Loose Core Research Articles

Facile Preparation of Cellulose Beads with Tunable Graded Pores and High Mechanical Strength.

Cellulose-based hierarchical porous beads exhibit significant application potential in adsorption and separation systems due to their degradation and biocompatibility. However, the current fabrications of cellulose beads show poor mechanical properties and a difficult-to-regulate hierarchical porous structure, reducing their lifespan of use and limiting their application in fine separation. Here, we reported the facile creep-drop method to prepare cellulose beads that enabled systemic regulation of the macro-size, micropore structures, and mechanical properties by optimizing injection nozzle diameter, the composition of the coagulation bath, the temperature of the coagulation bath, and cellulose concentration. Notably, during the molding process, the H2SO4-Na2SO4 composite solidification bath endowed cellulose beads with a dense shell layer and a loose core layer, which achieved the integration of mechanical properties and high porosity. The cellulose beads exhibited high porosity (93.38-96.18%) and high sphericity (86.78-94.44%) by modulating the shell thickness of the cellulose beads. In particular, the cellulose beads exhibited excellent mechanical properties with a high compressive strength of 544.24 kPa at a 5% cellulose concentration. It is expected that these cellulose beads with tunable microstructures can realize their potential for applications in the fields of wastewater treatment, chemical engineering, bioengineering, medicine, and pharmaceuticals.

A Review of Power Transformer Vibration and Noise Caused by Silicon Steel Magnetostriction

As an important hub equipment in the power system, the power transformer has increasingly higher requirements for its reliability and safety with the rapid development of the power industry. However, the vibrations and noise generated during its operation not only cause a series of issues such as loose windings, core wear, and fatigue of fasteners, but also lead to anxiety and unease among nearby residents, severely affecting their physical and mental health. Therefore, the generation mechanism and control measures of vibration and noise in power transformers have attracted extensive attention from scholars. The magnetostrictive effect of silicon steel sheets is one of the important factors in the causes of transformer vibration and noise. This paper focuses on the magnetostrictive effect of silicon steel sheets. Firstly, the micro mechanism of magnetostriction is introduced. Secondly, based on current research status, the vibration and noise mechanism caused by magnetostriction are summarized, and the influencing factors, measurement, modeling, and noise reduction methods based on magnetostriction are summarized. Finally, based on the current research status and future development trends, prospects are proposed, aiming to provide references for subsequent research on transformer vibration and noise.

Diagnosis of Loose Core Fault in Saturable Reactor of Thyristor Valve Based on Vibration Signal Time-Frequency Analysis and CNN

The saturable reactor is one of the core components of the thyristor valve, which plays a role in suppressing current changes and balancing voltage distribution. Due to long-term operation under complex working conditions, the vibration of the reactor can cause the core to loosen. To detect faults in time, a fault diagnosis method is proposed for loose cores in saturable reactors of thyristor valves based on time-frequency analysis of vibration signals and convolutional neural networks. The two-dimensional time-frequency spectrum of the vibration signal is obtained using the short-time Fourier transform method. A convolutional neural network model structure is designed to extract time-frequency spectrum features and diagnose different levels of core looseness faults through learning and training. An experiment was carried out to collect core fault information under variable working conditions. The effectiveness of the method proposed in this paper was verified based on experimental data validation. Through comparative experiments, it was found that the proposed method is superior to existing fault diagnosis methods.

An Efficient and Stable Embedded Multi-U-Shaped Column Rotary Transformer

Inductively Coupled Power Transfer (ICPT) technology can be used to solve the problems of high friction loss, overheating and low stability caused by conductive slip rings on rotating power supply equipment. An improved Embedded Multi-U-Shaped Column Rotary Transformer (EMUCRT) based on the ICPT and spliced core structure is proposed in this paper. It has a higher coupling coefficient and can transmit power more efficiently and stably due to the primary magnetic core is embedded inside the secondary magnetic core. Compared with the one-piece core structure, the spliced type is more flexible due to its loose magnetic core arrangement. Its primary coil can still transmit energy stably after offset by a certain distance, so the structure has good anti-misalignment ability. On the basis of analyzing its magnetic core arrangement and magnetic circuit model, the optimal magnetic core specifications and quantities of primary and secondary side are obtained by theoretical calculation and simulation. In order to verify the high efficiency, rotational stability and anti-misalignment characteristics of the structure, an experimental prototype was built. The results show that EMUCRT can transmit energy efficiently and has good rotational and offset stability.

