GaN/AlGaN structure. The inserted AlN layer effectively modulates the valence band of p-GaN near the p-GaN/AlN interface and decreases the activation energy, as confirmed by density functional theory and the 1D Poisson–Schrodinger equation. Moreover, a high-performance p-channel GaN FinFET with a high output current of 114 mA/mm is fabricated, and the polarization-enhanced epitaxial layer shows great promise for next-generation digital ICs.

Equation of state for helium-xenon gas mixture studied by molecular dynamics simulations

In the framework of long term behaviour of nuclear spent fuels, the performance codes need physical data base such as fission gas equation of state in order to assess the pressure inside fission gas bubbles. The high pressures expected in such bubbles require gas equation of state, which are not easily accessible in the literature. Therefore, molecular dynamics simulations are carried out to assess the helium-xenon mixture equation of state in these extreme conditions. First, we confirm that the interatomic potentials used, which are key parameter for the molecular dynamics simulations, reproduce very well the existing experimental data of the pure gases. This first study provides also new data for the melting point of pure xenon in the range of 500 K to 1500 K under high pressures. Second, the simulations of the helium-xenon mixture confirm that this mixture behaves like van der Waals compounds with the presence of a gas-gas separation in equilibrium at high densities. The miscibility gap, already known for temperatures under 333 K, was extended at higher temperatures until 800 K for high gas densities. This behaviour yields to the creation of an heterogeneous gas mix, which could curb the creation of bubble containing this mixture. Finally, we have fitted with the Happel formalism a new equation of state for the helium-xenon gas mixture, which is valid over temperatures ranging from 200 K to 1000 K and for atomic volumes higher than 0.27×10−28 m3 (i.e., densities lower than 6.1×104 mol.m−3).

Origin of medium-range atomic correlation in simple liquids: Density wave theory

The atomic pair-distribution function of simple liquid and glass shows exponentially decaying oscillations beyond the first peak, representing the medium-range order (MRO). The structural coherence length that characterizes the exponential decay increases with decreasing temperature and freezes at the glass transition. Conventionally, the structure of liquid and glass is elucidated by focusing on a center atom and its neighboring atom shell characterized by the short-range order (SRO) and describing the global structure in terms of overlapping local clusters of atoms as building units. However, this local bottom-up approach fails to explain the strong drive to form the MRO, which is different in nature from the SRO. We propose to add an alternative top-down approach based upon the density wave theory. In this approach, one starts with a high-density gas state and seeks to minimize the global potential energy in reciprocal space through density waves using the pseudopotential. The local bottom-up and global top-down driving forces are not mutually compatible, and the competition and compromise between them result in a final structure with the MRO. This even-handed approach provides a more intuitive explanation of the structure of simple liquid and glass.

Unveiling the Formation of the Massive DR21 Ridge

We present new 13CO (1−0), C18O (1−0), HCO+ (1−0), and H13CO+ (1−0) maps from the IRAM 30 m telescope and a spectrally resolved [C ii] 158 μm map observed with the SOFIA telescope toward the massive DR21 cloud. This traces the kinematics from low- to high-density gas in the cloud, which allows us to constrain the formation scenario of the high-mass star-forming DR21 ridge. The molecular line data reveal that the subfilaments are systematically redshifted relative to the dense ridge. We demonstrate that [C ii] unveils the surrounding CO-poor gas of the dense filaments in the DR21 cloud. We also show that this surrounding gas is organized in a flattened cloud with curved redshifted dynamics perpendicular to the ridge. The subfilaments thus form in this curved and flattened mass reservoir. A virial analysis of the different lines indicates that self-gravity should drive the evolution of the ridge and surrounding cloud. Combining all results, we propose that bending of the magnetic field, due to the interaction with a mostly atomic colliding cloud, explains the velocity field and resulting mass accretion on the ridge. This is remarkably similar to what was found for at least two nearby low-mass filaments. We tentatively propose that this scenario might be a widespread mechanism to initiate star formation in the Milky Way. However, in contrast to low-mass clouds, gravitational collapse plays a role on the parsec scale of the DR21 ridge because of the higher density. This allows more effective mass collection at the centers of collapse and should facilitate massive cluster formation.

