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A heat generating device including a heat generating element and an enclosing material entirely enclosing the heat generating element. The heat generating element includes a base sheet formed of a fibrous sheet containing superabsorbent polymer particles and hydrophilic fibers and a layer of a heat

A heat generating device including a heat generating element and an enclosing material entirely enclosing the heat generating element. The heat generating element includes a base sheet formed of a fibrous sheet containing superabsorbent polymer particles and hydrophilic fibers and a layer of a heat generative composition containing oxidizable metal particles on a side of the base sheet. The enclosing material includes a first cover sheet and a second cover sheet bonded together in their peripheral portions to provide a space therebetween in which the heat generating member is placed. The heat generating element in the space is in a non-fixed state to the enclosing material. The first cover sheet has air permeability in part and is disposed on the side of the layer of the heat generative composition. The heat generating device is capable of releasing steam from the side of the first cover sheet during use.

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1. A heat generating device comprising a heat generating element and an enclosing material entirely enclosing the heat generating element, the heat generating element comprising:a base sheet formed of a fibrous sheet containing superabsorbent polymer particles and hydrophilic fibers, anda layer of a

1. A heat generating device comprising a heat generating element and an enclosing material entirely enclosing the heat generating element, the heat generating element comprising:a base sheet formed of a fibrous sheet containing superabsorbent polymer particles and hydrophilic fibers, anda layer of a heat generative composition containing oxidizable metal particles located on a side of the base sheet,the enclosing material comprising a first cover sheet and a second cover sheet bonded to each other in their peripheral portions to provide a space therebetween in which the heat generating member is placed,the heat generating element placed in the space being in a non-fixed state to the enclosing material,the first cover sheet having air permeability in a part thereof and being located on the side of the layer of the heat generative composition, andthe heat generating device being configured to release steam from the side of the first cover sheet while in use, whereinwater being supplied from the base sheet to the heat-generative layer. 2. The heat generating device according to claim 1, wherein the second cover sheet has lower air permeability than the first cover sheet. 3. The heat generating device according to claim 1, wherein the layer of the heat generative composition is located on only one side of the base sheet. 4. The heat generating device according to claim 1, wherein the layer of the heat generative composition is located between the base sheet and another base sheet, the two base sheets being the same or different from each other. 5. The heat generating device according to claim 3, wherein the base sheet has, in its surface on which the layer of the heat generative composition is not provided, a lower water content than the layer of the heat generative composition, thereby supplying water for heat generation to the heat generating layer. 6. The heat generating device according to claim 1, wherein the heat generative composition has its lower part embedded in the base sheet. 7. The heat generating device according to claim 1, showing a change of 350% or less in three-point bending load after the end of heat generation relative to that before the onset of heat generation. 8. A method for making a heat generating element comprising: a base sheet formed of a fibrous sheet containing superabsorbent polymer particles and hydrophilic fibers, anda layer of a heat generative composition containing oxidizable metal particles, an electrolyte, and water located on the base sheet, whereinwater being supplied from the base sheet to the heat-generative layer,the method comprising steps of:applying a coating material containing the oxidizable metal particles and not containing the electrolyte to a side of the base sheet, andadding an aqueous electrolyte solution containing the electrolyte to that side of the base sheet having the coating material applied thereto. 9. The method for making a heat generating element according to claim 8, further including the step of sucking the base sheet from the other side during applying the coating material or after applying the coating material and before adding the aqueous electrolyte solution. 10. A method for making a heat generating device comprising a step of making a heat generating element by the method according to claim 8 and a step of entirely enclosing the resulting heat generating element in an enclosing material, wherein the step of making a heat generating element is a step of making a heat generating element having the layer of the heat generative composition in a non-flowable state, andthe step of enclosing is a step of enclosing the heat generating element having the layer of the heat generative composition in the non-flowable state in the enclosing material. 11. A method for making a heat generating element comprising: a base sheet formed of a fibrous sheet containing superabsorbent polymer particles and hydrophilic fibers, anda layer of a heat generative composition containing oxidizable metal particles, an electrolyte, and water located on the base sheet, whereinwater being supplied from the base sheet to the heat-generative layer,the method comprising steps of:adding an aqueous electrolyte solution containing the electrolyte to a side of the base sheet, andapplying a coating material containing the oxidizable metal particles and not containing the electrolyte to that side of the base sheet having the aqueous electrolyte solution added thereto. 12. A method for making a heat generating device comprising a step of making a heat generating element by the method according to claim 11 and a step of entirely enclosing the resulting heat generating element in an enclosing material, wherein the layer of the heat generative composition is made in a non-flowable state prior to enclosing the heat generating element in the enclosing material. 13. A method for making a heat generating element comprising: a base sheet, anda layer of a heat generative composition containing oxidizable metal particles, an electrolyte, and water on the base sheet,the method comprising steps of:(a) adding the electrolyte in a solid state to a side of the base sheet, and(b) applying a coating material containing the oxidizable metal particles and water and not containing the electrolyte to the side of the base sheet,the steps being carried out in a reverse order or simultaneously. 14. The method for making a heat generating element according to claim 13, wherein the water content is 18% to 48% by mass based on the total mass of the coating material. 15. The method for making a heat generating element according to claim 13, wherein the base sheet contains a superabsorbent polymer, and the electrolyte is added to the coated surface being in a wetted state.