An EHV Transformer Fault Diagnosis Method Based on Acoustic Perception and Sparse Autoencoder-BNCNN

The EHV transformer is one of the core transmission equipment of China’s EHV/UHV transmission system, and its reliability directly affects the operational stability of the power grid. In the long-term operation process, affected by sudden short-circuit, resonance, and aging, it will produce faults such as loose winding, loose iron core, inverted discharge, and shim block shedding. Most of the methods for transformer fault diagnosis need off-line monitoring and cause some damage to transformer equipment. At present, the use of transformer acoustic signal for diagnosis and identification is in the initial stage, and there is still room for improvement in the accuracy of fault diagnosis, and how to classify the unbalanced data samples also deserves attention. Therefore, an EHV transformer fault diagnosis method based on acoustic perception and sparse autoencoder-BNCNN is proposed in this paper. Firstly, the sparse autoencoder is trained to ensure the processing ability of the network on imbalanced data. Then, the high-step CNN network is optimized using the BN layer, the data is fed into the improved CNN network for feature extraction, and the classifier outputs the fault diagnosis results. Finally, the transformer acoustic data are processed, and the dataset is formed to evaluate the performance of the proposed method. The experimental results show that the proposed method not only improves the accuracy substantially compared with other methods but also has the ability to handle imbalanced data.

‘Another wall of turf’

Excavation of a well-preserved stretch of the rampart of the Antonine Wall east of Watling Lodge, at Laurieston, allowed for the first micromorphological study of its earthen building materials. This revealed that the rampart core, as well as the cheeks, were constructed in well layered courses of turf blocks – but sourced from different grasslands. The evidence differs from macroscopic observations made at other sections in this area where the rampart material had been interpreted as representing a loose earth core with clay cheeks. Our results show that even when the characteristic striped sections indicative of turf are not visible in the field, thin-section analysis can confirm the use of intact soil blocks with the remains of grassed surfaces. It now seems possible that the visible variation between materials in the eastern and western sectors of the Antonine Wall may simply be due to different types of turf used, varying in subsoil composition and topsoil formation, and representing differences in landscape management and survival of vegetation. Combined with macroscopic field recording to identify Roman building practices, our analysis of this section at Laurieston also shows the care that was taken to construct a level, well-draining base for the rampart to avoid slumping and moisture build-up. Further excavations and thin-section analysis elsewhere along the Wall are now needed to confirm whether turf was more extensively used than so far anticipated for the eastern sector and whether the Antonine Wall could possibly have been built completely of turf. Such conclusions would suggest a much more standardised construction process and more extensive grassland exploitation than considered up until now. While our results demonstrate the importance of micromorphological analysis for understanding this earthen UNESCO World Heritage site, our interdisciplinary approach may also have wider relevance for research on linear earthworks in different geographical and chronological settings.

Loose Cores and Cycles in Random Hypergraphs

Inspired by the study of loose cycles in hypergraphs, we define the loose core in hypergraphs as a structurewhich mirrors the close relationship between cycles and $2$-cores in graphs. We prove that in the $r$-uniform binomial random hypergraph $H^r(n,p)$, the order of the loose core undergoes a phase transition at a certain critical threshold and determine this order, as well as the number of edges, asymptotically in the subcritical and supercritical regimes.
 Our main tool is an algorithm called CoreConstruct, which enables us to analyse a peeling process for the loose core. By analysing this algorithm we determine the asymptotic degree distribution of vertices in the loose core and in particular how many vertices and edges the loose core contains. As a corollary we obtain an improved upper bound on the length of the longest loose cycle in $H^r(n,p)$.

Self-assembly of cyclic grafted copolymers with rigid rings and their potential as drug nanocarriers

Enhancing the performance of polymer micelles by purposeful regulation of their structures is a challenging topic that receives widespread attention. In this study, we systematically conduct a comparative study between cyclic grafted copolymers with rigid and flexible rings in the self-assembly behavior via dissipative particle dynamics (DPD) simulation. With a focus on the possible stacking ways of rigid rings, we propose the energy-driven packing mechanism of cyclic grafted copolymers with rigid rings. For cyclic grafted copolymers with large ring size (14 and 21-membered rings), rigid rings present a novel channel-layer-combination layout, which is determined by the balance between the potential energy of micelles (Emicelle) and the interaction energy between water and micelles (Eint). Based on this mechanism, we further regulate a series of complex self-assembling structures, including curved rod-like, T-shape, annular and helical micelles. Compared with flexible copolymers, cyclic grafted copolymers with rigid rings provide a larger and loose hydrophobic core and higher structural stability with micelles due to the unique packing way of rigid rings. Therefore, their micelles have a great potential as drug nanocarriers. They possess a better drug loading capacity and disassemble more quickly than flexible counterparts under acidic tumor microenvironment. Furthermore, the endocytosis kinetics of rigid micelles is faster than the flexible counterparts for the adsorption and wrapping process. This study may provide a reasonable idea of structural design for polymer micelles to enhance their performance in biomedical applications.