ТЕРМОХИМИЧЕСКАЯ РЕКУПЕРАЦИЯ ТЕПЛОТЫ В ГАЗОТУРБИННОЙ УСТАНОВКЕ

Natural gas every year wins an increasing share in the global fuel balance. Countries where cheap natural gas is available are seeking to increase the efficiency of its use in order to increase the volume of its use. One of the most promising methods for increasing the efficiency of natural gas is the heat recovery of gas waste. The subject of the study is a gas turbine plant due to the fact that the number of power plants with gas turbines tends to increase. In this paper, options for increasing the efficiency of using natural gas in gas turbines at a constant flow rate are investigated. Variants of implementation of thermochemical heat recovery of gas wastes based on steam reforming of methane and thermal heat recovery of gas wastes based on steam generation are considered. The results of a study based on a mathematical model of a gas turbine plant using thermal and thermochemical waste gas heat recovery are presented.
 A constructive calculation of the methane steam reforming reactor has been carried out. The design and regime parameters of the heat exchanger are determined. A 3D-model of the conversion reactor has been developed, which provides high gas density and the possibility of thermal elongation of heat exchange tubes

Propagation of surface magnetoplasmon polaritons in a symmetric waveguide with two-dimensional electron gas

The properties of surface magnetoplasmon polaritons (SMPPs) in a symmetric structure, composed of two semi-infinite regions of high-density two-dimensional electron gas (2DEG) separated by a thin film in Voigt configuration, are investigated. The normal and absorption dispersion relations for the transverse magnetic polarization are derived by correlating Maxwell’s equation and the boundary conditions. It is demonstrated that the features of SMPPs are greatly influenced by the external magnetic field, collision frequency of 2DEG, the dielectric constant, and the thickness of the thin film, suggesting that the locations and propagation lengths of SMPPs can be governed accordingly. It is shown that the symmetry of the physical geometry preserves the symmetry of the dispersion relations of SMPPs. Furthermore, it is discovered that as the external magnetic field increases, the penetration depth of SMPPs decreases, while their energy loss reduces, implying that plasmons can propagate for longer distances. Additionally, it is observed that SMPPs in the symmetric configuration have a longer lifetime than those in the asymmetric configuration.

ALMA-IMF

Context.Among the most central open questions regarding the initial mass function (IMF) of stars is the impact of environment on the shape of the core mass function (CMF) and thus potentially on the IMF.Aims.The ALMA-IMF Large Program aims to investigate the variations in the core distributions (CMF and mass segregation) with cloud characteristics, such as the density and kinematic of the gas, as diagnostic observables of the formation process and evolution of clouds. The present study focuses on the W43-MM2&MM3 mini-starburst, whose CMF has recently been found to be top-heavy with respect to the Salpeter slope of the canonical IMF.Methods.W43-MM2&MM3 is a useful test case for environmental studies because it harbors a rich cluster that contains a statistically significant number of cores (specifically, 205 cores), which was previously characterized in Paper III. We applied a multi-scale decomposition technique to the ALMA 1.3 mm and 3 mm continuum images of W43-MM2&MM3 to define six subregions, each 0.5–1 pc in size. For each subregion we characterized the probability distribution function of the high column density gas,η-PDF, using the 1.3 mm images. Using the core catalog, we investigate correlations between the CMF and cloud and core properties, such as theη-PDF and the core mass segregation.Results.We classify the W43-MM2&MM3 subregions into different stages of evolution, from quiescent to burst to post-burst, based on the surface number density of cores, number of outflows, and ultra-compact HII presence. The high-mass end (>1M⊙) of the subregion CMFs varies from close to the Salpeter slope (quiescent) to top-heavy (burst and post-burst). Moreover, the second tail of theη-PDF varies from steep (quiescent) to flat (burst and post-burst), as observed for high-mass star-forming clouds. We find that subregions with flat secondη-PDF tails display top-heavy CMFs.Conclusions.In dynamical environments such as W43-MM2&MM3, the high-mass end of the CMF appears to be rooted in the cloud structure, which is at high column density and surrounds cores. This connection stems from the fact that cores and their immediate surroundings are both determined and shaped by the cloud formation process, the current evolutionary state of the cloud, and, more broadly, the star formation history. The CMF may evolve from Salpeter to top-heavy throughout the star formation process from the quiescent to the burst phase. This scenario raises the question of if the CMF might revert again to Salpeter as the cloud approaches the end of its star formation stage, a hypothesis that remains to be tested.

Synergistically tailoring the distribution of precursor gas for uniform internal DLC coating of long tube

The uniformity of DLC film on the inner surface of a long pipe prepared by the Plasma Enhanced Chemical Vapor Deposition (PECVD) technique. Four gas inlet modes were used to deposit DLC coating in the tube with a diameter of 100 mm and 2000 mm in length. Firstly, the finite element method was explored to investigate the effects of pores distribution on the precursor gas density in the tube. The result showed the gas density distribution of case (iv) was uniform. Then, the film thickness, surface roughness, contact angle, the film structure, hardness were measured by a scanning electron microscopy, atom force microscopy, Raman spectrometer and nano indenter, respectively. The results revealed that the optimizing the gas pore distribution improved the uniformity of film thickness, roughness and hardness, which correspond with the finite element method. Furthermore, the thickness for case(iv) is about 4 μm with a uniformity of 3.8 %. The regions of thicker film were with higher gas density. This study indicates a feasible solution to obtain uniform DLC film in long tube by synthetically tailoring the input mode of precursor gas.

Multi-pass Differential Photoacoustic Sensor for Real-Time Measurement of SF6Decomposition Component H2S at the ppb Level

We designed and implemented a photoacoustic (PA) sensor for H2S detection in SF6 background gas based on a multi-pass differential photoacoustic cell (MDPC) and a near-infrared distributed feedback (DFB) laser. In the MDPC apparatus, two resonators with identical geometric parameters were vertically and symmetrically embedded. The differential processing algorithm of two phase-reversed signals realized the effective enhancement of the PA signal and suppressed the flow noise in the dynamic sampling process. In addition, the λ/4 buffer chamber in the MDPC was utilized as a muffler to further reduce the flow noise and realize the dynamic detection of H2S. The collimated excitation light was reflected 30 times in a multi-pass structure constituted of two gold-plated concave mirrors, and an absorption path length of 4.92 m was achieved. Due to the high gas density of SF6, the relationship between the signal-to-noise ratio (SNR) and the gas flow was different between SF6 and N2 background gases. The maximum flow rate of the characteristic gas components detected in the SF6 background is 150 standard cubic centimeters per minute (SCCM), which is lower than 350 SCCM in N2. The linearity property was analyzed, and the results show that the sensitivity of the sensor to H2S in the SF6 background was 27.3 μV/ppm. With the structure, parameters, temperature, gas flow, and natural frequency of the MDPC been optimized, a minimum detection limit (MDL) of 11 ppb was reached with an averaging time of 1000 s, which furnished an effective preventive implement for the safe operation of gas insulation equipment.

A novel p-GaN HEMT with AlInN/AlN/GaN double heterostructure and InAlGaN back-barrier

In this paper, a novel p-type gallium nitride (p-GaN) high electron mobility transistor (HEMT) is proposed and investigated for high power electronics applications. In the proposed device, an AlInN/AlN/GaN double heterostructure has been used. In addition, InAlGaN back-barrier has also been employed to improve the DC performance of the device. The double AlInN/GaN heterostructures offer low resistance, resulting to a high two-dimensional electron gas (2DEG) density. As a result the current drive efficiency of the device is significantly improved. On the other hand, an inserted InAlGaN back-barrier confines the electrons in the channel which further enhances the performance of the device. To validate the simulation results, calibration of the device models has been performed with experimental results. Results show that the proposed device exhibits low on-resistance (Ron), high drain current (Ids) and high transconductance (gm) as compared to the state-of-the-art devices. Also, the processing steps have been proposed for the possible fabrication of the proposed device. Finally, the optimization of the device design parameter is performed to achieve the optimal device performance with respect to Ron.