The Two Phase Transitions of Hydrophobically End-Capped Poly(N-isopropylacrylamide)s in Water.

High-sensitivity differential scanning calorimetry (HS-DSC) thermograms of aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions present a sharp unimodal endotherm that signals the heat-induced dehydration/collapse of the PNIPAM chain. Similarly, α,ω-di-n-octadecyl-PNIPAM (C18-PN-C18) aqueous solutions exhibit a unimodal endotherm. In contrast, aqueous solutions of α,ω-hydrophobically modified PNIPAMs with polycyclic terminal groups, such as pyrenylbutyl (Py-PN-Py), adamantylethyl (Ad-PN-Ad), and azopyridine- (C12-PN-AzPy) moieties, exhibit bimodal thermograms. The origin of the two transitions was probed using microcalorimetry measurements, turbidity tests, variable temperature 1H NMR (VT-NMR) spectroscopy, and 2-dimensional NOESY experiments with solutions of polymers of molar mass (Mn) from 5 to 20 kDa and polymer concentrations of 0.1 to 3.0 mg/mL. The analysis outcome led us to conclude that the difference of the thermograms reflects the distinct self-assembly structures of the polymers. C18-PN-C18 assembles in water in the form of flower micelles held together by a core of tightly packed n-C18 chains. In contrast, polymers end-tagged with azopyridine, pyrenylbutyl, or adamantylethyl form a loose core that allows chain ends to escape from the micelles, to reinsert in them, or to dangle in surrounding water. The predominant low temperature (T1) endotherm, which is insensitive to polymer concentration, corresponds to the dehydration/collapse of PNIPAM chains within the micelles, while the higher temperature (T2) endotherm is attributed to the dehydration of dangling chains and intermicellar bridges. This study of the two phase transitions of telechelic PNIPAM homopolymer highlights the rich variety of morphologies attainable via responsive hydrophobically modified aqueous polymers and may open the way to a variety of practical applications.

Micelles with a Loose Core Self‐Assembled from Coil‐g‐Rod Graft Copolymers Displaying High Drug Loading Capacity

AbstractHigh drug loading capacity is one of the critical demands of micellar drug‐delivery vehicles; however, it is a challenging work. Herein, it is demonstrated that micelles self‐assembled from poly(ethylene glycol)‐graft‐poly(γ‐benzyl‐l‐glutamate) (PEG‐g‐PBLG) coil‐g‐rod graft copolymers display high drug‐loading capacity for doxorubicin (DOX) model drugs. As revealed by a combination study of experiments and dissipative particle dynamics simulations, the high drug‐loading capacity of the micelles is related to the loose core structure of the micelles. In these micelles, the hydrophobic PBLG grafts randomly disperse in the micelle core due to their rigid nature and the coil‐g‐rod topology of the graft copolymers, which results in a loose core of the micelles. The structure of the graft copolymer, including the length of rod grafts, the length of coil backbone, and the grafting ratio of the rod grafts affecting the arrangement of the rod grafts in the micelle core has influence on the drug‐loading capacity of the micelles. Besides, the strong π–π stacking interaction between graft copolymers and DOX also plays an important part in achieving high drug‐loading capacity. In vitro studies reveal that these drug‐loaded micelles show good biocompatibility, and the DOX can be gradually released from the micelles.