X-Ray Unveiling Events in a z ≈ 1.6 Active Galactic Nucleus in the 7 Ms Chandra Deep Field-South

We investigate the extreme X-ray variability of a z = 1.608 active galactic nucleus in the 7 Ms Chandra Deep Field-South (XID 403), which showed two significant X-ray brightening events. In the first event, XID 403 brightened by a factor of >2.5 in ≲6.1 rest-frame days in the observed-frame 0.5–5 keV band. The event lasted for ≈5.0–7.3 days, and then XID 403 dimmed by a factor of >6.0 in ≲6.1 days. After ≈1.1–2.5 yr in the rest frame (including long observational gaps), it brightened again, with the 0.5–5 keV flux increasing by a factor of >12.6. The second event lasted over 251 days, and the source remained bright until the end of the 7 Ms exposure. The spectrum is a steep power law (photon index Γ = 2.8 ± 0.3) without obscuration during the second outburst, and the rest-frame 2–10 keV luminosity reaches ; there is no significant spectral evolution within this epoch. The infrared-to-UV spectral energy distribution of XID 403 is dominated by the host galaxy. There is no significant optical/UV variability and R-band (rest-frame ≈2500 Å) brightening contemporaneous with the X-ray brightening. The extreme X-ray variability is likely due to two X-ray unveiling events, where the line of sight to the corona is no longer shielded by high-density gas clumps in a small-scale dust-free absorber. XID 403 is probably a high-redshift analog of local narrow-line Seyfert 1 galaxies, and the X-ray absorber is a powerful accretion disk wind. On the other hand, we cannot exclude the possibility that XID 403 is an unusual candidate for tidal disruption events.

A comparative investigation of the effect of gas type on foam strength and flow behavior in tight carbonates

Foam generation technique has been practically applied to overcome gas mobility issue during enhanced oil recovery (EOR) processes. Incorporated with the utilized gas, the capability of foaming agent to generate persistent foam determines the flow performance under reservoir conditions. In this study, we first identified the surfactants with excellent phase behavior and foaming properties and, second, evaluated their bulk foam performance with different gases, including N2, CO2, and hydrocarbon gas (CH4) under high-pressure and high-temperature conditions. In bulk, the foam generated with the high-density gas, i.e., CO2, exhibited the greatest foamability but the lowest foam stability. With amidoamine oxide-based surfactant, CH4-foam was found to be more stable than N2-foam, while the opposite was observed for sulfobetaine-based surfactant. The most effective surfactant was later used to perform flow experiments on two tight carbonate rocks, including Minnesota Norther Cream Buff and Edwards limestone, with permeabilities of ∼0.8 and 16 mD, respectively. The core-flooding experiments showed that the optimum quality of CO2-foam was lower than that of N2- and CH4-foams. For a given flow rate and fixed foam quality, the steady-state strength followed the descending order of CH4-, N2-, and CO2-foam. However, in general, the difference in foam strength between N2- and CH4-foams was insignificant when the total flow rate was varied. We also found that the variation in rock permeability had a greater effect on the strength of the CO2-foam compared to those generated with N2 and CH4. In the presence of oil, the shear-thinning behavior of all foams was found more prominent. The increase in oil saturation had a more adverse effect on the strength of N2- and CH4-foams, particularly in the higher permeability rock. The results indicated that the foam strength, which is key in the effectiveness of foam EOR applications, is greatly influenced by several factors such as gas density, gas solubility in water, the presence of oil, and rock permeability.

On the surface oxidation and band alignment of ferroelectric Sc0.18Al0.82N/GaN heterostructures

Ferroelectric scandium aluminum nitride (ScAlN), owing to its unique properties and seamless integration with the mainstream GaN technology, has attracted considerable attention as one of the most promising materials for next-generation multifunctional electronics. To date, however, a detailed understanding of the fundamental surface properties and the band alignment of ScAlN/III-nitride heterostructure has remained elusive. In this work, with the use of high-resolution X-ray photoelectron spectroscopy measurements, we demonstrate the existence of a considerable oxidation layer on ScAlN when it is exposed to air, which shows a significant impact on the material characterization and electronic structure evaluation. By excluding the possible impact from the surface oxide layer, the band alignment of Sc0.18Al0.82N/GaN can be correctly determined. Simulation results further showed that the Sc0.18Al0.82N barrier layer could offer excellent charge carrier confinement and a high density of two-dimensional electron gas (2DEG) at the heterostructure interface, which is needed for GaN-based high electron mobility transistors (HEMTs) with enhanced performance. This work not only identifies the band alignment between Sc0.18Al0.82N and GaN for future novel ScAlN/III-nitride device applications but also highlights the importance of considering the prevalent surface oxidation issue in the emerging ferroelectric nitride family.