Fabrication and Evaluation of Weakly Cemented Three-dimensional Large-size Loose Sandstone Core

In order to realize the three-dimensional physical simulation experiment of loose sandstone reservoirs, one of the key points lies in how to obtain the three-dimensional large-size cores that can represent the actual reservoir conditions. However, due to the characteristics of small cementation strength, easy to loose, the large-scale cores drilling on site are difficult to achieve, so the preparation of weakly cemented three-dimensional large-scale loose sandstone cores is proposed. Through the design of the core mold, based on the particle size group and physical parameters of the target reservoir, and considering the macroscopic characteristics such as the actual reservoir thickness and the shale interlayer, weakly cemented three-dimensional large-scale core and saturated probe core are used. The physical properties of formed cores are evaluated and tested. The results show that compared with the actual reservoir, the porosity and permeability error of the core model is kept at about 10%, the precision is high, the homogeneity in the layer is good, the average cementation index of each layer is 1.6, which is a weak cementation category, and the degree of cementation consistent with the target reservoir. The prepared core can be done well in reducing the physical properties and macroscopic structure of the actual reservoir, and provides reliable core source support for the three-dimensional physical simulation experiment of the loose sandstone reservoir.

Bovine beta-casein micelles as delivery systems for hydrophobic flavonoids

The milk protein β-casein (β-CN) is an intrinsically unstructured amphiphilic protein that self-assembles into micelles. Naringenin is the main hydrophobic flavanone in grapefruit and has several beneficial biological effects: it exhibits, for example, antioxidant, anticancer and anti-inflammatory activity. This paper shows that naringenin can be encapsulated in β-CN micelles. Fluorescence spectroscopy, molecular docking modelling, dynamic light scattering (DLS), static light scattering (SLS) and isothermal titration calorimetry (ITC) were applied to characterize the effect of naringenin on the protein association behavior and properties of the resulting micelles. Naringenin binds to β-CN at both pH 7 and pH 2, promotes the formation of micelles with a well-defined size distribution and stabilizes the micelles. It was found that naringenin-containing β-CN micelles have a lower critical micelle concentration (CMC) and a larger aggregation number (Nagg) compared to pure β-CN micelles. SLS and multi-angle DLS results suggest considerable differences between the structures of pure β-CN micelles and naringenin-containing β-CN micelles. In the presence of naringenin spherical micelles were formed with a relatively loose core (“hollow sphere”), while the pure β-CN micelles are smaller and seem to be elliptic. Notably, by uptake of naringenin in the micelles, the concentration of naringenin in aqueous solution could be raised considerably. These findings lead to the conclusion that β-CN micelles are very promising as effective delivery nano-vehicles for hydrophobic bioactive compounds.

Preparation of Hollow Nitrogen Doped Carbon via Stresses Induced Orientation Contraction.

Hollow structured materials are widely applicable in various fields. Although many routes have been explored for getting such materials, a strategy mainly based on physical effect is still deficient. Herein, a "stresses induced orientation contraction" mechanism for preparation of hollow structures is reported. The composites constructed by zeolite imidazolate framework-8 (ZIF8) cores and polymerized dopamine (PDA) shells, upon annealing, form intensive interfacial interactions, which drag the ZIF8 cores outward to restrain their shrinkage. The gradually accumulated stresses in the central position of ZIF8 dodecahedron nanoparticles, then destroy the ZIF8 crystalline cores to form the hollow structures. In this stress-based route for creating hollow interiors with core-shell composites as the starting materials, three critical factors are necessary: 1) an intensive core-shell interfacial interaction; 2) the distinctly higher shrinkage degree of the cores than the shells; and 3) the relatively loose core structures. In oxygen reduction reaction (ORR) tested with three-electrode solution system and Zn-O2 battery, the achieved hollow nitrogen doped carbon (NC) demonstrates ultrahigh catalytic activities. This work gives an absolutely novel strategy for preparation of hollow structures, which may afford the exploration of a wider range of materials system with hollow interiors.

Structure and saccharification of corn stover pretreated with sulfur trioxide micro-thermal explosion and dilute alkali (STEX-DA)

Due to its different organizational structures, dense hard husk, and loose soft core, corn stover (CS) is more resistant to transformation into monosaccharides for biofuel production in comparison to rice straw or wheat straw. In this paper, before pretreatment with sulfur trioxide micro-thermal explosion and dilute alkali (STEX-DA), CS was cut into 2 cm to 3 cm lengths in the transverse direction and cross opening in the vertical direction. During the process of STEX-DA, the structures of cores of corn stover (CS-C) and husks of corn stover (CS-H) were separately studied via scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR) analysis. Furthermore, the components content and reducing sugars yield were calculated. The results showed that the CS-C achieved 66.7% of lignin removal rate, and the reducing sugars yield (calculated by original dry weight of straw) was 70.8%, while the CS-H displayed values of 64.7% and 55.3%, respectively. This result indicated that STEX-DA pretreatment could facilitate corn stover usage as a renewable energy source.

Rheological analysis of core-stabilized Pluronic F127 by semi-interpenetrating network (sIPN) in aqueous solution

A poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (Pluronic F127) micelle system was stabilized using an ultraviolet-induced semi-interpenetrating network (sIPN). The sIPN structure within the micelle cores was found to stabilize the micelles against low temperatures, but affected the resulting material properties. In this study, the rheological properties of Pluronic F127 with sIPN (F127-sIPN) and without sIPN (F127) were compared. The presence of the sIPN structure increased the gelation temperature (Tgel) at the same concentration, and unlike F127, F127-sIPN exhibited strong heating rate dependent and thermodynamically irreversible behaviors. Hard gels containing various concentrations of F127-sIPN and F127 were investigated at 40 °C. At concentrations above 18 wt. %, both F127-sIPN and F127 exhibited similar linear viscoelastic properties due to the tight, ordered core–shell micelles packing, but the two systems exhibited different behaviors below 18 wt. % concentration. To investigate this difference, hard gels with 16 wt. % F127-sIPN and F127 were selected, and two types of nonlinear rheological tests were conducted, i.e., large amplitude oscillatory shear (LAOS) and strain-rate frequency superposition (SRFS) tests. The cage modulus of F127-sIPN obtained from LAOS testing showed it maintained its elastic contribution over the large deformation region meaning that a loose core network still existed. The relaxation time spectrum of F127-sIPN obtained by SRFS testing indicated it had two relaxation modes (fast and slow) whereas that for F127 had only a fast mode. The slow relaxation mode of F127-sIPN is associated with crosslinking of the sIPN. Since these behaviors were not observed in linear rheological tests, it was concluded that nonlinear rheological tests provide more structural information about hard gels.

Prediction of phase angles from dynamic modulus data and implications for cracking performance evaluation

The need for a viscoelastic characterisation of hot mix asphalt is increasing as advanced testing and modelling is incorporated through mechanistic-empirical pavement design and performance-based specifications. Viscoelastic characterisation includes measurement of the mixture stiffness and relative proportion of elastic and viscous response. The most common method is to measure the complex modulus, where dynamic modulus represents the stiffness and the phase angle represents the relative extent of elastic and viscous response. Determination of phase angle from temperature and frequency sweep tests has been challenging, unreliable and prone to error due to a high degree of variability and sensitivity to signal noise. There are also large amounts of historical dynamic modulus data that are either missing phase angle measurements or have poorly measured phase angle data that inhibit their use in further evaluation. This paper evaluates the robustness of phase angle estimation from stiffness data for asphalt mixtures. The objectives of the study are to: (1) evaluate the procedure of estimating phase angle from the slope of log-log stiffness master curve fitted with a generalised logistic sigmoidal curve and compare it with lab measurements and the Hirsch model; (2) assess the effect of measured and predicted phase angles on a mixture Black Space diagram; (3) evaluate the effect of using predicted phase angles on SVECD fatigue analysis particularly regarding damage characteristics curves and fatigue coefficients and (4) evaluate the impact on layered viscoelastic pavement analysis for critical distresses (LVECD) pavement fatigue performance evaluation due to the use of predicted phase angles. Three sets of independent mixtures were evaluated in this study comprising a wide range of mixture conditions. The results indicate good agreement between measured and predicted phase angle values in terms of shape and peak master curve values. In terms of magnitude, the values from both matched very well for certain sets of mixtures and subsequently manifested in similar performance predictions. However, for other sets of mixtures, a considerable difference was observed between measured and predicted phase angle values as well as SVECD and LVECD results. The differences may be attributed to the use of different types of linear variable displacement transducers (loose core versus spring loaded). Another possible explanation for the difference could be the contribution of plastic strain, which may create a difference in phase angles of 1–2°.

Improving control of contamination from waste rock piles

Waste rock piles are made of heterogeneous, coarse-grained rock extracted from mines to reach the ore. The internal structure of a pile has a major impact on water and oxygen movement and hence the production of acid mine drainage or contaminated neutral drainage. This paper illustrates potential avenues to minimise the infiltration of precipitation into the core zone of rock piles by applying a compacted layer on top of each bench, made of finer-grained non-reactive waste rock. Several configurations and characteristics (without and with cover, inclined and horizontal covers, varying hydrogeological properties of the cover material) are evaluated using the numerical three-dimensional fully integrated variably saturated flow model HydroGeoSphere. In these simulations of a single bench, the compacted layer is represented as being homogeneous and isotropic while the loose core waste rock zone is represented using two approaches: (i) the classic equivalent porous media approach and (ii) a medium with randomly generated fractures to represent the effect of macropores on water flow. Short (10 d) and longer-term (1 a) simulations have been conducted with recharge events based on historic observations. The results provide guidelines for the design of efficient compacted layers leading to an improved environmental response of waste rock piles.