Spatial distributions and kinematics of shocked and ionized gas in M17

ABSTRACTMassive stars are formed in molecular clouds, and produce H ii regions when they evolve onto the main sequence. The expansion of H ii region can both suppress and promote star formation in the vicinity. M17 H ii region is a giant cometary H ii region near many massive clumps containing starless and protostellar sources. It is an appropriate target to study the effect of feedback from previously formed massive stars on the nearby star-forming environments. Observations of SiO 2-1, HCO+ 1-0, H13CO+ 1-0, HC3N 10-9, and H41 α lines are performed toward M17 H ii region with ambient candidates of massive clumps. In the observations, the widespread shocked gas surrounding M17 H ii region is detected: it probably originates from the collision between the expanding ionized gas and the ambient neutral medium. Some massive clumps are found in the overlap region of the shock and dense-gas tracing lines while the central velocities of shocked and high-density gases are similar. This suggests that part of massive clumps are located in the shell of H ii region, and may be formed from the accumulated neutral materials in the shell. In addition, by comparing the observations towards M17 H ii region with the simulation of cometary H ii region, we infer the presence of one or more massive stars travelling at supersonic velocity with respect to the natal molecular cloud in the H ii region.

Transportation and deformation of high-speed aluminum nanoparticles in inert gas with molecular dynamics study

In addition to complex deformation, high-speed nanoparticles in gas are also accompanied by significant size and interfacial effects. In this work, we simulate the transportation behavior of high-speed aluminum nanoparticles in helium gas with the classical molecular dynamics method. The evolution of aerothermodynamic quantities of solid particles and liquid particles is revealed, and different temperature rise effects are found. Furthermore, the melting of aluminum particles induced by high aerodynamic drag force is discovered, and the melting threshold conditions are proposed. In low-density (0.002 g/cm3) and high-density (0.02 g/cm3) gas, the initial velocity at which particles start to melt is 6 and 4 km/s, respectively. During the deformation of solid particles, the evolution of dislocation motion is discussed, and the evolution of the development characteristics of the molten layer is given. During the deformation of the liquid particles, vibration deformation and bag deformation modes are observed. The threshold conditions for deformation mode transitions are also given. Only in high-density gas, bag deformation occurs when the initial velocity of particles (D > 5 nm) exceeds 6 km/s. The local mechanical quantity of gas is used to explain the variation of the drag force of the particles. Moreover, the drag force model is corrected according to temperature and deformation effects. Within a certain period, the model results overestimate the drag force, and the error with the simulation results is about 25%. This provides a model reference for high-speed nanoparticle dynamics and two-phase flow problems.

Circumplanetary disk ices

Context. The subsurface oceans of icy satellites are among the most compelling among the potentially habitable environments in our Solar System. The question of whether a liquid subsurface layer can be maintained over geological timescales depends on its chemical composition. The composition of icy satellites is linked to that of the circumplanetary disk (CPD) in which they form. The CPD accretes material from the surrounding circumstellar disk in the vicinity of the planet, however, the degree of chemical inheritance is unclear. Aims. We aim to investigate the composition of ices in chemically reset or inherited circumplanetary disks to inform interior modeling and the interpretation of in situ measurements of icy solar system satellites, with an emphasis on the Galilean moon system. Methods. We used the radiation-thermochemical code ProDiMo to produce circumplanetary disk models and then extract the ice composition from time-dependent chemistry, incorporating gas-phase and grain-surface reactions. Results. The initial sublimation of ices during accretion may result in a CO2 -rich ice composition due to efficient OH formation at high gas densities. In the case of a Jovian CPD, the sublimation of accreted ices results in a CO2 iceline between the present-day orbits of Ganymede and Callisto. Sublimated ammonia ice is destroyed by background radiation while drifting towards the CPD midplane. Liberated nitrogen becomes locked in N2 due to efficient self-shielding, leaving ices depleted of ammonia. A significant ammonia ice component remains only when ices are inherited from the circumstellar disk. Conclusions. The observed composition of the Galilean moons is consistent with the sublimation of ices during accretion onto the CPD. In this scenario, the Galilean moon ices are nitrogen-poor and CO2 on Callisto is endogenous and primordial. The ice composition is significantly altered after an initial reset of accreted circumstellar ice. The chemical history of the Galilean moons stands in contrast to the Saturnian system, where the composition of the moons corresponds more closely with the directly inherited circumstellar disk material.