Intramicrogel Complexation of Oppositely Charged Compartments As a Route to Quasi-Hollow Structures

We have predicted using computer simulations and have detected with SAXS measurements that pH-sensitive core–shell polyampholyte microgels can form a dense layer (“skin”) at the core–shell interface. The microgels have cationic core and neutral shell at low pH, whereas the core becomes neutral and the shell becomes anionic at high pH. The core and shell are oppositely charged at intermediate pH values. The layer formation is a result of the electrostatic complexation between oppositely charged subchains. We have studied microgels with different core–shell ratios and fractions of ionizable groups and analyzed radial distribution of polymer volume fraction and volume fractions of cationic and anionic groups. We have demonstrated that in many cases complexation of oppositely charged subchains (intermediate pH values) or swelling of charged core with neutral shell (low pH) are responsible for the formation of quasi-hollow structures with a loose core of strongly swollen subchains and dense shell of interpenet...

Mechanical Properties of the Fibrin Network on the Macroscopic and Microscopic Scales

The scaffold of the thrombus is a three-dimensional network of fibrin, a fibrous protein that determines the mechanical properties of the whole clot and plays thereby an important role in hemostasis. With our atomic force microscope (AFM)-based force spectroscopy, called nano-thrombelastography, we followed the nanoscale changes in the viscoelastic properties during fibrin network formation and streptokinase (STK)-induced fibrinolysis in human plasma-derived clot.During nano-thrombelastography we measure the forces acting on an AFM cantilever which is submerged into a plasma or fibrinogen droplet during its cyclic vertical travel. The time-dependent evolution of force resembles well a macroscopic thrombelastogram: force increases gradually with the formation of the elastic fibrin network during coagulation. By measuring the area of the hysteresis loop formed by the cyclic force curves we could follow the changes in the viscous properties of the fibrin network. Effect of STK manifests in a rapid decrease in thrombus elasticity, whereas thrombus viscosity only decayed with a delay.For morphological experiments, clot formation was initiated by adding thrombin in 0,5 IU final concentration to fibrinogen solution on mica surface. In dried samples, scanning AFM images of mature fibrin fibers appeared as 17-nm-high and 12-196-nm-wide filaments. Upon lowering the sodium and chloride concentration of the solution, we observed fibrin networks wherein fibers had increased diameter. We carried out force mapping on native fibers with a diameter between 100-250 nm in Hepes-buffer. Force curves were recorded for each 100-µm2 area previously scanned. The local Young-modulus was calculated by using the Hertz model. The force-volume maps revealed that fibrin fibers exceeding 200 nm diameter have a dense surface layer and a loose core, suggesting that the thickening of the fibrils involves a distinct mechanical maturation.

Solvothermal Synthesis of Single-Crystal Magnetite Hollow Sub-Microspheres: A Novel Formation Mechanism and Magnetic Properties

In this paper, single-crystal magnetite hollow sub-microspheres with a narrow diameter distribution are synthesized through a simple solvothermal process in ethylene glycol in the presence of urea and a small amount of water. The determining role of water in the solvothermal synthesis is studied. It is found that a small amount of water is crucial for the formation of the magnetite hollow spheres. A novel formation mechanism of the magnetite hollow spheres is proposed based on the bubble-assisted Ostwald ripening. It is believed that the appropriate amount of CO2 gas bubbles produced in situ by urea hydrolysis is crucial for the formation of hollow spheres. Because of the existence of gas microbubbles, magnetite solid spheres with a loose core and compact shell form, which is the key factor for the following inside-out Ostwald ripening and the formation of the hollow spheres. Thus, by simple changing of the water dosage, magnetite hollow spheres with different diameters and shell thicknesses are obtained controllably. The magnetic properties of the obtained magnetite hollow spheres are studied. It is found that the saturation magnetization of the magnetite hollow sub-microspheres decreases with the increasing shell thickness, whereas the coercivity and remanent magnetization increase with increasing shell thickness.